Theory of Evolution by Natural Selection
CHAPTER 5
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
5.1 Adaptations Are a Product of Natural Selection
Stated more precisely, natural selection is the differential success (survival and reproduction) of individuals within the population that results from their interaction with their environment. As outlined by Darwin, natural selection is a product of two conditions: (1) that variation occurs among individuals within a population in some “heritable” characteristic, and (2) that this variation results in differences among individuals in their survival and reproduction as a result of their interaction with the environment. Natural selection is a numbers game. Darwin wrote:
Among those individuals that do reproduce, some will leave more offspring than others. These individuals are considered more fit than the others because they contribute the most to the next generation. Organisms that leave few or no offspring contribute little or nothing to the succeeding generations and so are considered less fit.
The fitness of an individual is measured by the proportionate contribution it makes to future generations. Under a given set of environmental conditions, individuals having certain characteristics that enable them to survive and reproduce are selected for, eventually passing those characteristics on to the next generation. Individuals without those traits are selected against, failing to pass their characteristics on to future generations. In this way, the process of natural selection results in changes in the properties of populations of organisms over the course of generations, by a process known as evolution.
An adaptation is any heritable behavioral, morphological, or physiological trait of an organism that has evolved over a period of time by the process of natural selection such that it maintains or increases the fitness (long-term reproductive success) of an organism under a given set of environmental conditions. The concept of adaptation by natural selection is central to the science of ecology. The study of the relationship between organisms and their environment is the study of adaptations. Adaptations represent the characteristics (traits) that enable an organism to survive, grow, and reproduce under the prevailing environmental conditions. Adaptations likewise govern the interaction of the organism with other organisms, both of the same and different species. How adaptations enable an organism to function in the prevailing environment—and conversely, how those same adaptations limit its ability to successfully function in other environments—is the key to understanding the distribution and abundance of species, the ultimate objective of the science of ecology.
5.2 Genes Are the Units of Inheritance
By definition, adaptations are traits that are inherited—passed from parent to offspring. So to understand the evolution of adaptations, we must first understand the basis of inheritance: how characteristics are passed from parent to offspring and what forces bring about changes in those same characteristics through time (from generation to generation).
At the root of all similarities and differences among organisms is the information contained within the molecules of DNA (deoxyribonucleic acid). You will recall from basic biology that DNA is organized into discrete subunits—genes—that form the informational units of the DNA molecule. A gene is a stretch of DNA coding for a functional product (ribonucleic acid: RNA). The product is usually messenger RNA (mRNA) and mRNA ultimately results in the synthesis of a protein. The alternate forms of a gene are called alleles (derived from the term allelomorphs, which in Greek means “different form”). The process is called gene expression in which DNA is used in the synthesis of products such as proteins. All of the DNA in a cell is collectively called the genome.
Genes are arranged in linear order along microscopic, threadlike bodies called chromosomes. The position occupied by a gene on the chromosome is called the locus (Latin for place). In most multicellular organisms, each individual cell contains two copies of each type of chromosome (termed homologous chromosomes). In the process of asexual reproduction, both chromosomes are inherited from the single parent. In sexual reproduction, one is inherited from its mother through the ovum and one inherited from its father through the sperm. At any locus, therefore, every diploid individual contains two copies of the gene—one at each corresponding position in the homologous chromosomes. These two copies are the alleles of the gene in that individual. If the two copies of the gene are the same, then the individual is homozygous at that given locus. If the two alleles at the locus are different, then the individual is heterozygous at the locus. The pair of alleles present at a given locus defines the genotype of an individual; therefore, homozygous and heterozygous are the two main categories of genotypes.
5.3 The Phenotype Is the Physical Expression of the Genotype
The outward appearance of an organism for a given characteristic is its phenotype. The phenotype is the external, observable expression of the genotype. When an individual is heterozygous, the two different alleles may produce an individual with intermediate characteristics or one allele may mask the expression of the other ( Figure 5.2). In the case in which one allele masks the expression of the other, the allele that is expressed is referred to as the dominant allele, whereas the allele that is masked is called the recessive allele. If the allele is recessive, it will only be expressed if the individual is homozygous for that allele (homozygous recessive). If the physical expression of the heterozygous individual is intermediate between those of the homozygotes, the alleles are said to be incomplete dominance, and each allele has a specific value (proportional effect) that it contributes to the phenotype.
Phenotypic characteristics that fall into a limited number of discrete categories, such as the example of flower color presented in Figure 5.2, are referred to as qualitative traits. Even though all genetic variation is discrete (in the form of alleles), most phenotypic traits have a continuous distribution. These traits, such as height or weight, are referred to as quantitative traits. The continuous distribution of most phenotypic traits occurs for two reasons. First, most traits have more than one gene locus affecting them. For example, if the phenotypic characteristic of flower color illustrated in Figure 5.2 is controlled by two loci rather than a single locus (each with two alleles—A:a and B:b), there are nine possible genotypes ( Figure 5.3). In contrast to the three distinct flower colors (phenotypes) produced in the case of a single locus, there is now a range of flower colors varying in hue between dark red and white depending on the number of alleles coding for the production of red pigment (see Figure 5.2). The greater the number of loci, the greater is the range of possible phenotypes. The second factor influencing phenotypic variation is the environment.
5.4 The Expression of Most Phenotypic Traits Is Affected by the Environment
The expression of most phenotypic traits is influenced by the environment; that is to say, the phenotypic expression of the genotype is influenced by the environment. Because environmental factors themselves usually vary continuously—temperature, rainfall, sunlight, level of predation, and so on—the environment can cause the phenotype produced by a given genotype to vary continuously. To illustrate this point, we can use the example of flower color controlled by two loci presented previously (and in Figure 5.3). Pigment production during flower development can be affected by temperature. If temperatures below some optimal value or range function to reduce the expression of the A and B alleles in the production of red pigment, fluctuations in temperatures over the period of flower development in the population of plants will function to further increase the range of flower colors (shades between red and white) produced by the nine genotypes.
Interpreting Ecological Data
Q1. Which of the two genotypes (G1 or G2) exhibits the greater norm of reaction?
Q2. What would the line look like for a genotype that did not exhibit phenotypic plasticity?
Q3. Is there any environment in which the two genotypes will express the same phenotype?
Q4. Is it possible for the two genotypes to exhibit the same phenotype?
The ability of a genotype to give rise to different phenotypic expressions under different environmental conditions is termed phenotypic plasticity. The set of phenotypes expressed by a single genotype across a range of environmental conditions is referred to as the norm of reaction ( Figure 5.4). Note that we are not talking about different genotypes adapted to different environmental conditions, but about a single genotype (set of alleles) capable of altering the development or expression of a phenotypic trait in response to the conditions encountered by the individual organism. The result is the improvement of the individual’s ability to survive, grow, and reproduce under the prevailing environmental conditions (i.e., increase fitness). For example, the bodies of many species of insects change in color in response to the prevailing temperature during development ( Figure 5.5). Development under colder temperatures typically results in darker coloration. Darker coloration most likely facilitates increased absorption of solar radiation, allowing them to compensate for the lower temperature (see Chapter 7 for discussion of thermoregulation in animals).
Some of the best examples of phenotypic plasticity occur among plants. The size of the plant, the ratio of reproductive tissue to vegetative tissue, and even the shape of the leaves may vary widely at different levels of nutrition, light, moisture, and temperature. An excellent illustration of phenotypic plasticity in plants is the work of Sonia Sultan of Wesleyan University. Sultan’s research focuses on phenotypic plasticity in plant species in response to resource availability. In a series of greenhouse experiments, she examined the developmental response of the herbaceous annual Polygonum lapathifolium (common name curlytop knotweed) to different light environments. Sultan grew different individuals of the same genotype for eight weeks at two light levels: low light (20 percent available photosynthetically active radiation [PAR]) and high light (100 percent available PAR). Individuals of the same genotype grown under low-light conditions produced less biomass (slower growth rate), but produced far more photosynthetic leaf area per unit of biomass through changes in biomass allocation, morphology, and structure ( Figure 5.6). Individuals grown under low-light conditions produced large, thin leaves and few branches. In contrast, the larger high-light plants grew narrow leaves on many more branches. This response is referred to as developmental plasticity. As such, these changes are irreversible. After the adult plant develops, these patterns of biomass allocation (proportions of leaf, stem, and root) will remain largely unchanged, regardless of any changes in the light environment.
In contrast to developmental plasticity, other forms of phenotypic plasticity in response to prevailing environmental conditions are reversible. For example, fish have an upper and lower limit of tolerance to temperature (see Chapter 7). They cannot survive at water temperatures above and below these limits. However, these upper and lower limits change seasonally as water temperatures warm and cool. This pattern of seasonal change in temperature tolerance is illustrated in the work of Nann Fangu and Wayne Bennett of the University of West Florida. Fangu and Bennett measured seasonal changes in the temperature tolerances of Atlantic stingrays (Dasyatis sabina) that inhabit shallow bays of the Florida coast. Their data for individuals inhabiting St. Josephs Bay on the Gulf Coast of Florida show a systematic shift in the critical minimum and maximum temperatures with seasonal changes in the ambient environmental (water) temperature ( Figure 5.7). As water temperatures change seasonally, shifts in enzyme and membrane structure allow the individual’s physiology to adjust slowly over a period of time, influencing heart rate, metabolic rate, neural activity, and enzyme reaction rates. These reversible phenotypic changes in an individual organism in response to changing environmental conditions are referred to as acclimation.
Acclimation is a common response in both plant and animal species involving adjustments relating to biochemical, physiological, morphological, and behavioral traits.
5.5 Genetic Variation Occurs at the Level of the Population
Adaptations are the characteristics of individual organisms—a reflection of the interaction of the genes and the environment. They are the product of natural selection. Although the process of natural selection is driven by the success or failure of individuals, the population—the collective of individuals and their alleles—changes through time, as individuals either succeed or fail to pass their genes to successive generations. For this reason, to understand the process of adaptation through natural selection, we must first understand how genetic variation is organized within the population.
A species is rarely represented by a single, continuous interbreeding population. Instead, the population of a species is typically composed of a group of subpopulations—local populations of interbreeding individuals, linked to each other in varying degrees by the movement of individuals (see Sections 8.2 and 19.7 for discussion of metapopulations). Thus, genetic variation can occur at two hierarchical levels, within subpopulations and among subpopulations. When genetic variation occurs among subpopulations of the same species, it is called genetic differentiation.
Interpreting Ecological Data
Q1. What type of data do the original measures of beak depth represent? (See Chapter 1, Quantifying Ecology 1.1.)
Q2. How have the original measurements of beak depth been transformed for presentation purposes in Figure 5.8?
Q3. What is the range (maximum – minimum values) of beak depths observed for the sample of individuals presented in Figure 5.8? (Categories are in units of 0.2 mm.)
The sum of genetic information (alleles) across all individuals in the population is referred to as the gene pool. The gene pool represents the total genetic variation within a population. Genetic variation within a population can be quantified in several ways. The most fundamental measures are allele frequency and genotype frequency . The word frequency in this context refers to the proportion of a given allele or genotype among all the alleles or genotypes present at the locus in the population.
5.6 Adaptation Is a Product of Evolution by Natural Selection
We have defined evolution as changes in the properties of populations of organisms over the course of generations (Section 5.1). More specifically, phenotypic evolution can be defined as a change in the mean or variance of a phenotypic trait across generations as a result of changes in allele frequencies. In favoring one phenotype over another, the process of natural selection acts directly on the phenotype. But in doing so, natural selection changes allele frequencies within the population. Changes in allele frequencies from parental to offspring generations are a product of differences in relative fitness (survival and reproduction) of individuals in the parental generation.
The work of Peter Grant and Rosemary Grant provides an excellent documented example of natural selection. The Grants have spent more than three decades studying the birds of the Galápagos Islands, the same islands whose diverse array of animals so influenced the young Darwin when he was a naturalist aboard the expeditionary ship HMS Beagle. Among other events, the Grants’ research documented a dramatic shift in a physical characteristic of finches inhabiting some of these islands during a period of extreme climate change.
Recall from our initial discussion in Section 5.1 that natural selection is a product of two conditions: (1) that variation occurs among individuals within a population in some heritable characteristic and (2) that this variation results in differences among individuals in their survival and reproduction. Figure 5.8 shows variation in beak size in Darwin’s medium ground finch (Geospiza fortis) on the 40-hectare islet of Daphne Major, one of the Galápagos Islands off the coast of Ecuador. Heritability of beak size in this species was established by examining the relationship between the beak size of parents and their offspring ( Figure 5.9).
(After Boag and Grant 1984.)
During the early 1970s, the island received an average rainfall of between 127 and 137 millimeters (mm) per year, supporting an abundance of seeds and a large finch population (1500 birds). In 1977, however, a periodic shift in the climate of the eastern Pacific Ocean—called La Niña—altered weather patterns over the Galápagos, causing a severe drought (see Chapter 2, Section 2.9). That season, only 24 mm of rain fell. During the drought, seed production declined drastically. Small seeds declined in abundance faster than large seeds did, increasing the average size and hardness of seeds available ( Figure 5.11). The decline in food (seed) resources resulted in an 85 percent decline in the finch population as a result of mortality and possible emigration ( Figure 5.12a). Mortality, however, was not equally distributed across the population (Figure 5.12b). Small birds had difficulty finding food, whereas large birds, especially males with large beaks, had the highest rate of survival because they were able to crack large, hard seeds.
The graph in Figure 5.12b represents a direct measure of the differences in fitness (as measured by survival) among individuals in the population as a function of differences in phenotypic characteristics (beak size), the second condition for natural selection. The phenotypic trait that selection acts directly upon is referred to as the target of selection; in this example, it is beak size. The selective agent is the environmental cause of fitness differences among organisms with different phenotypes, or in this case, the change in food resources (abundance and size distribution of seeds).
The increased survival rate of individuals with larger beaks resulted in a shift in the distribution of beak size (phenotypes) in the population (Figure 5.13). This type of natural selection, in which the mean value of the trait is shifted toward one extreme over another (Figure 5.14a), is called directional selection. In other cases, natural selection may favor individuals near the population mean at the expense of the two extremes; this is referred to as stabilizing selection (Figure 5.14b). When natural selection favors both extremes simultaneously, although not necessarily to the same degree, it can result in a bimodal distribution of the characteristic(s) in the population (Figure 5.14c). Such selection, known as disruptive selection, occurs when members of a population are subject to different selection pressures.
Interpreting Ecological Data
Q1. Figure 5.12b shows the survival of ground finches as a function of beak size during the period of drought. How does the graph in Figure 5.12b relate to this figure?
Q2. How do the patterns of relative fitness shown in the graphs on the left-hand column give rise to the corresponding patterns of selection illustrated by the arrows in the graphs shown in the right-hand column?
The work of Beren Robinson of Guelph University in Canada provides an excellent example of disruptive selection. In studying the species of threespine stickleback (Gasterosteus aculeatus), which occupies Cranby Lake in the coastal region of British Columbia, Robinson found that individuals sampled from the open-water habitat (limnetic habitat) differed morphologically from individuals sampled from the shallower nearshore waters (benthic habitat). In a series of experiments, Robinson established that these individuals represented distinct phenotypes that are products of natural selection promoting divergence within the population. He initially established that morphological differences between the two forms were heritable, rather than an expression of phenotypic plasticity in response to the two different habitats or diets. He reared offspring of the two forms under identical laboratory conditions (environmental conditions and diet) and although there was some degree of phenotypic plasticity, differences in most characteristics remained between the two forms. On average, the benthic form (BF) had (1) shorter overall body length, (2) deeper body, (3) wider mouth, (4) more dorsal spines, and (5) fewer gill rakers than did the limnetic form (LF) ( Figure 5.15a).
The two habitats in the lake—benthic and limnetic—provide different food resources; so to determine the agent of selection that caused divergence within the population, Robinson conducted feeding trials in the laboratory to test for trade-offs in the foraging efficiency of the two forms on food resources found in the two habitats. The foraging success of individual fish was assessed in two artificial habitats, mimicking conditions in the limnetic and benthic environments. Two food types were used in the trials. Brine shrimp larvae (Artemia), a common prey found in open water, were placed in the artificial limnetic habitats. Larger amphipods, fast-moving arthropods with hard exoskeletons that forage on dead organic matter on the sediment surface, were placed in the artificial benthic habitats.
Results of the foraging trials revealed distinct differences in the foraging success of the two morphological forms (phenotypes; Figure 5.15b). The LF individuals were most successful at foraging on the brine shrimp larvae. They had a higher consumption rate and required only half the number of bites to consume as compared to the BF individuals. In contrast, BF individuals had a higher intake rate for amphipods and on average consumed larger amphipods than did LF individuals.
Robinson was able to determine that the higher intake rate of brine shrimp larvae by LF individuals was related to this form’s greater number of gill rakers, and greater mouth width was related to the higher intake rate of amphipods by BF individuals. Therefore, he found that foraging efficiency was related to morphological differences between the two forms, suggesting that divergent selection in the two distinct phenotypes represents a trade-off in characteristics related to the successful exploitation of these two distinct habitats and associated food resources.
5.7 Several Processes Other than Natural Selection Can Function to Alter Patterns of Genetic Variation within Populations
Natural selection is the only process that leads to adaptation because it is the only one in which the changes in allele frequency from one generation to the next are a product of differences in the relative fitness (survival and reproduction) of individuals in the population. Yet not all phenotypic characteristics represent adaptations, and processes other than natural selection can be important factors influencing changes in genetic variation (allele and genotype frequencies) within populations. For example, mutation is the ultimate source of the genetic variation that natural selection acts upon. Mutations are heritable changes in a gene or a chromosome. The word mutation refers to the process of altering a gene or chromosome as well as to the product, the altered state of the gene or chromosome. Mutation is a random force in evolution that produces genetic variation. Any altered phenotypic characteristic resulting from mutation may be beneficial, neutral, or harmful. Whether a mutation is beneficial depends on the environment. A mutation that enhances an organism’s fitness in one environment could harm it in another. Most of the mutations that have significant effect, however, are harmful, but the harmful mutations do not survive long. Natural selection eliminates most deleterious genes from the gene pool, leaving behind only genes that enhance (or at least do not harm) an organism’s ability to survive, grow, and reproduce in its environment.
Another factor that can directly influence patterns of genetic variation within a population is a change in allele frequencies as a result of random chance—a process known as genetic drift. Recall from basic biology that the recombination of alleles in sexual reproduction is a random process. The offspring produced in sexual reproduction, however, represent only a subset of the parents’ alleles. If the parents have only a small number of offspring, then not all of the parents’ alleles will be passed on to their progeny as a result of the random assortment of chromosomes at meiosis (the process of recombination). In effect, genetic drift is the evolutionary equivalent of sampling error, with each successive generation representing only a subset or sample of the gene pool from the previous generation.
In a large population, genetic drift will not affect each generation much because the effects of the random nature of the process will tend to average out. But in a small population, the effect could be rapid and significant. To illustrate this point, we can use the analogy of tossing a coin. With a single toss of the coin, the probability of each of the two possible outcomes, heads or tails, is equal, or 50 percent. Likewise, with a series of four coin tosses, the probability of the outcome being two heads and two tails is 50 percent. But each individual outcome in the coin tosses is independent; therefore, in a series of four coin tosses, there is also a probability of 0.0625, or 6.25 percent, that the outcome will be four heads. The probability of the outcome being all heads drops to 9.765 × 10−4 if the number of tosses is increased to 10, and this probability drops to 8.88 × 10−16 for 50 tosses. Likewise, the probability of heterozygous (Aa) individuals in the population producing only homozygous (either aa or AA) offspring under a system of random mating decreases with increasing population size.
Patterns of genetic variation within a population can also be influenced by the movement of individuals into, or out of, the population. Recall from the discussion of genetic variation in Section 5.5 that the population of a species is typically composed of a group of subpopulations—local populations of interbreeding individuals that are linked to one another in varying degrees by the movement of individuals (see Chapter 8). Migration is defined as the movement of individuals between local populations, whereas gene flow is the movement of genes between populations (see Chapter 8). Because individuals carry genes, the terms are often used synonymously; however, if an individual immigrates into a population but does not successfully reproduce, the new genes are not established in the population. Migration is a potent force in reducing the level of population differentiation (genetic differences among local populations; see Section 5.5).
One of the most important principles of genetics is that under conditions of random mating, and in the absence of the factors discussed thus far—natural selection, mutation, genetic drift, and migration—the frequency of alleles and genotypes in a population remains constant from generation to generation. In other words, no evolutionary change occurs through the process of sexual reproduction itself. This principle, referred to as the Hardy–Weinberg principle, is named for Godfrey Hardy and Wilhelm Weinberg, who each independently published the model in 1908 (see Quantifying Ecology 5.1). Mating is random when the chance that an individual mates with another individual of a given genotype is equal to the frequency of that genotype in the population. When individuals choose mates nonrandomly with respect to their genotype—or more specifically, select mates based on some phenotypic trait—the behavior is referred to as assortative mating. Perhaps the most recognized and studied form of assortative mating is female mate choice. Female mate choice is the behavior in which females exhibit a bias toward certain males as mates based on specific phenotypic traits (often secondary sex characteristics), such as body size or coloration (see Chapter 10, Section 10.11).
Positive assortative mating occurs when mates are phenotypically more similar to each other than expected by chance. Positive assortative mating is common, and one of the most widely reported examples relates to the timing of reproduction. Plants mate assortatively based on flowering time. In populations of plants with an extended flowering time, early flowering plants are often no longer flowering when late flowering plants are in bloom.
The genetic effect of positive assortative mating is an increase in the frequency of homozygotes with a decrease in the frequency of heterozygotes in the population. Think of a locus where AA individuals tend to be larger than Aa, which in turn are larger than aa individuals. With positive assortative mating, AA will mate with AA, and aa with other aa. All of these matings will produce only homozygous offspring. Even mating between Aa individuals will result in half of the offspring being homozygous. The genetic effects of positive assortative mating are only at the loci that affect the phenotypic characteristic by which the organisms are selecting mates.
Negative assortative mating occurs when mates are phenotypically less similar to each other than expected by chance. Though not as common as positive assortative mating, negative assortative mating results in an increase in the frequency of heterozygotes.
A special case of nonrandom mating is inbreeding. Inbreeding is the mating of individuals in the population that are more closely related than expected by random chance. Unlike positive assortative mating, inbreeding increases homozygosity at all loci. Inbreeding affects all loci equally because related individuals are genetically similar by common ancestry, and they are therefore more likely to share alleles throughout the genome than unrelated individuals.
Inbreeding can be detrimental. Offspring are more likely to inherit rare, recessive, deleterious genes. These genes can cause decreased fertility, loss of vigor, reduced fitness, reduced pollen and seed fertility in plants, and even death. These consequences are referred to as inbreeding depression.
As we have seen from the preceding discussion, nonrandom mating changes genotypic frequencies from one generation to the next, but assortative mating does not directly result in a change of allele frequencies within a population. The other three processes discussed—mutation, migration, and genetic drift, together with natural selection—alter the allele frequencies, and therefore result in a shift in the distribution of genotypes (and potentially phenotypes) within the population. As such, all four processes function as agents of evolution. However, natural selection is special among the four evolutionary processes because it is the only one that leads to adaptation. The other three can only speed up or slow the development of adaptations.
Quantifying Ecology 5.1 Hardy–Weinberg Principle
The Hardy–Weinberg principle states that both allele and genotype frequencies will remain the same in successive generations of a sexually reproducing population if certain criteria are met: (1) mating is random, (2) mutations do not occur, (3) the population is large, so that genetic drift is not a significant factor, (4) there is no migration, and (5) natural selection does not occur.
If we have only two alleles at a locus, designated as A and a, then the usual symbols for designating their frequencies are p and q, respectively. Because frequencies (proportions) must sum to 1, then:
p+q=1orq=1−pp+q=1 or q=1 −p
Genotypic frequencies are typically designated by uppercase letters. In the case of a locus with two alleles, P is the frequency of AA, H is the frequency of Aa, and Q is the frequency of aa. As with gene frequencies, genotype frequencies must sum to 1:
P+H+Q=1P+H+Q=1
Given a population having the genotypic frequencies of
P=0.64,H=0.32,and Q =0.04P=0.64,H=0.32, and Q =0.04
we can calculate the allele frequencies as follows:
P=P+H/2=0.64+(0.32/2)=0.8P=P+H/2=0.64+(0.32/2)=0.8
q=Q+H/2=0.04+(0.32/2)=0.2q=Q+H/2=0.04+(0.32/2)=0.2
With a population consisting of the three genotypes just described (AA, Aa, and aa), there are six possible types of mating ( Table 1 ). For example, the mating AA × AA occurs only when an AA female mates with an AA male, with the frequency of occurrence being P × P (or P 2) under the conditions of random mating. Similarly, an AA × Aa mating occurs when an AA female mates with an Aa male (proportion P × H) or when an Aa female mates with an AA male (proportion H × P). Therefore, the overall proportion of AA × Aa matings is PH + HP = 2PH. The frequencies of these and the other four types of matings are given in the second column of Table 1.
To calculate the offspring genotypes produced by these matings, we must first examine the offspring produced by each of the six possible pairings of parental genotypes (Table 1). Because homozygous AA genotypes produce only A-bearing gametes (egg or sperm), and homozygous aa genotypes produce only a-bearing gametes, the mating of AA × AA individuals will produce only AA offspring, and likewise, the mating of aa × aa individuals will produce only offspring with genotype aa. In addition, the mating of AA × aa individuals will produce only heterozygous offspring (Aa).
In contrast to homozygous individuals, heterozygous individuals produce both A- and a-bearing gametes.
Therefore, the mating of a heterozygous individual with a homozygous (either AA or aa) or another heterozygous individual will produce offspring of all three possible genotypes (AA, Aa, and aa). The relative frequencies of offspring genotypes depend on the specific combination of parents (Figure 1). The offspring frequencies presented in Figure 1 are based on the assumption that an Aa heterozygote individual produces an equal number of A- and a-bearing gametes (referred to as Mendelian segregation).
Using the data presented in Figure 1 and the frequencies of the different types of matings in column 2 of Table 1, the genotype frequencies of the offspring, denoted as P’ (AA), H’ (Aa), and Q’ (aa), are presented in column 3 of Table 1. The new genotype frequencies are calculated as the sum of the products shown at the bottom of Table 1. For each genotype, the frequency of each mating producing the genotype is multiplied by the fraction of the genotypes produced by that mating.
We can now calculate the allele frequencies (p') for the generation of offspring (designated as q') using the formula presented previously:
p′=p′+H′/2=0.64+(0.32/2)=0.8p'=p'+H'/2=0.64+(0.32/2)=0.8
q′+Q′+H′/2=0.04+(0.32/2)=0.2q'+Q'+H'/2=0.04+(0.32/2)=0.2
Note that both the genotype and allele frequencies of the offspring generation are the same as those of the parental generation.
of these matings are shown.
In natural populations the assumptions of the Hardy–Weinberg principle are never fully met. Mating is not random, mutations do occur, individuals move between local populations, and natural selection does occur. All of these circumstances change the frequencies of genotypes and alleles from generation to generation, acting as evolutionary forces in a population. The beauty of the Hardy–Weinberg principle is that it functions as a null model, where deviations from the expected frequencies can provide insight into the evolutionary forces at work within a population.
How would the frequency of heterozygotes in the population change if the frequency for the A allele in the example described was p = 0.5?
When the frequency of an allele is greater than 0.8, most of these alleles are contained in homozygous individuals (as illustrated in the preceding example). When the frequency of an allele is less than 0.1, in which genotype are most of these alleles?
The example of natural selection in the population of medium ground finches as described previously represents a shift in the distribution of phenotypes in the population inhabiting the island of Daphne Major in response to environmental changes that occurred over time (period of drought). This shift in the mean phenotype (beak size) reflects a change in genetic variation (allele and genotype frequencies) within the population. Natural selection can also function to alter genetic variation among local populations as a result of local differences in environmental conditions—the process of genetic differentiation (see Section 5.5).
Species having a wide geographic distribution often encounter a broader range of environmental conditions than do those species whose distribution is more restricted. The variation in environmental conditions can give rise to a corresponding variation in morphological, physiological, and behavioral characteristics (phenotypes). Significant differences often exist among local populations of a single species inhabiting different regions. The greater the distance between populations, the more pronounced the differences often become as each population adapts to the locality it inhabits. The changes in phenotype across the landscape therefore reflect the changing nature of natural selection operating under the different local environmental conditions. Geographic variation within a species in response to changes in environmental conditions can result in the evolution of clines, ecotypes, and geographic isolates or subspecies.
A cline is a measurable, gradual change over a geographic region in the average of some phenotypic character, such as size and coloration. Clines are usually associated with an environmental gradient that varies in a continuous manner across the landscape, such as changes in temperature or moisture with elevation or latitude. Continuous variation in the phenotypic character across the species distribution results from gene flow from one population to another along the gradient. Because environmental constraints influencing natural selection vary along the gradient, any one population along the gradient will differ genetically to some degree from another—the difference increasing with the distance between the populations.
Clinal differences exist in size, body proportions, coloration, and physiological adaptations among animals. For example, the fence lizard (Sceloporus undulatus) is one of the most widely distributed species of lizards in North America, ranging throughout the eastern two-thirds of the United States and into northern Mexico. Across its range, the fence lizard exhibits a distinct gradient of increasing body size with latitude ( Figure 5.16). Lizards from northern latitudes are larger than lizards from southern latitudes. Furthermore, lizards from higher elevations in geographically proximal areas exhibit larger body size than lizards from lower elevations. Thus, mean body size increases along an environmental gradient of decreasing mean annual temperature.
Similar clines are observed in plant species. Alicia Montesinos-Navarro of the University of Pittsburgh and colleagues examined phenotypic variation in Arabidopsis thaliana, a small annual flowering plant species that is native to Europe, Asia, and northwestern Africa. The researchers examined 17 natural populations that occupy an altitudinal gradient in the region of northeastern Spain. Along the gradient, precipitation increases, but maximum spring temperature and minimum winter temperature decrease with altitude. Examination of the local populations revealed a systematic variation in a variety of phenotypic characteristics. Aboveground mass, number of rosette leaves at bolting (a measure of size at reproduction), developmental time, and number of seeds and seed weight increased with altitude (Figure 5.17). Although these changes in phenotypic characteristics are clearly in response to the gradient of environmental conditions with altitude, how can the researchers be sure that the changes in phenotype represent changes in allele and genotype frequencies between populations rather than phenotypic plasticity?
Recall from Section 5.4 that phenotypic plasticity is the ability of a single genotype to produce different phenotypes under different environmental conditions (norms of reaction; see Figure 5.4). A common approach used to determine if observed phenotypic differences between local populations represent differences in allele frequencies (genetic differentiation) or phenotypic plasticity is the common garden experiment. In this experiment individuals (genotypes) from the different populations are grown under controlled environmental conditions—a common garden. If the phenotypic differences observed in the local populations are maintained in individuals grown in the common garden, the differences in phenotype represent genetic differences between the populations (genetic differentiation). If the individuals from the different populations no longer exhibit differences in phenotypic characteristics, then the differences observed in the local populations in their natural environments are a function of phenotypic plasticity. When Montesinos-Navarro and her colleagues grew genotypes from the 17 local populations under uniform controlled conditions (the common garden experiment), the phenotypic differences were maintained, revealing that the A. thaliana cline represents adaptations to local environmental conditions along the altitudinal gradient.
Clinal variation may show marked discontinuities. Such abrupt changes, or step clines, often reflect abrupt changes in local environments. Such variants are called ecotypes. An ecotype is a population adapted to its unique local environmental conditions (see this chapter, Field Studies: Hopi Hoekstra). For example, a population inhabiting a mountaintop may differ from a population of the same species in the valley below. This is the case with the weedy herbaceous annual Diodia teres (with the common name poorjoe) that occurs in a wide variety of habitats in eastern North America (Figure 5.18). In the southeastern United States, there are two distinct ecotypes: one occurs in inland agricultural fields and the other in coastal sand-dune habitats. The populations differ strikingly in morphology. Among many differences, the coastal population has heavier stem pubescence (covered with short, soft, erect hairs), a more flattened growth habit, and larger seed size relative to the inland population. To understand the patterns of local adaptation acting on these two distinct ecotypes, Nicholas Jordan of the University of Minnesota undertook a series reciprocal transplant studies. In these studies, seeds from the two ecotypes were planted in each of the two distinct habitats (inland and coastal). By comparing patterns of survival, growth, and reproduction of the two ecotypes in the two different habitats, Jordan was able to analyze selection for and against native and introduced individuals. Results of the study reveal two important facts regarding the two ecotypes. First, phenotypic differences between the two ecotypes were maintained in both environments indicating that the ecotypes represent genetic differences between the two populations rather than phenotypic plasticity. Second, in each of the two habitats, the native ecotype performed better than the introduced ecotype in comparisons of survival, growth, and seed production. Each of the two ecotypes exhibited a greater relative fitness in its native habitat indicating that the phenotypic differences represent adaptations to the two different local environments (see Figure 5.18).
Although ecotypes typically represent distinct genetic populations (with respect to the phenotypic characteristics that relate to the local adaptations), gene flow occurs to varying degrees between adjacent populations, and often, zones of hybridization (mating between ecotypes) can be found. In some cases, however, geographic features such as rivers or mountain ranges that impede the movement of individuals (or gametes) can restrict gene flow between adjacent populations. For example, the southern Appalachian Mountains are noted for their diversity of salamanders. This diversity is fostered in part by a rugged terrain, an array of environmental conditions, and the limited ability of salamanders to disperse ( Figure 5.19). Populations become isolated from one another, preventing a free flow of genes. One species of salamander, Plethodon jordani, formed a number of semi-isolated populations, each characteristic of a particular part of the mountains. These subpopulations make up geographic isolates, in which some extrinsic barrier—in the case of the salamanders, rivers and mountain ridges—prevents the free flow of genes among subpopulations. The degree of isolation depends on the efficiency of the extrinsic barrier, but rarely is the isolation complete. These geographic isolates are often classified as subspecies, a taxonomic term for populations of a species that are distinguishable by one or more characteristics. Unlike clines, for subspecies we can draw a geographic line separating the subpopulations into subspecies. Nevertheless, it is often difficult to draw the line between species and subspecies.
Field Studies Hopi Hoekstra
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts
A key focus of evolutionary ecology is on identifying traits (phenotypic characteristics) that are ecologically important and determining how those traits affect the relative fitness of individuals in different environments. Color is one phenotypic characteristic that has been shown to have a major influence on the way in which organisms interact with their environment. Color plays a central role in a wide variety of ecological processes relating to survival and reproduction and can therefore directly affect an individual’s fitness. One of the more widely studied adaptive roles of color is cryptic coloration, that is, coloring that allows an animal to blend into the surrounding environment and therefore avoid detection by potential predators (see Chapter 14, Section 14.10). Geographic variation in habitat, such as the background color of surface substrate (soils, rocks, or snow cover), vegetation, or water, can present different adaptive environments and selective pressures resulting in localized differences in patterns of body color. In mammals, some of the most extreme variations in coat color occur in deer mice (genus Peromyscus). These mice occur throughout most of continental North America. One species of deer mouse that exhibits a large degree of variation in coat color over short geographic distances is Peromyscus polionotus. This species of Peromyscus occurs throughout the southeastern United States where it is commonly referred to as “oldfield mouse” because it inhabits abandoned agricultural fields. Understanding the evolution of variations in coat color within this species has been a focus of studies by the evolutionary ecologists Hopi Hoekstra and her students at Harvard University.
In the southeastern United States, oldfield mice (P. polionotus) typically occupy overgrown fields with dark soil and have a dark brown coat, which serves to camouflage the mice from predators (Figure 1, left). In the last few thousand years, however, these mice have colonized the sand dunes of Florida’s coasts. Here the sands are lighter in color (white sands) than the inland soil, and there is much less vegetation cover. These beach-dwelling mice, known as “beach mice,” have reduced pigmentation and have a much lighter color (compared to the inland populations) that blends well into the light-colored sand (Figure 1, right). Using a combination of field studies, classic genetics, and modern molecular biology, Hoekstra and her students are working to understand how, through changes in pigmentation genes, these mice have adapted to this new environment.
Although it may seem straightforward that being light colored would be a selective advantage over darker pigmentation on the white sand dunes of coastal Florida, evidence is needed to establish that differences in pigmentation among local populations represent adaptations. To accomplish this task, Hoekstra and colleague Sacha Vignieri undertook a series of field experiments to quantify the selective advantage of having pigmentation patterns that better match the surrounding environment. The experimental approach involved placing darker-colored mice from an inland, oldfield population (Lafayette Creek Wildlife Area, Florida) into the coastal dune environment (Topsail Hill State Park, Florida) and lighter-colored individuals from the dune environment into the inland environment. Rates of survival for the transplanted individuals could then be compared with those of local populations whose coat color better matched the surrounding environment. Using live mice for such an experiment, however, can present a number of major problems, including capturing hundreds of mice, transporting and releasing them into new locations, and then the difficulty of determining the fate of the mice over the experimental period. To avoid these problems, the researchers used a unique approach of creating model mice using nonhardening plasticine (a type of modeling clay). Although simple, this method has several advantages over using live mice. First, because plasticine preserves evidence of predation attempts (tooth, beak, or claw imprints), it is possible to quantify both predation rate and predator type. Additionally, using models, they were able to deploy a large number of individuals within a given environment. Finally, this experimental approach allowed them to focus on variation in a single trait of interest—coat color—controlling for other traits such as behavioral differences.
Some 250 models of P. polionotus were made, half of which were painted to mimic the coat color and pattern of the darker oldfield mouse and half the light-colored beach mouse (Figure 2). Each afternoon, the researchers set out the light and dark models in a straight line and in random order about 10 m apart in a habitat known to be occupied by either beach or mainland mice (and hence their natural predators). To determine the difference in color (brightness) between the model and the substrate on which it was placed, soil samples from around each deployed model were collected and measured for brightness (light reflection). The researchers would then return the following day and record which models showed evidence of predation. The shape of the imprint of the model left by the predator (beak or tooth marks) and the surrounding tracks gave clues as to the type of predator. By documenting predation events in both habitats, the researchers could determine how differences in color between the model and the background environment (soil) influenced rates of predation.
Results of the experiment revealed that models that were both lighter and darker than their local environment experienced a lower rate of survival (greater rate of predation) than models that were better matched in color to the soil on which they were placed (see Figure 2). Seventy five percent of all predation events occurred on mice that did not match their substrate, representing a large selective disadvantage. In the light-substrate beach environment, most attacked mice were dark, but some light models also were attacked. By analyzing the soil samples that were collected at the location where each model was placed, the researchers found that these light-colored models were all much lighter than their local substrate. In other words, selection acts against mice that are either too dark or too light relative to their background. This result demonstrates that in addition to predation acting as an agent of selection resulting in significant differences in pigmentation between inland and beach populations, there is also selection for subtle color phenotypes within a habitat.
In addition to establishing the role of natural selection in the evolution of phenotypic variation in color among local populations of P. polionotus, Hoekstra and her colleagues have also identified the genetic basis for these observed differences. Their work has revealed several interesting patterns. First, they have found that most of the differences in mouse fur color are caused by changes in just a handful of genes; this means that adaptation can sometimes occur via a few large mutational steps. For example, they identified a single DNA base-pair mutation in a pigment receptor, the presence or absence of which accounts for about 30 percent of the color differences between dark mainland mice and light-colored beach mice on the Florida coast. To date, this is one of the few examples of how a single DNA change can have a profound effect on the survival of individuals in nature.
Second, they have shown that the same adaptive characteristics can evolve by several different genetic pathways. Beach mice are not just restricted to Florida’s Gulf Coast but are also found some 200 miles away on the Atlantic coast. They have shown that mice on the eastern coastal dunes have also evolved to possess light-colored fur but through different mechanisms. The pigment receptor mutation causing light color in the Gulf Coast mice is absent in the East Coast beach mice. Thus, similar evolutionary changes can sometimes follow different paths.
Bibliography
Hoekstra, H. E. 2010. From mice to molecules: “The genetic basis of color adaptation.” In In the light of evolution: Essays from the laboratory and field. (Ed. J. B. Losos). Greenwood Village, CO: Roberts and Co. Publishers.
Hoekstra, H. E., R. J. Hirschmann, R. A. Bundey, P. A. Insel, and J. P. Crossland. 2006. A single amino acid mutation contributes to adaptive beach mouse color pattern. Science 313:101–104.
Mullen, L. M., S. N. Vignieri, J. A. Gore, and H. E. Hoekstra. 2009. Adaptive basis of geographic variation: Genetic, phenotypic and environmental differentiation among beach mouse populations. Proceedings of the Royal Society B 276:3809–3818.
Vignieri, S. N., J. Larson, and H. E. Hoekstra. 2010. The selective advantage of cryptic coloration in mice. Evolution 64:2153–2158.
Natural selection acts on phenotypic variation within a species or population. In the case of Peromyscus polionotus, what is the cause of this variation?
What is the “selective agent” in this example of natural selection?
5.9 Adaptations Reflect Trade-offs and Constraints
If Earth were one large homogeneous environment, perhaps a single phenotype, a single set of characteristics might bestow upon all living organisms the ability to survive, grow, and reproduce. But this is not the case. Environmental conditions that directly influence life vary in both space and time (Part One, The Physical Environment). Patterns of temperature, precipitation, and seasonality vary across Earth’s surface, producing a diversity of unique terrestrial environments (Chapter 2). Likewise, variations in depth, salinity, pH, and dissolved oxygen define an array of freshwater and marine habitats (Chapter 3). Each combination of environmental conditions presents a unique set of constraints on the organisms that inhabit them—constraints on their ability to maintain basic metabolic processes that are essential to survival and reproduction. Therefore, as features of the environment change, so will the set of traits (phenotypic characteristics) that increase the ability of individuals to survive and reproduce. Natural selection will favor different phenotypes under different environmental conditions. This principle was clearly illustrated by the example of Darwin’s medium ground finch in Section 5.6, in which a change in the resource base (abundance, size, and hardness of seeds) over time resulted in a shift in the distribution of phenotypes within the population, as well as the example of the two distinct phenotypes of the threespine stickleback adapted to the limnetic and benthic zones of lake ecosystems in the lakes of the Pacific Northwest of North America (also see this chapter, Field Studies: Hopi Hoekstra). Simply stated, the fitness of any phenotype is a function of the prevailing environmental conditions; the characteristics that maximize the fitness of an individual under one set of environmental conditions generally limit its fitness under a different set of conditions. The limitations on the fitness of a phenotype under different environmental conditions are a function of trade-offs imposed by constraints that can ultimately be traced to the laws of physics and chemistry.
This general but important concept of adaptive trade-offs is illustrated in the example of natural selection for beak size in the population of Darwin’s medium ground finch (G. fortis) presented in Section 5.6. Recall from Figure 5.10 that the ability to use different seed resources (size and hardness) is related to beak size. Individuals with small beaks feed on the smallest and softest seeds, and individuals with larger beaks feed on the largest and hardest seeds. These differences in diet as a function of beak size reflect a trade-off in morphological characteristics (the depth and width of the beak) that allow for the effective exploitation of different seed resources. This pattern of trade-offs is even more apparent if we compare differences in beak morphology and the use of seed resources for the three most common species of Darwin’s ground finch that inhabit Santa Cruz island in the Galápagos.
The distributions of beak size (phenotypes) for individuals of the three most common species of Darwin’s ground finches are shown in Figure 5.20a . As their common names suggest, the mean value of beak size increases from the small (Geospiza fuliginosa) to the medium (G. fortis) and large (Geospiza magnirostris) ground finch. In turn, the proportions of various seed sizes in their diets (Figure 5.20b) reflect these differences in beak size, with the average size and hardness of seeds in the diets of these three populations increasing as a function of beak size. Small beak size restricts the ability of the smaller finch species (G. fuliginosa) to feed on larger, harder seed resources. In contrast, large beak size allows individuals of the largest species, G. magnirostris, to feed on a range of seed resources from small, softer seeds to larger, harder seeds. However, because they are less efficient at exploiting the smaller seed resources, these larger-beaked individuals restrict their diet to the larger, harder seeds. The profitability—defined as the quantity of food energy gained per unit of time spent handling these small seeds (see Section 14.7)—is extremely low for the larger birds and makes feeding on smaller seeds extremely inefficient for these individuals. This inefficiency is directly related to the greater metabolic (food energy) demands of the larger birds, which illustrates a second important concept regarding the role of constraints and trade-offs in the process of natural selection: individual phenotypic characteristics (such as beak size) often are components of a larger adaptive complex involving multiple traits and loci. The phenotypic trait of beak size in Darwin’s ground finches is but one in a complex of interrelated morphological characteristics that determine the foraging behavior and diet of these birds. Larger beak size is accompanied by increased body size (length and weight) as well as specific changes in components of skull architecture and head musculature ( Figure 5.21), all of which are directly related to feeding functions.
In summary, the beak size of a bird sets the potential range of seed types in the diet. This relationship between morphology and diet represents a basic trade-off that constrains the evolution of adaptations in Darwin’s finches relating to their acquisition of essential food resources. In addition, the example of Darwin’s ground finches illustrates how natural selection operates on genetic variation at the three levels we initially defined in Section 5.5: within a population, among subpopulations of the same species, and among different species. Natural selection operated in the local population of medium ground finches on the island of Daphne Major during the period of drought in the mid-1970s, increasing the mean beak size of birds in this population in response to the shift in the abundance and quality of seed resources. In addition, natural selection has resulted in differences in mean beak size between populations of the medium ground finch inhabiting the islands of Daphne Major and Santa Cruz (see Chapter 13, Figure 13.17). The larger mean beak size for the population on Santa Cruz is believed to be a result of competition from the population of small ground finch present on the island (G. fuliginosa does not occupy Daphne Major). The presence of the smaller species on the island has the effect of reducing the availability of smaller, softer seeds (see Figure 5.20) and increasing the relative fitness of G. fortis individuals with larger beaks that can feed on the larger, harder seeds (see Figure 5.10).
Population genetic studies have also shown that natural selection is the evolutionary force that has resulted in the genetic differentiation of various species of Darwin’s finches inhabiting the Galápagos Islands ( Figure 5.22). The process in which one species gives rise to multiple species that exploit different features of the environment, such as food resources or habitats, is called adaptive radiation. The different features of the environment exert the selection pressures that push the populations in various directions, and reproductive isolation, the necessary condition for speciation to occur, is often a by-product of the changes in morphology, behavior, or habitat preferences that are the actual targets of selection. In this way, the differences in beak size and diet among the three species of ground finch are magnified versions of the differences observed within a population, or among populations inhabiting different islands.
(From Patel 2006.)
In the chapters that follow, we will examine this basic principle of trade-offs as it applies to the adaptation of species and explore how the nature of adaptations changes with changing environmental conditions. We will explore various adaptations of plant and animal species, respectively, to key features of the physical environment that directly influence the basic processes of survival and assimilation in Chapters 6 and 7, and trade-offs involved in the evolution of life history characteristics (adaptations) relating to reproduction in Chapter 10. The role of species’ interactions as a selective agent in the process of natural selection will be examined later in Part Four (Species Interactions).
Throughout our discussion, adaptation by natural selection is a unifying concept, a mechanism for understanding the distribution and abundance of species. We will explore the selective forces giving rise to the adaptations that define the diversity of species as well as the advantages and constraints arising from those adaptations under different environmental conditions. Finally, we will examine how the trade-offs in adaptations to different environmental conditions give rise to the patterns and processes observed in communities and ecosystems as environmental conditions change in space and time.
Ecological Issues & Applications Genetic Engineering Allows Humans to Manipulate a Species’ DNA
For a millennia, humans have been using the process of selective breeding to modify the characteristics of plant and animal species. By selecting individuals that exhibit a desired trait, and mating them with individuals exhibiting the same trait (or traits), breeders produce populations with specific physical and behavioral characteristics (phenotypes). This process of selective breeding is analogous to natural selection—the differential fitness of individuals within the population resulting from differences in some heritable characteristic(s). Unlike natural selection, however, humans function as the agent of selection rather than the environment. Darwin referred to selective breeding as “artificial selection,” and his understanding of this process was instrumental in his development of the idea of natural selection.
Although the process of selective breeding has provided us with the diversity of domesticated plants and animals upon which we depend for food, the process has one major limitation. The array of characteristics that can be selected for are limited to the genetic variation (alleles) that exists within the population (species). For example, red flower color can only be selected for if the allele coding for the production of red pigment exists within the plant population (species). Modern genetic techniques, however, have removed this fundamental constraint. It is now possible to transfer DNA (genes) from one species to another.
The process of directly altering an organism’s genome is referred to as genetic engineering. The primary technology used in genetic engineering is genetic recombination; the development of recombinant DNA or rDNA by combining the genetic material from one organism into the genome of another organism (generally of a different species). The resulting modified gene is called a transgene, and the recipient of the recombinant DNA is called a transgenic organism.
The process of genetic engineering using rDNA requires the successful completion of a series of steps (Figure 5.23). DNA extraction is the first step in the process. To work with DNA, scientists must extract it from the donor organism (the organism that has the desired trait). During the process of DNA extraction, the complete sequence of DNA from the donor organism is extracted at once. The next task is to separate the single gene of interest from the rest of the DNA using specific enzymes that “cut” the desired segment of DNA from the larger strand. Copies of the gene can then produced using cloning techniques. Once the gene has been cloned, genetic engineers begin the third step, designing the gene to work once it is inside a recipient organism. This is done by using other enzymes that are capable of adding new segments called promoters (which start a sequence) and terminators (which stop a sequence). A promoter is a region of DNA that initiates the transcription of a particular gene (the first step of gene expression, in which a particular segment of DNA is copied into RNA). The terminator is a section of genetic sequence that marks the end of gene. The next step is to insert the new gene into the cell of the recipient organism. The process in which changes in a cell or organism are brought about through the introduction of new DNA is called transformation.
Transformation is accomplished through a variety of techniques, but two main approaches are used for plant species: the “gene gun” method and the Agrobacterium method. The gene gun method fires gold particles carrying the foreign DNA into plant cells. Some of these particles pass through the plant cell wall and enter the cell nucleus, where the transgene integrates itself into the plant chromosome. The Agrobacterium method involves the use of soil-dwelling bacteria known as Agrobacterium tumefaciens that cause crown gall disease in many plant species. This bacterium has a plasmid, or loop of nonchromosomal DNA, that contains tumor-inducing genes (T-DNA), along with additional genes that help the T-DNA integrate into the host genome. When the bacteria infect the plant, the plasmid is integrated into the plant’s chromosomes, becoming part of the plant’s genome. For genetic engineering purposes, the tumor-inducing part of the plasmid is removed so that it will not harm the plant. The desired gene from the donor organisms is then inserted into the bacteria’s plasmid. The bacteria can now be used as a delivery system that will transfer the transgene into the plant.
An organism whose genetic material has been altered using genetic engineering techniques (including transgenic organisms) are commonly referred to as genetically modified organisms (GMOs). Genetic engineering has been used to produce a wide variety of GMOs. Organisms that have been genetically modified include microorganisms such as bacteria and yeast, insects, plants, fish, and mammals. GMOs are used in biological and medical research, production of pharmaceutical drugs, experimental medicine (e.g., gene therapy), but perhaps their most widespread application has been in agriculture (Figure 5.24). In agriculture, genetically engineered crops have been created that possess desirable traits such as resistance to pests or herbicides, increased nutritional value, or production of pharmaceuticals.
In addition to the ethical and health concerns that genetically modified crop species have raised, the practice of genetic engineering and the production of transgenic species (GMOs) has raised considerable concern among ecologists. There is little concern about gene transfer between major agricultural species such as corn, soybean, and rice and native plant populations as a result of the lack of close relatives capable of cross-pollination. However, other crop species, such as members of the genus Brassica (member of the mustard family) are represented by a variety of domesticated and wild (native) species and subspecies that are capable of cross-pollination. For example, Brassica napus (rapeseed) used in the production of rapeseed oil has been genetically modified to tolerate herbicides (used to kill weeds in agricultural fields), and the transfer of herbicide-tolerant traits by pollen to weedy relatives (other members of the genus Bassica) has been recorded. Brassica includes a number of important crop species such as turnips, cabbage, and broccoli.
A major application of genetic engineering in agriculture is the development of insect-resistant crop strains. Perhaps the most widely grown genetically modified crop plant is Bt corn. Bacillus thuringiensis, or Bt, is a common soil bacterium whose genome contains genes for several proteins toxic to insects. For decades, Bt has been sprayed on fields as an organic pesticide. Starting in the mid-1990s, several varieties of corn were genetically engineered to incorporate Bt genes’ encoding proteins, which are toxic to various insect pests. Some strains of Bt produce proteins that are selectively toxic to caterpillars, such as the southwestern corn borer, whereas others target mosquitoes, root worms, or beetles. The insecticide proteins are contained within the plant tissues, which are fatal to the pest species when ingested. Concerns have been raised over the potential impacts of Bt corn and other insect-resistant genetically modified crop species on nontarget insect species or to predators that feed on these insects.
The use of transgenes to confer disease resistance to crops represents another possible ecological risk. If genes that code for viral resistance are transferred to crops, there is a potential for transfer to wild plants, creating the potential for the natural development of new plant viruses of increased severity.
Summary
Adaptation 5.1
Characteristics that enable an organism to thrive in a given environment are called adaptations. Adaptations are a product of natural selection. Natural selection is the differential fitness of individuals within the population that results from their interaction with their environment, where the fitness of an individual is measured by the proportionate contribution it makes to future generations. The process of natural selection results in changes in the properties of populations of organisms over the course of generations by a process known as evolution.
Genes 5.2
The units of heredity are genes, which are linearly arranged on threadlike bodies called chromosomes. The alternative forms of a gene are alleles. The pair of alleles present at a given locus defines the genotype. If both alleles at the locus are the same, the individual is homozygous. If the alleles are different, the individual is heterozygous. The sum of heritable information carried by the individual is the genome.
Phenotype 5.3
The phenotype is the physical expression of the genotype. The manner in which the genotype affects the phenotype is termed the mode of gene action. When heterozygous individuals exhibit the same phenotype as one of the homozygotes, the allele that is expressed is termed dominant and the masked allele is termed recessive. If the physical expression of the heterozygote is intermediate between the homozygotes, the alleles are said to be codominant.
Even though all genetic variation is discrete, most phenotypic traits have a continuous distribution because (1) most traits are affected by more than one locus, and (2) most traits are affected by the environment.
Phenotypic Plasticity 5.4
The ability of a genotype to give rise to a range of phenotypic expressions under different environmental conditions is termed phenotypic plasticity. The range of phenotypes expressed under different environmental conditions is termed the norm of reaction. If the phenotypic plasticity occurs during the growth and development of the individual and represents an irreversible characteristic, it is referred to as developmental plasticity. Reversible phenotypic changes in an individual organism in response to changing environmental conditions are referred to as acclimation.
Genetic Variation 5.5
Genetic variation occurs at three levels: within subpopulations, among subpopulations of the same species, and among different species. The sum of genetic information across all individuals in the population is the gene pool. The fundamental measures of genetic variation within a population are allele frequency and genotype frequency.
Natural Selection 5.6
Natural selection acts on the phenotype, but in doing so it alters both genotype and allele frequencies within the population. There are three general types of natural selection: directional selection, stabilizing selection, and disruptive selection. The target of selection is the phenotypic trait that natural selection acts upon, whereas the selective agent is the environmental cause of fitness differences among individuals in the population.
Processes Influencing Genetic Variation 5.7
Natural selection is the only evolutionary process that can result in adaptations; however, some processes can function to alter patterns of genetic variation from generation to generation. These include mutation, migration, genetic drift, and nonrandom mating. Mutations are heritable changes in a gene or chromosome. Migration is the movement of individuals between local populations. This movement results in the transfer of genes between local populations. Genetic drift is a change in allele frequency as a result of random chance.
Nonrandom mating on the basis of phenotypic traits is referred to as assortative mating. Assortative mating can be either positive (mates are more similar than expected by chance) or negative (dissimilar). A special case of nonrandom mating is inbreeding—the mating of individuals that are more closely related than expected by chance.
Genetic Differentiation 5.8
Natural selection can function to alter genetic variation between populations; this result is referred to as genetic differentiation. Species having a wide geographic distribution often encounter a broader range of environmental conditions than do species whose distribution is more restricted. The variation in environmental conditions often gives rise to a corresponding variation in many morphological, physiological, and behavioral characteristics as a result of different selective agents in the process of natural selection.
Trade-offs and Constraints 5.9
The environmental conditions that directly influence life vary in both space and time. Likewise, the objective of selection changes with environmental circumstances in both space and time. The characteristics enabling a species to survive, grow, and reproduce under one set of conditions limit its ability to do equally well under different environmental conditions.
Genetic Engineering Ecological Issues & Applications
Genetic engineering is the process of directly altering an organism’s genome. The primary technology used in genetic engineering is genetic recombination—the combining of genetic material from one organism into the genome of another organism, generally of a different species. The result of this process is recombinant DNA (rDNA). The resulting modified gene is called a transgene, and the recipient of the rDNA is called a transgenic organism.
