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Week #12 Lab: Evolution by Genetic Drift

Introduction: You are hiking in the mountains & accidentally step on a group of beetles that is on the trail. This species of beetle has genetically-determined body color & displays a lot of genetic variation for the trait – a bunch of them are blue, some are yellow, & a few are green. Just by chance, you accidently squash all of the green beetles with your boot & only blue and yellow beetles remain. Due to that chance event, the beetle population now contains a greater percentage of the alleles that code for blue & yellow body color, while the alleles for green body color have been eliminated from the population. You just caused evolution by genetic drift in the beetle population!

Genetic drift, like natural selection, can cause the allele frequencies for genes to change in populations. That is, it can cause populations to evolve. But in natural selection, organisms live or die (&/or reproduce or don’t) because of their genes. Natural selection is NOT a random process. In genetic drift, however, organisms live or die (&/or reproduce or don’t) regardless of their genes – it’s simply due to chance. Genetic drift IS a random process.

Genetic drift influences the evolution of all traits, in all populations, all of the time. Even if natural selection is acting on a trait in a population, chance differences in survival & reproduction (genetic drift) will always occur, & therefore will always effect the evolution of the trait to some extent. How powerful a role genetic drift (chance) will play in the evolution of a trait depends on the size of the population. In large populations, genetic drift tends to be a relatively weak force of evolution: the loss of just a few alleles due to chance will barely change the allele percentages of a very large group. But if/when populations are very small, such as during population bottlenecks or founder events, genetic drift can be a very powerful force of evolution: the loss of just a few alleles will profoundly change the allele percentages of a very small group.

Both natural selection & genetic drift have caused human traits to evolve over time. While much of the genetic variation in human traits around the world can be explained by natural selection, much can also be explained by genetic drift. Both fossils & DNA provide strong evidence that our species, Homo sapiens, evolved in Africa approximately 200,000 years ago. As small groups of humans emigrated out of Africa & founded populations in other regions & on other continents, these founder events created populations that were genetically different from the original, source population.

A famous local example of founder effects is the unusually high percentage of genetically-deaf people that existed on the island of Martha’s Vineyard for over 200 years. Martha’s Vineyard was founded by a small group of immigrants from the county of Kent in southern England in the late 1600’s into the early 1700’s. The first known deaf settler on record was Jonathan Lambert, who immigrated to the Massachusetts Bay Colony in 1694 & settled on the island. As his form of deafness was heritable, he passed it to his children, who passed it their children, & so on, causing in an increase in the percentage of deaf people in the community. The high numbers of deaf people resulted in the formation of a unique from of sign language known as Martha’s Vineyard Sign Language (MVSL). Census data show that the incidence of deaf islanders peaked at 1 in 155 islanders; whereas the incidence on the mainland was roughly 1 in almost 6,000 people. When a deaf school in Connecticut opened in the 1800’s, many deaf children left the island for school & ended up staying on the mainland to live. By the 1950’s, MVSL had been merged with the newly-formed American Sign Language (ASL) & the last native-born practitioner of MVSL died, along with the language. By the late 1900’s the incidence of deafness & the impact of the founder event declined due to increased gene flow as people continued to immigrate & emigrate to & from the island.

Using the same materials that you used in last week’s lab, follow the procedure below to simulate genetic drift by a founder event in a moth population.

Learning Objectives: This lab is designed to help you to…

• Understand how genetic drift can act as a force of evolution.

• Distinguish between natural selection & genetic drift.

• Practice problem-solving skills by calculating allele percentages in a population.

• Analyze planetary issues (evolution by genetic drift) & their effect on life & work.

Laboratory Activity #1: Simulating Genetic Drift due to Founder Effects

Will a founder event (genetic drift) cause your moth population to evolve (change genetically & phenotypically)? What do you think will happen to the percentages of the B & b alleles as a small part of the population emigrates from the main population & establishes (founds) a new one? State your hypothesis & prediction below.

Hypothesis:

Prediction:

Procedure:

1. Prepare your lab supplies. Using scissors, cut out the 45 black, 45 grey, & 45 white squares from the sheets labelled “Moths” at the end of this lab. These colored squares will represent the “moths” in your simulation. Next, cut out the 10 larger black & 10 larger white squares from the sheet labelled “Gametes” at the end of this lab. Crumple these squares into balls (or fold in half twice) with the colored side showing on the outside. These little, colored balls will represent the different alleles for the color gene carried by the “gametes” (eggs & sperm) produced by the “moths” in your simulation.

2. Set up your moth population. Randomly place 15 black, 15 grey, & 15 white moths (colored squares) on the “mainland” habitat provided. The phenotypes & allele percentages of this mainland moth population are already recorded for you in Table 1 below.

3. Simulate a founder event. Close your eyes & randomly pick four moths from the population. Move these four moths from the “mainland” habitat to the “island” habitat provided, to simulate them flying off to an uninhabited island (with no bird predators).

4. Collect the appropriate “gametes” (colored balls of paper) from the four founder island moths based upon their gentoypes, & place the eight gametes in a container – this is the “gene pool” on the island. Like last week, simulate sex & reproduction of the founders by picking two gametes at a time from the gene pool & placing a baby moth of the corresponding genotype/color on the island habitat. Continue this process until you have a population of 45 new baby moths, but remember to return the gametes to the gene pool container after each pick!

5. These moths are an annual species, so assume the original four founder moths die after all that sex! Record the colors of just the baby moths in this newly-founded island population in Table 1 below.

Results: Table 1 – Colors & allele frequencies of mainland vs. island moths

Data Analysis: Count up & record the #B & # b alleles in the island population. When counting up the alleles, remember that each black moth carries two B alleles & each grey moth carries one B allele; each white moth carries two b alleles & each grey moth carries one b allele. Thus, the total number of alleles should equal 90 alleles (#B alleles + #b alleles = 90). If they do not, you’ve done something wrong, & will need to go back & check your numbers before calculating allele percentages!

Calculate the percentage of B & b alleles using the equations below. Remember, the total number of alleles each generation is 90. Record the percentages in the last two columns of Table 1.

% B alleles = # B alleles x 100

total # alleles

% b alleles = # b alleles x 100

total # alleles

Discussion:

1. Did the founding of an island population of moths cause evolution? That is, did the phenotypes & allele percentages of the newly-founded island population differ from those of the mainland population from which they came? Explain.

2. If over time, more & more moths from the mainland flew to the island & reproduced, would this gene flow cause the island population to become more or less similar to that of the mainland population? Explain.

White “Moths” - Cut out the 45 squares below. Each square represents one adult moth that is homozygous for the white color allele (phenotype = white & genotype = bb).

Grey “Moths” - Cut out the 45 squares below. Each square represents one adult moth that is heterozygous for the color gene (phenotype = grey & genotype = Bb).

Black “Moths” – Cut out the 45 squares below. Each square represents one adult moth that is homozygous for the black color allele (phenotype = black & genotype = BB).

White “Gametes” – Cut out each of the 10 white squares below. Roll up each square into a ball (or fold twice). Each will represent a haploid gamete (egg or sperm) carrying a white (b) allele for the color gene.

Black “Gametes” – Cut out each of the 10 black squares below. Roll up each square into a ball (or fold twice) with the black color on the outside. Each will represent a haploid gamete (egg or sperm) carrying a black (B) allele for the color gene.

Mainland Habitat

Island Habitat

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