How does natural selection drive evolution?

Over time, genes spontaneously mutate or change, causing changes in individual organisms. If those changes allow the organism to bear more offspring than others of its' species these then will be passed on.

The environment will be what selects these changes. If the environment is benign then very little change will be seen. If not, the genes that give an advantage to a species will be "selected" to pass on their genes. The other genes will become lost or reduced and found in fewer and fewer individuals.

It is thought that dark eyes, dark hair and darker skin will be found more common as the years go on. There is really no reason anymore for light skin genes to remain. People can get vitamin D in other ways besides by sunlight.

It is important to remember that evolution works neither to directly benefit individuals. It is simply a process by which certain genes spread. It can even harm a species, if the species becomes adapted to too narrow an environment, the entire species can disappear.

Fonte: http://www.portaleducacao.com.br/biologia/artigos/7297/evolucao-e-selecao-natural