Introduction To Genetics - Genes and Evolution

Genes and Evolution

Further information: Evolution and Introduction to evolution

A population of organisms evolves when an inherited trait becomes more common or less common over time. For instance, all the mice living on an island would be a single population of mice. If over a few generations, white mice went from being rare, to being a large part of this population, then the coat color of these mice would be evolving. In terms of genetics, this is called a change in allele frequency—such as an increase in the frequency of the allele for white fur.

Alleles become more or less common either just by chance (in a process called genetic drift), or through natural selection. In natural selection, if an allele makes it more likely for an organism to survive and reproduce, then over time this allele becomes more common. But if an allele is harmful, natural selection makes it less common. For example, if the island was getting colder each year and was covered with snow for much of the time, then the allele for white fur would become useful for the mice, since it would make them harder to see against the snow. Fewer of the white mice would be eaten by predators, so over time white mice would out-compete mice with dark fur. White fur alleles would become more common, and dark fur alleles would become more rare.

Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties. So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those that are useful, causes adaptation. This is when organisms change in ways that help them to survive and reproduce.