Mechanisms of Evolutionary Change

A population is a group of individuals of one species living in the same area and able to interbreed. Its gene pool is all the alleles of all the genes in that population. An allele frequency is the proportion of all copies of a gene that are one particular allele. Because each diploid individual carries two copies of each gene, a population of $N$ individuals holds $2N$ copies. At the population level, evolution is a change in allele frequencies over generations.

A mutation is a change in the DNA sequence. It is the only process that creates brand-new alleles, so it is the ultimate source of genetic variation; the other mechanisms only rearrange or sort alleles that already exist. Most mutations are neutral or harmful, and a few are beneficial in a particular environment. In animals, only mutations in the cells that give rise to eggs or sperm can be passed to offspring. Mutation rates per gene are very low, so mutation on its own changes allele frequencies slowly, but over millions of individuals and many generations it supplies the variation that selection and drift act on.

Gene flow is the movement of alleles between populations, when individuals migrate and breed in a new population or when pollen or seeds are carried from one population to another. Gene flow can bring new alleles into a population and tends to make neighbouring populations more genetically similar. When gene flow between two populations stops, for example because a river changes course or a highway divides a habitat, the populations can begin to drift apart genetically, which is the first step toward speciation.

Genetic drift is a random change in allele frequencies caused by chance events in which individuals happen to survive and reproduce. Toss a fair coin 1000 times and you will get close to 50% heads; toss it 10 times and 70% heads is not surprising. In the same way, drift has a small effect in large populations and a large effect in small ones, where an allele can be lost or can become the only allele (fixed) within a few generations. Drift does not produce adaptation, because it ignores whether an allele is helpful. Two special cases are common:

• Bottleneck effect: a disaster or human activity (a flood, fire, disease or overhunting) sharply reduces a population's size, and the survivors carry a random, reduced sample of the original gene pool. Even if the population later grows large again, the lost alleles do not come back. Hunting in the 1800s reduced northern elephant seals to a very small number; although the population has since grown enormously, its genetic diversity remains very low.
• Founder effect: a small group leaves a large population and starts a new one (colonizing an island, for example). The founders' allele frequencies may differ from the source population purely by chance, so a rare allele can become common. The Old Order Amish community in Pennsylvania, founded by a small number of settlers, has a much higher frequency of the allele for Ellis-van Creveld syndrome, a rare inherited condition, than the general population.

Sexual selection is a form of natural selection in which traits increase in frequency because they improve mating success. In intrasexual selection, members of one sex (often males) compete directly, as when bull moose clash antlers. In intersexual selection (mate choice), members of one sex choose mates based on traits such as the peacock's large, colourful tail. Sexual selection can favour traits that actually reduce survival, such as a tail that makes a peacock easier for predators to catch, as long as the gain in mating success outweighs the cost. Non-random mating more generally means individuals do not pair up by chance. In assortative mating, individuals tend to mate with others that have similar phenotypes; in inbreeding, relatives mate with each other. Inbreeding increases the proportion of homozygous offspring, so harmful recessive traits appear more often.