Figure 3.

Evolutionary mechanisms at the population level drive gene diversification. (a) There are four main evolutionary forces. Mutation introduces new variants into the population. Gene flow introduces new variants through migration from divergent populations. Genetic drift explains the increase or decrease of variant frequency in a population due to random processes and is linked to population size. Natural selection increases the frequency of genetic variants that increase population fitness, including survival and reproduction. (b) Duplications are the most common type of mutation underlying the evolution of novel gene families. Tandem duplications produce identical adjacent sequences. Retroduplications result in a retrocopy of the gene devoid of introns and with a polyA tail. Whole-genome duplication entails complete chromosome duplication. (c) Different types of natural selection decrease, shift, or increase genetic variation in a population. Stabilizing selection decreases genetic variation, favoring an average phenotype. Directional selection favors a particular phenotype, causing the frequency of variants to continuously shift in one direction. Diversifying selection (or disruptive selection) increases genetic variation as it favors two or more phenotypes, each providing selective advantages.