Evolution is allele frequency change
Figure 6.2B shows that the average horn size of male bighorn sheep has decreased over time, but it does not reveal the cause of that decline. A clue to the cause comes from an additional observation (Coltman et al.
2003): horn size is an inherited trait. This means that rams with large horns tend to have offspring that have large horns and that rams with small horns tend to have offspring that have small horns. Because trophy hunting selectively eliminates rams with large horns, it favors rams whose genetic characteristics lead to the production of small horns. Hence, it seems likely that trophy hunting is causing the genetic characteristics of the bighorn sheep population to change, or evolve, over time— a conclusion supported in a recent analysis of data from a bighorn sheep population subjected to intense hunting for 23 years (Pigeon et al. 2016).As suggested by the trophy-hunting example, biologists often define evolution in terms of genetic change. To make such a definition more precise (and to introduce terms that will be used throughout this chapter), let's review some basic principles of genetics:
• Genes are composed of DNA, and they specify how to build (encode) proteins.
• A given gene can have two or more forms (known as alleles) that result in the production of different versions of the protein that the gene encodes.
• We can designate the genotype (genetic makeup) of an individual with letters that represent the individual's two copies of each gene (one inherited from its mother, the other from its father). For example, if a gene has two alleles, designated A and a, the individual could be of genotype AA, Aa, or aa.
With these principles as background, we can define evolution as change over time in the frequencies (proportions) of different alleles in a population. To illustrate how this definition is applied, consider a population of 1,000 individuals and a gene with two alleles (A and a). Suppose there are 360 individuals of genotype AA, 480 of genotype Aa, and 160 of genotype aa. The frequency of the a allele in this population is 0.4, or 40%;1 hence, since there are only two alleles in the population (A and a), the frequency of the A allele must be 1 - 0.4 = 0.6, or 60%. If the frequency of the a allele were to change over time, say, from 40% to 71%, then the population would have evolved at that gene. (In scientific studies, researchers often use an approach based on the Hardy-Weinberg equation to test whether a population is evolving at one or more genes.)