Populations evolve, individuals do not
Natural selection acts as a sorting process, favoring individuals with some heritable traits (e.g., bighorns with small horns) over others (e.g., bighorns with large horns). Individuals with the favored traits tend to leave more offspring than do individuals with other traits.
As a result, from one generation to the next, a greater proportion of the individuals in the population will have the traits favored by natural selection. When these traits have a genetic basis, this process can cause the allele frequencies of the population to change over time, thereby causing the population to evolve. But the individuals in the population do not evolve—either they have the traits favored by selection or they don't.1
- Each of the 1,000 individuals in the population has two alleles, giving a total of 2,000 alleles in the population. Each of the 360 individuals of genotype AA has zero a alleles, each of the 480 individuals of genotype Aa has one a allele, and each of the 160 individuals of genotype aa has two a alleles. Thus, the frequency of the a allele is (0 ? 360 + 1 ? 480 + 2 ? 160)/2,000 = 0.4. The frequency of the a allele can also be calculated using genotype frequencies, in which case we have [(0 ? 0.36) + (1 ? 0.48) + (2 ? 0.16)]∕2 = 0.4, where 360/1,000 = 0.36 is the frequency of genotype AA, 0.48 is the frequency of genotype Aa, and 0.16 is the frequency of genotype aa.