The Forces of Evolutionary Change--Microevolution
|15.1 Why Is Evolution Much More Likely Than Not?
1. A gene pool includes all the genes in a population. Allele movement between populations is gene flow. Inherited characteristics of the individuals in a population reflect allele frequencies.
2. In Hardy–Weinberg equilibrium, evolution is not occurring because allele frequencies do not change from generation to generation. In this idealized state, we can calculate the proportion of genotypes and phenotypes in a population by inserting known allele frequencies into an algebraic equation: p2 + 2pq + q2. The equation also can reveal allele frequency changes when we know the proportion of genotypes in a population.
15.2 How Does Mate Choice Influence Evolution?
3. When allele frequencies change, evolution occurs.
4. Nonrandom mating causes certain alleles to predominate because a particular phenotype is more attractive to the opposite sex.
5. In genetic drift, small populations separate from larger ancestral populations and establish a new gene pool, with different allele frequencies. The founder effect and population bottlenecks are forms of genetic drift.
15.3 How Does Mutation Fuel Evolution?
6. The deleterious alleles in a population constitute its genetic load.
7. Mutation alters allele frequencies by changing one allele into another and providing new phenotypes for evolution to act on.
8. Harmful recessive alleles are selected against in homozygotes, but heterozygotes maintain them and mutation introduces them into the gene pool.
15.4 How Does Natural Selection Mold Evolution?
9. In directional selection, an extreme phenotype becomes more prevalent in a population. Industrial melanism is an example.
10. In disruptive selection, extreme expressions survive at the expense of intermediate forms.
11. In stabilizing selection, an intermediate phenotype has an advantage.
12. Balanced polymorphism is a form of stabilizing selection that maintains deleterious recessive alleles because heterozygotes are protected against another medical condition.