Chapter 5: Mendelian Genetics in Populations I: Selection and MutationOverview |
In chapter 3 we considered the logical structure of Darwin’s theory of evolution by natural selection and reviewed evidence that the theory provides an accurate mechanistic explanation of descent with modification. As Darwin himself recognized, however, his theory is incomplete without an accurate understanding of the mechanism of inheritance. In Chapter 5 you will develop a more complete understanding of the mechanism of evolution.
5.1 Mendelian Genetics in Populations: The Hardy–Weinberg Equilibrium Principle Most people are susceptible to HIV. Their best hope of avoiding infection is to avoid contact with the virus. There are, however, a few individuals who remain uninfected despite repeated exposure. Given that individuals homozygous for a particular allell are much less likely to contract HIV, will the global AIDS epidemic cause an increase in the frequency of the allele in human populations? If so, how fast will it happen?
In section 5.1 you will learn the tools needed to answer this question.
5.2 Selection In the population model we used to derive the Hardy–Weinberg equilibrium principle, first on our list of assumptions was that all individuals survive at equal rates and contribute equal numbers of gametes to the gene pool. Systematic violations of this assumption are examples of selection. Selection happens when individuals with particular phenotypes survive to reproductive age at higher rates than individuals with other phenotypes, or when individuals with particular phenotypes produce more offspring during reproduction than individuals with other phenotypes.
Turn to section 5.2 of your textbook to learn about how phenotypes can exhibit differences in reproductive success that are heritable.
5.3 Patterns of Selection: Testing Predictions of Population Genetic Theory In the 1927 case of Buck v. Bell, the United States Supreme Court upheld the state of Virginia’s sterilization statute by a vote of eight to one. Drafted on the advice of eugenicists, the sterilization law was intended to improve the genetic quality of future generations by allowing the forced sterilization of individuals afflicted with hereditary forms of insanity, feeblemindedness, and other mental defects. The court’s decision in Buck v. Bell reinvigorated a compulsory sterilization movement dating from 1907 (Kevles 1995). By 1940, 30 states had enacted sterilization laws, and by 1960 over 60,000 people had been sterilized without their consent (Reilly 1991; Lane 1992). In hindsight, the evidence that these individuals suffered from hereditary diseases was weak.
In section 5.3 of your textbook you can learn about the evolutionary logic behind compulsory sterilization.
5.4 Mutation Cystic fibrosis is among the most common serious genetic diseases among people of European ancestry, affecting approximately 1 newborn in 2,500. In spite of the fact that cystic fibrosis was lethal for most of human history, in some populations as many as 4% of individuals are carriers.
Turn to section 5.4 of your textbook to learn how alleles that cause a lethal genetic disease can remain this common.