Chapter 7: Population Genetics 3: Multiple Loci and SexOverview |
The models we have used until now track allele frequencies at just one locus at a time. We have only been able to consider the evolution of traits that are (or appear to be) controlled by a single gene. The genomes of real organisms, of course, contain hundreds or thousands of loci. And many traits are determined by the combined influence of numerous genes. In Chapter 7 we will take our models of the mechanics of evolution closer to real organisms by considering two or more loci simultaneously.
7.1 Evolution at Two Loci: Linkage Equilibrium and Linkage Disequilibrium Section 7.1 of you textbook will expand on your understanding of the one-locus version of Hardy–Weinberg analysis to consider two loci simultaneously. In principle, we could focus on any pair of loci in an organism’s genome. Our discussion will be easier to understand, however, if we focus on a pair of loci located on the same chromosome. That is, we will consider two loci that are physically linked. We will imagine that locus A has two alleles, A and a, and that locus B has two alleles, B and b.
Turn to section 7.1 of your textbook to compare and contrast the effect of one versus two loci on the mechanics of evolution.
7.2 Practical Reasons for Measuring Linkage Disequilibrium In the introduction to this chapter, we promised rewards awaiting readers who mastered the abstractions of Section 7.1. Two such rewards are these: Measurements of linkage disequilibrium provide clues that are useful in reconstructing the history of genes and populations; and linkage disequilibrium can be used to identify alleles that have recently been favored by positive selection.
In section 7.2 you learn about the rewards of measuring linkage disequilibrium.
7.3 The Adaptive Significance of Sex Sexual reproduction is complicated, costly, and dangerous. Searching for a mate takes time and energy, and may increase the searcher’s risk of being killed by a predator. Once found, a potential mate may demand additional exertion or investment before agreeing to cooperate. Sex itself may expose the parties to sexually transmitted diseases. And after all that, the mating may prove to be infertile. Why not avoid all the trouble and risk and simply reproduce asexually instead?
Refer to section 7.3 of your textbook to learn the answer to this question.