Lecture 10. Natural Selection II

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Unformatted text preview: ity Problem: Which equilibria are stable and which are unstable? Calculate for the internal equilibrium. wAA = 1 wAa = 0.2 waa = 0.6 ∆p p internal equilibrium Equilibria for heterozygote inferiority Solu+on: wAA = 1 wAa = 0.2 waa = 0.6 ∆p stable equilibrium p unstable internal equilibrium stable equilibrium Can you recognize the four types of selec4on from the graphs? 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 20 40 60 80 100 20 40 60 80 20 40 60 80 100 p 0.8 1.0 0.8 0.6 0.6 0.4 0.4 0.2 0.2 20 40 60 80 100 genera:on (t) 100 Stable equilibria with more than two alleles •  the presence of more than two alleles at a locus complicates analyses (e.g., with n alleles, there are n(n+1)/2 genotypes and their fitnesses) •  simple generaliza+ons from two‐allele theory do not generally carry‐over to mul+ple allele cases •  for example, even if every heterozygote is more fit than every homozygote, stable mul+ple‐allele polymorphisms (internal equilbria) are rare •  mul+‐allele polymorphisms at a locus are therefore unlikely to be maintained simply by heterozygote advantage; more complex forms of selec+on are likely responsible (e.g. diversifying selec+on, spa+al varia+on in selec+on) Summary of the Model •  Our model considers selec+on at a single locus with only two alleles •  Selec+on results from differences in zygote‐to‐adult viability; all other components of fitness are assumed equal •  The model makes explicit predic+ons about the long term result of natural selec+on for a given set of fitnesses •  What does the model tell us about how gene+c varia+on can be maintained by selec+on? How Realis4c are the Assump4ons •  Random ma+ng and absence of gene flow – seem reasonable •  Muta+on and drid are negligible – might be true in the sort term, not true in the long term Dominance and Selec4on •  Some+mes we are interested in the degree of dominance between two alleles. Genotype A1A1 A1A2 A2A2 Fitness 1 1 ‐ h s 1 ‐ s • Where A1...
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