TODAY. . . SelectionDirectionalStabilizingDisruptiveMore HWGenetic DriftAllele fixationSelection• Natural: Increase in the frequency of alleles that increase reproductive success• Sexual: A specific type of natural selection; increase in the frequency of alleles that enhance one’s ability to obtain matesSomething to keep in mind. . . We have simplified what is really going on with respect to genetic diversity! There may be many alleles for the same gene in any given population.Homework question1. Construct a version of the Hardy-Weinberg equations that could be applied to a gene with three alleles.Genetic variation• The number and relative frequency of alleles that are present in a population• Some types of selection increase variation, other types reduce it.• To the extent that phenotype is determined by genotype, a reduction in phenotypic diversity will reduce genetic diversity4. In a population of leatherback sea turtles, two alleles that are incompletely dominant exist for a gene that affects shell thickness and weight (S –thick, heavy shell; s – thin, light shell). You sample 100 individuals from the population: 37 are SS, 8 are ss, and 55 are Ss.A)Is this population in Hardy-Weinberg equilibrium? Why or why not?B) Create a hypothesis to explain why this population is, or is not, as the case may be, in Hardy-Weinberg equilibrium. Which Hardy-Weinberg assumptions could be violated, if any?Heterozygote advantageWhen heterozygotes have a fitness advantage over homozygotes, keeping genetic variation in a populationMore on Selection. . .Directional Selection• Allele frequencies change in one direction• Variation is reduced• Mean value of trait increases or decreasesGreat Plains Cliff Swallows• Population enduresextended cold weather• Food source dies• Swallows die• Who’s left? Why?• Body fat storeshelped carry themthrough thin timesStabilizing Selection• Extreme phenotypes are lost (tails of distribution reduced)– Reduces variation• Mean value of trait remains unchangedStabilizing Selection• Birth weight in humans• High mortality rates for very large and for very small*• Nutrition influenced?• Extreme phenotypesare favored;Phenotypes near mean value are eliminated• Mean does not change• Distributions becomes bimodal– Variation increasedDisruptive SelectionDisruptive Selection• Black-billed seedcrackers• Large-billed individualsspecialize on large seeds, small-billed on small seeds• Intermediate sizeshandle seedsinefficientlyGenetic Drift• Undirected change in allele frequencies– due to sampling error (random mating)• In very large populations, sampling error plays a small role; in small populations, it matters a lot!Consequences of Drift• No allele is more fit than any other (no natural selection)– drift is random with respect to fitness• BUT, some alleles clearly “won” the reproduction lottery– They randomly increased their frequency in thepopulation• In finite populations equally fit alleles are atrisk of disappearing = loss (freq = 0)– Over time drift can produce random loss orfixation (freq = 1) of allelesConsequences of DriftDepends on. . .• Population size!– founder effect (colonization of new habitat by few individuals with a random and reduced sample of alleles from the source population)– bottlenecks (rapid, large reduction in population size)• Probability that an allele will become fixed in the population = frequency of that allele in the population– @ each generation, probability of fixation changes!Importance in naturalpopulations?• Small: zoos, endangered species• Large: pseudogenes and silent mutationshave no effect on fitness (invisible, for now, to selection), but are an important source of raw material for future selection events; Drift can eliminate these from populations.• Reduces genetic diversityAfter allele fixation, the only way to increase genetic diversity is to introduce new alleles!How?Effect on average fitness– Genetic drift reduces allelic diversity, so the arrival of new alleles might increase fitness– BUT, If populations are well adapted to their environment, then new (non-adaptive) alleles might reduce average fitnessGene Flow(immigration and emigration of new alleles)Mutation1. The only creator of novelty (new alleles)2. Restores genetic diversity3. Fitness of mutations is randoma. Most mutations are deleterious (negativeeffect on fitness)b. Sometimes they’re beneficial, and may spread through the