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Populations Genetics Are populations evolving? How? Why? H-W equations let us answer a specific aspect of this question: Are gene (allele) and/or genotype frequencies changing over time? HW equations By definition: p + q = 1 p = freq. A allele q = freq. a allele Prediction under HW equilibrium: p2 + 2pq + q2 = 1 p2 = freq. AA genotype 2pq = freq. Aa genotype q2 = freq. aa genotype figure 21-07.jpg Use HW to do two things: Calculate allele frequencies (given genotype freq’s) Predict genotype frequencies (given allele freq’s) So what? If ACTUAL genotype frequencies match predictions, then… • the population is in HW equilibrium • allele and genotype freq. constant across generations If ACTUAL genotype frequencies differ from predictions, then… • population is not in HW equilibrium • some evolutionary force is acting! Evolutionary forces that violate Hardy-Weinberg: • • • • • mutation migration non-random mating genetic drift selection How does each affect genetic variation within populations? Mutation – transformation of one allele into another • generates genetic variation • alone, not a strong evolutionary force • provides the “raw material” of genetic variation on which selection and drift can act Migration – movement of individuals between populations • “gene flow” between gene pools • maintains genetic variation within populations by bringing in new alleles • prevents populations from genetically diverging (and eventually becoming separate species) Non-random mating • does not change allele freq’s • DOES change genotype freq’s Assortative mating – individuals prefer mates with same genotype increases homozygosity at a particular locus Disassortative – individuals prefer mates with different genotypes increases heterozygosity at a particular locus Inbreeding – mating between close relatives • increases homozygosity across the genome • danger: “inbreeding depression” – inbred populations often show decreased fitness (due to greater risk of homozygous recessive disorders) (Freeman & Herron 2001) Genetic drift – random changes in allele frequencies between generations • due to sampling error • greatest effect in small populations – population bottlenecks – founder effect Genetic drift via population bottleneck or founder effect Simulation: Genetic drift at one locus http://darwin.eeb.uconn.edu/simulations/drift .html Selection – differential survival and reproduction of individuals with different genotypes • Non-random process (unlike genetic drift) • Natural selection involves… – More offspring are born than can survive – Competition/struggle for survival for limited resources – Variation between individuals that makes some better able to survive and reproduce – This variation is heritable/genetic (can be passed on) Result: Over many generations, the genotypes that are better able to survive and reproduce become more common in the population. Simulation: Selection at one locus http://darwin.eeb.uconn.edu/simulations/sele ction.html Galapagos finches Classify the following examples: • Human birth weight tends to stay around 7 lbs. (too big = trouble for mom; too small = trouble for newborn) • Many seeds have seed coats of medium thickness (if too thin, no protection; if too thick, germinating seed can’t break through) • Finches need either small beaks (to eat tiny seeds) or large beaks (to crack large seeds). Balanced Polymorphism (aka Heterozygote Advantage) Ex: sickle cell anemia Frequency-dependent selection Sexual Selection Peacock Experiments Sexual Selection • Intrasexual – Members of one sex (usually male) compete for access to the other sex – Ex: Sea lions, rams • Intersexual – Members of one sex (usually female) choose certain members of the opposite sex over others – Ex: Pea hens choosing pea cocks