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Lecture 11

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McGill University
Biology (Sci)
BIOL 469
Rex Brynen

Gene flow may change allele frequencies • Migration of individuals and movements of gametes between populations, gene flow, are common. • If the arriving individuals or gametes survive and reproduce in their new location, they may add new alleles to the gene pool of the population, or they may change the frequencies all alleles already present if they come from a population with different allele frequencies. Genetic drift may cause large changes in small populations • Genetic drift—random changes in allele frequencies—may produce large changes in allele frequencies from one generation to the next. • Populations that are normally large may pass through occasional periods when only a small number of individuals survive population bottlenecks; genetic variation can be reduced by genetic drift. • Genetic drift can have similar effects when a few pioneering individuals colonize a new region. o B/c of its small size, the colonizing population is unlikely to have all the alleles found among members of its source population. o The resulting change in genetic variation, founder effect, is equivalent to that in a large population reduced by a bottleneck. Non-random mating changes genotype frequencies • Mating patterns may alter genotype frequencies if individuals in a population choose other individuals of particular genotypes as mates (non-random mating). o For example, if females mate with males of the same genotype, then homozygous genotypes will be overrepresented and heterozygous genotypes underrepresented. • Self-fertilization (selfing), another form of non-random mating. • Sexual selection is a particularly important form of non-random mating that DOES change allele frequencies and often results in adaptations. What Evolutionary Mechanisms Result in Adaptation? • Adaptation occurs when some individuals in a population contribute more offspring to the next generation that others allele frequencies in the population change in a way that adapts individuals to the environment that influenced such reproductive success (natural selection). • Natural selection acts on the phenotype. • The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called its fitness. • Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but only changes in the relative success of different phenotypes within a population lead to changes in allele frequencies from one generation to another. • The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype. Natural selection produces variable results • Natural selection can act on characters with quantitative variation in any one several different ways: o Stabilizing selection—favouring average individuals o Directional selection –favouring individuals that vary in one direction from the mean pop. o Disruptive selection—favouring individuals that vary in opposite directions f
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