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evolutionary change on its smallest scale

population genetics

the study of how populations change genetically over time

modern synthesis

a comprehensive theory of evolution that integrated ideas from many other fields


a localized group of individuals that are capable of interbreeding and producing fertile offspring

gene pool

the aggregate of genes in a population at any one time is called the population's ______ _______

Hardy-Weinberg theorem

states that the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work

Hardy-Weinberg equilibrium

the condition describing a non-evolving population, which follows these 5 conditions: large population, no gene flow, no mutations, random mating, no natural selection


change in the nucleotide sequence of DNA

genetic drift

similar deviations from the expected result explain how allele frequencies can fluctuate unpredictably from one generation to the next

bottleneck effect

when a sudden change in the environment may drastically reduce the size of a population, the survivors may have passed through a restrictive "bottleneck", and their gene pool may no longer be reflective of the original population's gene pool

founder effect

when a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool isn't reflective of the source population

gene flow

genetic additions to and/or subtractions from a population resulting from the movement of fertile individuals or gametes

phenotypic polymorphism

a population is said to display ________ ________ for a character if two or more distinct morphs are each represented in high enough frequencies to be readily noticeable

average heterozygosity

the percent, on average, of a population's loci that are heterozygous in members of the population

geographic variation

differences between the gene pools of separate populations or population subgroups


a graded change in a trait along a geographic axis


the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

relative fitness

the contribution of a genotype to the next generation comparted to the contributions of alternative genotypes for the same locus

directional selection

__________ ________ is most common when a population's environment changes or when members of a population migrate to a new habitat with different environmental conditions from their former one

disruptive selection

________ ________ occurs when conditions favor individuals on both extremes of a phenotypic range over individuals with intermediate phenotype

stabilizing selection

_________ _________ acts against extreme phenotypes and favors intermediate variants

balancing selection; balanced polymorphism

_________ ________ occurs when natural selection maintains stable frequencies of two or more phtnotypic forms in a population, a state called ___________ _________

heterozygote advantage

if individuals who are heterozygous at a particular gene locus have greater fitness than the homozygous, natural selection will tend to maintain two or more alleles at that locus

frequency-dependent selection

the fitness of any one morph that declines if it becomes too common in the population

neutral variation

some of the genetic variation in populations probably has little or no impact on reproductive success, and thus natural selection doesn't affect those alleles


genes that have become inactivated by mutations

sexual selection

natural selection for mating success

sexual dimorphism

marked differences between the sexes in secondary sexual characteristics, which are not directly associated with reproduction

intrasexual selection

(selection "within the same sex") is a direct competition amon individuals of one sex for mates of the opposite sex

intersexual selection

individuals of one sex (usually females) are choosy in selecting their mates from the other sex

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