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evolutionary change below the species level; change in the allele frequencies in a population over generations

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 geographically seperate populations or population subgroups


a graded change in a character along a geographic axis


a change in the nucleotide sequence of an organism's DNA, ultimately creating genetic diversity; in a virus it can occur in the DNA or RNA


a localized group of individuals of the same species that can interbreed, producing fertile offspring

gene pool

the aggregate of all of the alleles for all of the loci in all individuals in a population

Hardy-Weinberg principle

the principle that frequencies of alleles and genotypes in a population 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 nonevolving population

adaptive evolution

evolution that results in a better match between organisms and their environments

genetic drift

a process in which chance events caues unpredicatble fluctuations in allele frequencies from one generation to the next; most pronounced in small populations

founder effect

genetic drift taht occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original pool

bottleneck effect

genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions; the surviving population is no longer genetically representative of the original population

gene flow

the transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gametes

relative fitness

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

directional selection

natural selection in which individuals at one end of the phenotypic range survive or reproduce more succesffully than do other individuals

disruptive selection

natural selection in which individuals on both extemes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes

stabilizing selection

natural selection in which intermediate phenotypes survive or reproduce more successfuly than do extreme phenotypes

sexual selection

a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates

sexual dimorphism

marked differences between the secondary sex characteristics of males and females

intrasexual selection

a direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex

intersexual selection

selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; mate choice

balancing selection

natural selection that maintains two or more phenotypic forms in a population

heterozygote advantage

greater reproductive success of heterozugoous individuals compared with homozygotes; tends to preserve variation in a gene pool

frequency-dependent selection

a decline in the reproductive success of individuals that have a phenotype that has become too common in a population

neutral variation

genetic variation that does not appear to provide a selective adavantage or disadvantage

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