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chapter 23


defining evolution on its smallest scale by focusing on evolutionary change in populations

genetic variation

differences among individuals in the composition of their genes or other DNA segments

average heterzygosity

the average percentage of loci that are heterozygous

geographic variation

differences in the genetic composition of separate populations


a graded change in a character along a geographic axis


a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring

gene pool

all copies of every type of allele at every locus in all members of the population

hardy-weinberg principle

the gene pool of a population that is not evolving; named for the British mathematician and German physician who independently derived it in 1908

genetic drift

chance events can also cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations

founder effect

when a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population

bottleneck effect

a population that passes through a "bottleneck" resulting in a severe drop in population size

gene flow

the transfer of alleles into or out of a population due to 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

directional selection

occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population's frequency curve for the phenotypic character in one direction of the other

disruptive selection

occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes

stabilizing selection

acts against both extreme phenotypes and favors intermediate variants

sexual selection

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

sexual dimorphism

a difference between the two sexes in secondary sexual characteristics

intrasexual selection

selection within the same sex, individuals of one sex compete directly for mates of the opposite sex

intersexual selection

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

neutral variation

differences in DNA sequences that do not confer a selective advantage or disadvantage

balancing selection

: occurs when natural selection maintains two or more forms in a population

heterozygote advantage

if individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes

frequency-dependent selection

the fitness of a phenotype depends on how common it is in the population

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