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The Evolution of Population

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