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Average heterozygosity(het'-er-o-zi-go'-si-te) The percent, on average, of a population’s loci that are heterozygous in members of the population.
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Balancing selectionNatural selection that maintains two or more phenotypic forms in a population.
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Bottleneck effectGenetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions. Typically, the surviving population is no longer genetically representative of the original population.
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ClineA graded change in a character along a geographic axis.
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Directional selectionNatural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.
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Disruptive selectionNatural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
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Founder effectGenetic drift that 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 population.
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Frequency-dependent selectionA decline in the reproductive success of individuals that have a phenotype that has become too common in a population.
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Gene flowThe transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gametes.
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Gene poolThe aggregate of all of the alleles for all of the loci in all individuals in a population. The term is also used in a more restricted sense as the aggregate of alleles for just one or a few loci in a population.
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Genetic driftA process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next. Effects of genetic drift are most pronounced in small populations.
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Geographic variationDifferences between the gene pools of geographically separate populations or population subgroups.
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Hardy-Weinberg equilibriumThe condition describing a nonevolving population (one that is in genetic equilibrium).
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Hardy-Weinberg principleThe 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.
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Heterozygote advantageGreater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool.
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Intersexual selectionSelection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice.
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Intrasexual selectionA direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex.
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MicroevolutionEvolutionary change below the species level; change in the allele frequencies in a population over generations
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Mutation(myu-ta´-shun) A change in the nucleotide sequence of an organism’s DNA, ultimately creating genetic diversity. Mutations also can occur in the DNA or RNA of a virus.
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Natural selectionA process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are organisms with other characteristics.
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Neutral variationGenetic variation that does not appear to provide a selective advantage or disadvantage.
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Point mutationA change in a gene at a single nucleotide pair.
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PopulationA localized group of individuals of the same species that can interbreed, producing fertile offspring.
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Punnett squareA diagram used in the study of inheritance to show the predicted results of random fertilization in genetic crosses.
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Quantitative characterA heritable feature that varies continuously over a range rather than in an either-or fashion.
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Recessive alleleAn allele whose phenotypic effect is not observed in a heterozygote.
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Relative fitnessThe contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.
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Sexual dimorphism(di-mor´-fizm) Marked differences between the secondary sex characteristics of males and females.
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Sexual selectionA form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.
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Sickle-cell diseaseA human genetic disease caused by a recessive allele that results in the substitution of a single amino acid in a globin polypeptide that is part of the hemoglobin protein; characterized by deformed red blood cells (due to protein aggregation) that can
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