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Three mechanisms that can cause allele frequency change

1. natural selection
2. genetic drift
3. gene flow


a change in allele frequencies in a population over generations

discrete characters

can be classified on an either-or basis

quantitative characters

vary along a continuum within a population

nucleotide variability

measured by comparing the sequences of pairs of individuals

geographic variation

differences between gene pools of separate populations or populations subgroups (some karyotypes are different, but their phenotypes are the same) (chance events)


geographic variation; graded change in a trait along a geographic axis; result of natural selection

mutation rates

low in animals and plants, lower in prokaryotes and higher in viruses

sexual reproduction

can shuffle existing alleles into new combinations

five conditions for hardy-weinberg

1. no mutations
2. random mating
3. no natural selection
4. extremely large population size
5. no gene flow
(can evolve at some loci while being in hardy-weinberg equilibrium at other loci)

genetic drift

allele frequencies fluctuate unpredictably from one generation to the next; the smaller sample the greater the chance of deviation from predicted results; reduce genetic variation through losses of alleles

the founder effect

a few individuals become isolated from a larger population; allele frequencies in the small founder population can be different from those in the larger parent population

bottleneck effect

a sudden reduction in population size due to a change in the environment; resulting gene pool may no longer be reflective of the original population's gene pool

effects of genetic drift

1. significant in small populations
2. causes allele frequencies to change at random
3. can lead to a loss of genetic variation within populations
4. dan cause harmful alleles to become fixed.

gene flow

movement of alleles among populations; can be transferred through the movement of fertile individuals or gametes; reduces differences between populations over time; more likely than mutation to alter allele frequencies directly; decreases and increases the fitness

relative fitness

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

directional selection

favors individuals at one end of the phenotypic range

disruptive selection

favors individuals at both extremes of the phenotypic range

stabilizing selections

favors intermediate variants and acts against extreme phenotypes

sexual selection

natural selection for mating success

sexual dimorphism

marked differences between the sexes in secondary sexual characteristics

intrasexual selection

competition among individuals of one sex for mates of the opposite sex

intersexual selection

individuals of one sex are choosy in selecting their mates

balancing selection

occurs when natural selection maintains stable frequencies maintains stable frequencies of two or more phenotypic forms in a population

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