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Chapter 23: The Evolution of Populations

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Three mechanisms that can cause allele frequency change
1. natural selection
2. genetic drift
3. gene flow
microevolution
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)
cline
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