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Key Terms

BiologyKey Terms

adaptive evolution
increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection
allele frequency
(also, gene frequency) rate at which a specific allele appears within a population
assortative mating
when individuals tend to mate with those who are phenotypically similar to themselves
bottleneck effect
magnification of genetic drift as a result of natural events or catastrophes
gradual geographic variation across an ecological gradient
directional selection
selection that favors phenotypes at one end of the spectrum of existing variation
diversifying selection
selection that favors two or more distinct phenotypes
evolutionary fitness
(also, Darwinian fitness) individual’s ability to survive and reproduce
founder effect
event that initiates an allele frequency change in part of the population, which is not typical of the original population
frequency-dependent selection
selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection)
gene flow
flow of alleles in and out of a population due to the migration of individuals or gametes
gene pool
all of the alleles carried by all of the individuals in the population
genetic drift
effect of chance on a population’s gene pool
genetic structure
distribution of the different possible genotypes in a population
genetic variance
diversity of alleles and genotypes in a population
geographical variation
differences in the phenotypic variation between populations that are separated geographically
good genes hypothesis
theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease
handicap principle
theory of sexual selection that argues only the fittest individuals can afford costly traits
fraction of population variation that can be attributed to its genetic variance
honest signal
trait that gives a truthful impression of an individual’s fitness
mating of closely related individuals
inbreeding depression
increase in abnormalities and disease in inbreeding populations
broader scale evolutionary changes seen over paleontological time
changes in a population’s genetic structure
modern synthesis
overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today
nonrandom mating
changes in a population’s gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others
population genetics
study of how selective forces change the allele frequencies in a population over time
population variation
distribution of phenotypes in a population
relative fitness
individual’s ability to survive and reproduce relative to the rest of the population
selective pressure
environmental factor that causes one phenotype to be better than another
sexual dimorphism
phenotypic difference between the males and females of a population
stabilizing selection
selection that favors average phenotypes
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