adaptation heritable trait or behavior in an organism that aids in its survival and reproduction in its present environment

adaptive evolution increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection

adaptive radiation speciation when one species radiates out to form several other species

allele frequency (also, gene frequency) rate at which a specific allele appears within a population

allopatric speciation speciation that occurs via geographic separation

allopolyploid polyploidy formed between two related, but separate species

aneuploidy condition of a cell having an extra chromosome or missing a chromosome for its species

assortative mating when individuals tend to mate with those who are phenotypically similar to themselves

autopolyploid polyploidy formed within a single species

behavioral isolation type of reproductive isolation that occurs when a specific behavior or lack of one prevents reproduction from taking place

bottleneck effect magnification of genetic drift as a result of natural events or catastrophes

cline gradual geographic variation across an ecological gradient

convergent evolution process by which groups of organisms independently evolve to similar forms

directional selection selection that favors phenotypes at one end of the spectrum of existing variation

dispersal allopatric speciation that occurs when a few members of a species move to a new geographical area

divergent evolution process by which groups of organisms evolve in diverse directions from a common point

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)

gametic barrier prezygotic barrier occurring when closely related individuals of different species mate, but differences in their gamete cells (eggs and sperm) prevent fertilization from taking place

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

gradual speciation model model that shows how species diverge gradually over time in small steps

habitat isolation reproductive isolation resulting when populations of a species move or are moved to a new habitat, taking up residence in a place that no longer overlaps with the other populations of the same species

handicap principle theory of sexual selection that argues only the fittest individuals can afford costly traits

heritability fraction of population variation that can be attributed to its genetic variance

homologous structures parallel structures in diverse organisms that have a common ancestor

honest signal trait that gives a truthful impression of an individual’s fitness

hybrid offspring of two closely related individuals, not of the same species

hybrid zone area where two closely related species continue to interact and reproduce, forming hybrids

inbreeding mating of closely related individuals

inbreeding depression increase in abnormalities and disease in inbreeding populations

macroevolution broader scale evolutionary changes seen over paleontological time

microevolution changes in a population’s genetic structure

modern synthesis overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today

natural selection reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change

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

postzygotic barrier reproductive isolation mechanism that occurs after zygote formation

prezygotic barrier reproductive isolation mechanism that occurs before zygote formation

punctuated equilibrium model for rapid speciation that can occur when an event causes a small portion of a population to be cut off from the rest of the population

reinforcement continued speciation divergence between two related species due to low fitness of hybrids between them

relative fitness individual’s ability to survive and reproduce relative to the rest of the population

reproductive isolation situation that occurs when a species is reproductively independent from other species; this may be brought about by behavior, location, or reproductive barriers

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

speciation formation of a new species

species group of populations that interbreed and produce fertile offspring

stabilizing selection selection that favors average phenotypes

sympatric speciation speciation that occurs in the same geographic space

temporal isolation differences in breeding schedules that can act as a form of prezygotic barrier leading to reproductive isolation

variation genetic differences among individuals in a population

vestigial structure physical structure present in an organism but that has no apparent function and appears to be from a functional structure in a distant ancestor

vicariance allopatric speciation that occurs when something in the environment separates organisms of the same species into separate groups