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change in a kind of organism over time; process by which modern organisms have descended from ancient organisms

natural selection

the principle that, among the range of inherited trait variations, those that lead to increased reproduction and survival will most likely be passed on to succeeding generations; also called "survival of the fittest"

homologous structures

structures that have different mature forms but develop from the same embryonic tissues

vestigial structures

remnant of a structure that may have had an important function in a species' ancestors, but has no clear function in the modern species

allele frequency

the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene

gene pool

all the genes, including all the different alleles for each gene, that are present in a population at any one time

founder effect

genetic drift that occurs when a few individuals become isolated from a larger population, with the result that the new population's gene pool is not reflective of the original population.

Hardy-Weinberg Principle

allele frequencies in a population will remain constant unless one or more factors cause the frequencies to change


group of similar organisms that can breed and produce fertile offspring

behavioral isolation

form of reproductive isolation in which two populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding

temporal isolation

form of reproductive isolation in which two populations reproduce at different times

geographic isolation

form of reproductive isolation in which two populations are separated physically by geographic barriers such as rivers, mountains, or stretches of water


species that no longer exist

convergent evolution

when two or more species NOT descended from a common ancestor develop similar traits as they adapt to the same type of environment

adaptive radiation

process by which a single species or small group of species evolves into several different forms that live in different ways; rapid growth in the diversity of a small group of organisms


a model of evolution in which gradual change over a long period of time leads to biological diversity

punctuated equilibrium

evolutionary model suggesting species often diverge in spurts of relatively rapid change, followed by long periods of little change

directional selection

form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve

stabilizing selection

form of natural selection by which the center of the curve remains in its current position; occurs when individuals near the center of a distribution curve have higher fitness than individuals at either end

disruptive selection

form of natural selection in which a single curve splits into two; occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle


inherited characteristic that increases an organism's chance of survival


how well an organism can survive and reproduce in its environment


the formation of new species as a result of evolution

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