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inherited characteristic that increases an organism's chance of survival


the gradual change in a species over time

artificial selection

selection by humans for breeding of useful traits from the natural variation among different organisms

natural selection

The process by which individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species


disappearance of a species from all parts of its geographical range


scientist who studies fossils

fossil record

chronological collection of life's remains in sedimentary rock layers


layers of sedimentary rock


the study of the past and present distribution of species


similarity in characteristics resulting from a shared ancestry

homologous structures

similar structures that related species have inherited from a common ancestor

vestigial organs

organs that serve no useful purpose for the organism; these suggest that sometime in the past they were useful

evolutionary tree

a diagram that reflects evolutionary relationships among groups of organisms


group of individuals of the same species that live in the same area

gene pool

the combined genetic information of all members of a particular population


evolution resulting from small specific genetic changes that can lead to a new subspecies


a change in the sequence of DNA

stabilizing selection

intermediate forms are favored and extreme forms are eliminated

directional selection

occurs when natural selection favors one of the extreme variations of a trait

disruptive selection

natural selection that favors individuals with either extreme of a trait

sexual dimorphism

marked differences between the sexes in secondary sexual characteristics, which are not directly associated with reproduction

sexual selection

results from differences in traits affecting access to mates

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