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a group of organisms that are physically similar and can mate with each other and produce offspring that can also mate and reproduce


the preserved remains or traces of an organism that lived in the past


a behavior or physical characteristic that allows an organism to survive or reproduce in its environment


the gradual change in a species over time

scientific theory

a well-tested concept that explains a wide range of observations

natural selection

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


any difference between individuals of the same species

homologous structures

body parts that are structurally similar in related species and that provide evidence for a common ancestor

branching tree

a diagram that shows how scientists think different groups of organisms are related

petrified fossil

a fossil formed when minerals replace all or part of an organism


a type of fossil formed when a shell or other hard part of an organism dissolves, leaving an empty space in the shape of the part


a type of fossil that forms when a mold becomes filled in with minerals that then harden

relative dating

a technique used to determine which of two fossils is older

radioactive dating

a technique used to determine the actual age of a fossil on the basis of the amount of a radioactive element it contains


the time it takes for half of the atoms in a radioactive element to decay

fossil record

the millions of fossils that scientists have collected


when no members of a species are alive


the theory that evolution occurs slowly but steadily

punctuated equilibria

the theory that species evolve during short periods of rapid change

radioactive element

an unstable element that breaks down into a different element

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