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Analogous structures  -  A trait that is morphologically and functionally similar to another, but which arose from a different ancestral condition.
Coevolution  -  Evolution that results from the relationships between animals. For example, one animal might prey on another, meaning that only the fastest of the second animal survive, creating a selection pressure. As the second animal gets extremely fast, only the fastest of the first predatory animal can catch enough food to survive, creating selection pressure on the predator to become faster.
Coevolutionary arms race  -  The back-and-forth evolution of defense and offense between predator or parasite and prey that often a can often result in a rapid burst of evolutionary change in both species.
Convergent evolution  -  Pattern of evolution in which two unrelated species gradually become similar to each other through adaptation to a common environment, often resulting in analogous structures. Compare with divergent evolution and parallel evolution.
Divergent evolution  -  Pattern of evolution in which two closely related species gradually become more and more dissimilar. Compare with convergent evolution and parallel evolution.
Parallel evolution  -  Pattern of evolution in which two species maintain the same degree of similarity while each undergoes change along an independent path. Compare with convergent evolution and divergent evolution.
Species  -  The most specific categorization for organisms. The term "species" refers to a group of organisms that shares the same gene pool and can successfully mate. A bulldog and a greyhound are of the same species because they can produce young. A cat and a bulldog, which experience reproductive isolation from each other and therefore cannot produce young, are separate species.