One might expect that plants and animals faced with similar environmental conditions and occupying a comparative ecological niche would perhaps, over time, derive the same solution to optimize their survival in their habitat. It is this principle that underlies the observable phenomenon known as evolutionary convergence. Convergence occurs when unrelated species, frequently found in entirely separate parts of the world, independently arrive at the same morphology or evolve striking similarities in some other aspect of their biology that are not the result of similarity via relatedness. One of many terrestrial examples include the euphorbs of South America and the cacti (or cactuses) of North America, that look and function in a very similar way, even though they evolved out of contact with each other, because each evolved under very similar conditions of water availability, temperature, and altitude. It makes sense that a limited range of options are available to an animal or plant in a particular environment, and that different lineages may evolve outward or functional similarities that could easily be taken, wrongly, as evidence of a common ancestor. Whales, that evolved from terrestrial mammals, have converged on the fish body plan because streamlined bodies and fins are the most efficient morphological adaptations for their way of life. Similarly, bats have evolved wings and fly, although they are not birds. They are not moths, either, that also have wings and fly at night. Speaking of moths, some have even developed ability to tune in on the echolocation frequencies used by bats, thus handily avoiding their predators.

The evolutionary opposite to convergence is divergence, whereby descendants from an ancestral stock 'radiate' into different niches, usually associated with invasion of a new land mass or a very diverse aquatic system, and form species with different morphology, behavior, or physiology. Classic examples of divergence include Darwin's finches of the Galapagos Islands and the radiation of marsupials from a now-extinct ancestor into Australia and New Guinea, where they filled the niches elsewhere occupied by placental mammals. Specializations that separate closely-related species are numerous among birds and fishes — in fishes the most extreme are found in coral reefs and, perhaps even more extreme, in the cichlid populations of tropical African lakes.