In biology, evolution by natural selection is the process in which organisms change their biological properties and characteristics over time; the theory suggests that all living organisms have evolved from other, preexisting life forms and have adapted to their environment. The theory of evolution was originally presented by English biologist and naturalist Charles Darwin in his 1859 scientific treatise On the Origin of Species.
According to many scientists, evolution can be divergent, convergent and parallel.
Divergent evolution happens when two related species evolve into vastly different species of organisms to better adapt to their environment. For instance, the evolution of humans and apes is considered to be divergent as both species have inherited their traits from a common primate ancestor, but have also developed different traits to better fit their natural habitat.
Convergent evolution is the independent and repeated evolution of similar biological properties and traits in different species that do not share a common ancestor as a result of the environment in which they live in. Sharks (fish) and dolphins (mammals), for instance, do not share a common ancestor, however they have very similar features, as both species live in water and therefore have adapted to survive in that environment. The separate ancestors of dolphins and sharks did not share any similar traits, nor similar environments or ecosystems.
Similar to convergent evolution, parallel evolution occurs when two or more different species develop similar anatomical traits to better adapt to their environment; however, the separate ancestors of these species also had similar features and lived in a similar environment. The similar biological traits and similar natural habitats of both the evolved species and their ancestors is, essentially, the main distinction between parallel and convergent evolution. The marsupial mammals, for instance, have evolved in parallel to the placental mammals.