Evidence for Evolution
Biogeography presents the most easily noticeable evidence for evolution. Species in different parts of the world are different, with many areas having their own indigenous ones. On the other hand, species close to another geographically are usually much more similar. Islands are examples of this--commonly groups of islands have closely related, but still different, species on them. This shows the migration of a group of individuals from one species to another location, which then forms a separate species. Another case, the fact that species inhabiting very similar environments located in different regions of the world are not similar, shows that the randomness of evolution produces different results even under similar circumstances. This is contrary to many previous beliefs (held by people as renowned as Christopher Columbus) that life in similar environments is similar, but which do not hold up to actual observations.
The fossil record also provides a large amount of evidence for evolution. The ages of fossils conform to the ideas of evolution. Prokaryotes, declared to have evolved first, are the oldest of fossils, followed by eukaryotes. Vertebrates come towards the very end, with humans one of the youngest of species. The fossil record also provides many examples of transitional fossils, which show a progression of intermediate species between one species and another. The differences in fossils of similar species over time show modifications amassing into much more visible changes.
Similarities between related species also supply evidence for evolution. Species often share the same underlying structure, for example the forelimbs of mammals are very similar, despite having very different functions. These homologous structures are a result of the two species resulting from a common ancestor. Likewise, vestigial organs (such as the appendix in humans) are left over from ancestral species, in which they served some important function, but this function became useless in subsequent descendants, and so the organ became less and less function.
Possibly the most important evidence for evolution comes from molecular biology. Common genes between species, even those as different as humans and bacteria, are often similar in function, and those among even more closely related species can be nearly identical. Proteins that do common tasks are also often very similar between species. This is a result of organisms sharing a common ancestor. Genes that are absolutely necessary for life are, as a result, prevalent in all life, and remain unchanged because any changes must be fatal almost all of the time.