Many types of evidence support evolution, including studies in molecular biology, such as DNA, and developmental biology. Anatomical comparisons between species are the most common type of evidence for evolution. An analogous structure shows how different species have evolved to become more similar from their ancient ancestors, whereas homologous structures show how similar species have changed over time.
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Speciation
Speciation refers to the evolution of one species into another over time. Could there be a reason why species would become more similar? Similar selection pressures in the environment are usually responsible for convergent evolution. Therefore, the two species live in similar environments and need to fill the same niche in different areas of the world.
Due to the same natural selection process operating in these environments, the same types of adaptations are favorable, and individuals with favorable adaptations survive long enough to pass along their genes. As a result, only individuals with favorable adaptations remain in the population.
Adaptations like these can sometimes change the structure of an individual. It is possible to gain, lose, or rearrange body parts depending on whether their function is the same as their original function. The result can be analogous structures in different species that occupy the same niche and environment.
Taxonomy
Carolus Linnaeus often grouped similar-looking species into similar groups when he first began classifying and naming species with taxonomy. As a result, species were incorrectly grouped according to their evolutionary origins. The appearance or behavior of two species does not necessarily indicate their close relationship.
There is no need for analogous structures to have followed the same evolutionary path. Analogous structures on one species may have evolved long ago, while analogous structures on another species may be relatively new. Before they are fully alike, they may go through different developmental and functional stages.
An analogous structure does not necessarily prove that two species have a common ancestor. It is more likely they came from different branches of the phylogenetic tree and may not be closely related.
Examples
The human eye is very similar to the eye of an octopus. The octopus’ eye is superior to the human’s in that it does not have a “blind spot.” This is the only structural difference between the two. The phylogenetic tree of life shows that the octopus and the human are not closely related.
Many animals have wings as an adaptation. There were wings on bats, birds, insects, and pterosaurs. In terms of homologous structures, a bat is more closely related to a human than to a bird or an insect. Although all of these species have wings and can fly, they are very different in other ways. Their locations just happen to fill the flying niche.
The color, placement of their fins, and overall shape of sharks and dolphins are very similar. Sharks, however, are fish, while dolphins are mammals. On the evolutionary scale, dolphins are more closely related to rats than sharks. This has also been proven by other types of evolutionary evidence, such as DNA similarities.
In order to determine which species have evolved from different ancestors to become more similar through their analogous structures, it takes more than just appearances. Analogous structures themselves are evidence for the theory of natural selection and the accumulation of adaptations over time, however.