There are many types of evidence for evolution, including studies in the molecular biology field (like DNA) and also in the developmental biology field. However, the most commonly used types of evidence for evolution are anatomical comparisons between species. While homologous structures show how similar species have changed from their ancient ancestors, analogous structures show how different species have evolved to become more similar.
Speciation is the change over time of one species into a new species. So why would different species become more similar? Usually the cause of convergent evolution is similar selection pressures in the environment. In other words, the environments in which the two different species live are similar and those species need to fill the same niche in different areas around the world. Since natural selection works in the same way in these types of environments, the same types of adaptations are favorable and those individuals with those favorable adaptations survive long enough to pass down their genes to their offspring. This continues until only individuals with favorable adaptations are left in the population.
Sometimes, these types of adaptations can change the structure of the individual. Body parts can be gained, lost, or rearranged depending on whether or not their function is the same as the original function of that part. This can lead to analogous structures in different species that occupy the same type of niche and environment in different locations.
When Carolus Linnaeus first began classifying and naming species with taxonomy, he often grouped similar looking species into similar groups. This led to incorrect groupings when compared to actual evolutionary origins of the species. Just because species look or behave the same does not mean they are closely related.
Analogous structures do not have to have the same evolutionary path. One analogous structure may have come into existence long ago, while the analogous match on another species may be relatively new. They may go through different developmental and functional stages before they are fully alike. Analogous structures are not necessarily evidence that two species came from a common ancestor. It is actually more likely they came from two separate branches of the phylogenetic tree and may not be closely related at all.
Examples of Analogous Structures
The eye of a human is very similar in structure to the eye of the octopus. In fact, the octopus eye is superior to the human eye in that it does not have a "blind spot". Structurally, that is really the only difference between the eyes. However, the octopus and the human are not closely related and reside far away from each other on the phylogenetic tree of life.
Wings are a popular adaptation for many animals. Bats, birds, insects, and pterosaur all had wings. A bat is more closely related to a human than a bird or insect based on homologous structures. Even though all these species have wings and can fly, they are very different in other ways. They just all happen to fill the flying niche in their locations.
Sharks and dolphins look very similar in their appearance due to color, placement of their fins, and over all body shape. However, sharks are fish and dolphins are mammals. This means that dolphins are more closely related to rats than they are sharks on the evolutionary scale. Other types of evolutionary evidence, like DNA similarities have proven this.
It takes more than looks to determine which species are closely related and which have evolved from different ancestors to become more similar through their analogous structures. However, analogous structures themselves are evidence for the theory of natural selection and the accumulation of adaptations over time.