Explanation:
The vertebrate forelimb of human, bird, and bat may not appear to have much in common, but a closer examination of the anatomy shows that the same bones were applied to a variety of different jobs in each of these species. Evolution doesn't plan ahead. It can only work with what's available. The color coding shows the way the bony elements were modified in each species. You might think that the two wings would be more similar in design because of the job they do but, in fact, the bird wing and human arm are more similar on the interior. However, the bird "hand" is reduced, whereas the bat and human hand have more in common. See if you can find a similar image of a dolphin or whale flipper for another comparison.
Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.
Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure \(\PageIndex{3}\) shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor.
