Homologous structures
Because evolution is descent with modification, there are several similarities between related organisms due to their common ancestry. Homologous structures, or variations of a structure found in a shared common ancestor, are an extension of this concept.
As shown in the picture above, the forelimbs of all mammals have the same basic arrangement of bones. And yet, each of the different forelimbs have a different function whether it be waking, flying, swimming, etc. There are so many homologous structures that mammals have in common, that it would be difficult to prove that they all evolved separately through convergent evolution. Instead, scientists use the mammals' homologous structures to prove that they are descendants from the same common ancestor.
Another concept to know about homologous structures are vestigial structures. Vestigial structures are simply leftover features from a common ancestor that modern organisms have no use for.
- Some examples include the appendix in humans, or the pelvic bones found in snakes from their walking ancestors.
- Another would be how all vertebrate embryos have a tail and a pharyngeal pouch.
- Finally are "pseudogenes" or genes that have lost their function. Yet, these same genes may still be functional in a related species.
analogous structures
Analogous structures are when species share features with the same function because of convergent evolution, not because of common ancestry. Thus, they are the exact opposite of homologous structures.
Note that convergent evolution is the independent evolution of similar features in unrelated species. Take the picture to the right. Butterfly and bat species do not share a common ancestor. However, their separate common ancestors evolved over the generations so that their modern descendants both have wings that share the same function for flight. Thus, these species underwent convergent evolution to yield the analogous wing structures. |