Introduction to Classification: On discovering an unknown organism, researchers classify it by looking for morphological and anatomical features that appear to have the same function as those found on other species. The next step is determining whether or not the similarities are due to an independent evolutionary development or to descent from a common ancestor.
Homologies: Homologies are anatomical features, of different organisms, that have a similar appearance or function because they were inherited from a common ancestor that also had them. For instance, the forelimb of a bear, the wing of a bird, and human’s arm have the same functional types of bones as did the common reptilian ancestor. Therefore, these bones are homologous structures. The more homologies two organisms possess, the more likely it is that they have a close genetic relationship. There can also be non homologous structural similarities between species. In these cases, the common ancestor did not have the same anatomical structures as its descendants. Instead, the similarities are due to independent development in the now separate evolutionary lines. Such misleading similarities are called homoplasies. Homoplastic structures can be the result of parallelism, convergence, or mere chance.
Parallelism: Parallelism or parallel evolution is a similar evolutionary development in different species lines after divergence from a common ancestor that did not have the characteristic but did have an initial anatomical feature that led to it. For example, some South American and African monkeys evolved relatively large body sizes independently of each other. Their common ancestor was a much smaller monkey otherwise similar to them in other features.
Convergence: Convergence or convergent evolution is the development of a similar anatomical feature in distinct species lines after divergence from a common ancestor that did not have the initial trait that led to it. The similar appearance of North American wolves which are placental mammals and Tasmanian wolves which are marsupials is an example. Australian marsupials are examples of convergent evolution with placental mammals elsewhere. Both parallelism and convergence are thought to be due primarily to separate species lines experiencing the same kinds of natural selection pressures over extended periods of time.
Analogies: Analogies are anatomical features that have the same form or function in different species that have no known common ancestor. The wings of a bird and a butterfly are analogous structures because they are outwardly similar in shape and function. However, their wings are quite different on the inside. Bird wings have an internal framework consisting of bones, while butterfly wings do not have any bones at all and are kept rigid mostly through fluid pressure. Analogies may be due to homologies or homoplasies, but the common ancestor, if any, is unknown.