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I'm sure you've learned these terms in your class:
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 your arm have the same functional types of bones as did our shared 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 nonhomologous 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 , 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 instance, some South American and African monkeys evolved relatively large body sizes independently of each other. Their common ancestor was a much smaller monkey but was otherwise reminiscent of the later descendant species. Apparently, nature selected for larger monkey bodies on both continents during the last 30 million years.
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 common ancestor is usually more distant in time than is the case with parallelism. The similar appearance and predatory behavior of North American wolves and Tasmanian wolves (thylacines) is an example. The former is a placental mammal like humans and the latter is an Australian marsupial like kangaroos. Their common ancestor lived during the age of the dinosaurs 125 million years ago and was very different from these descendants today. There are, in fact, a number of other Australian marsupials that are striking 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 long periods of time.
Analogies are anatomical features that have the same form or function in different species that have no known common ancestor. For instance, the wings of a bird and a butterfly are analogous structures because they are superficially similar in shape and function. Both of these very distinct species lines solved the problem of getting off of the ground in essentially the same way. 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.
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