Principles of Classification

Not surprisingly, biologists also classify organisms into different categories mostly by judging degrees of apparent similarity and difference that they can see. The assumption is that the greater the degree of physical similarity, the closer the biological relationship.

On discovering an unknown organism, researchers begin their classification by looking for 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. If the latter is the case, then the two species are probably closely related and should be classified into the same or near biological categories.

	Human arm bones (a common mammal, bird and reptile configuration)

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, analogies, or mere chance.

Parallelis, 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 is an example. The former is a placental mammal and the latter is an Australian marsupial. Their common ancestor lived during the age of the dinosaurs more than 100 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.

Tasmanian wolf or tiger (now extinct)		Mexican wolf

Examples of Convergent Evolution--ant eating mammals from four continents This link takes you to an external website. To return here, you must click the "back" button on your browser program.

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.

Problems in Classifying Organisms

Listing characteristics that distinguish one species from another has the effect of making it appear that the species and their distinctive attributes are fixed and eternal. We must always keep in mind that they were brought about by evolutionary processes that operated not merely at some time in the distant past, but which continue to operate in the present and can be expected to give rise to new forms in the future. Species are always changing. As a consequence, they are essentially only a somewhat arbitrarily defined point along an evolutionary line.

	Jaguar

It is also important to realize that most species are physically and genetically diverse. Many are far more varied than humans. When you think of an animal, such as the jaguar shown here, and describe it in terms of its specific traits (fur color patterns, body shape, etc.), it is natural to generalize and to think of all jaguars that way. To do so, however, is to ignore the reality of diversity in nature.

Another problem in classifying a newly discovered organism is in determining the specific characteristics that actually distinguish it from all other types of organisms. There is always a lively debate among researchers over defining new species because it is not obvious what are the most important traits. There are two schools of thought in resolving this dilemma. The first defines new species based on minor differences between organisms. This is the splitter approach. The second tends to ignore minor differences and to emphasize major similarities. This lumper approach results in fewer species being defined.

Ideally, this dispute could be settled by breeding experiments--if two organisms can mate and produce fertile offspring, they are probably members of the same species. However, we must be careful because members of very closely related species can sometimes produce offspring together, and a small fraction of those may be fertile. This is the case with mules, which are the product of mating between female horses and male donkeys. About one out of 10,000 mules is fertile. Does this mean that horses and donkeys are in the same species? Whatever the answer may be, it is clear that species are not absolutely distinct entities, though by naming them, we implicitly convey the idea that they are.

Tigons and Ligers--what happens when tigers and lions mate This link takes you to an external website. To return here, you must click the "back" button on your browser program.

Breeding experiments are rarely undertaken to determine species boundaries because of the practical difficulties. It is time consuming and wild animals do not always cooperate. Using this kind of reproductive data for defining species from the fossil record is impossible since we cannot go back in time to observe breeding patterns and results. Comparisons of DNA sequences are now becoming more commonly used as an aid in distinguishing species. If two animals share a great many DNA sequences, it is likely that they are at least closely related. Unfortunately, this usually does not conclusively tell us that they are members of the same species. Therefore, we are still left with morphological characteristics as the most commonly used criteria for identifying species differences.

The Linnaean scheme for classification of living things lumps organisms together based on presumed homologies. The assumption is that the more homologies two organisms share, the closer they must be in terms of evolutionary distance. The higher, more inclusive divisions of the Linnaean system are created by including together closely related clusters of the immediately lower divisions. The result is a hierarchical system of classification with the highest category consisting of all living things. The lowest category consists of a single species. Each of the categories above species can have numerous subcategories. In the example below, only two genera (plural of genus) are listed per family but there could be many more or only one.

order
family				family
genus		genus		genus		genus
species	species	species	species	species	species	species	species

Most researchers today take a cladistics approach to classification. This involves making a distinction between derived and primitive traits when evaluating the importance of homologies in determining placement of organisms within the Linnaean classification system. Derived traits are those that have changed from the ancestral form and/or function. An example is the foot of a modern horse. Its distant early mammal ancestor had five digits. The bones of these digits have been largely fused together in horses giving them essentially only one toe with a hoof. In contrast, primates have retained the primitive characteristic of having five digits on the ends of their hands and feet. Animals sharing a great many homologies that were recently derived, rather than only ancestral, are more likely to have a recent common ancestor. This assumption is the basis of cladistics.

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