Think about an elephant. Develop a mental image of it. How would you describe it to someone who has never seen one? Take a moment to consider carefully . . .
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your mental image was accurate.
Very likely your mental image was a visual one like the picture. Humans primarily emphasize traits that can be seen with their eyes since they mostly rely on their sense of vision. However, there is no reason that an elephant or any other organism could not be described in terms of touch, smell, and/or sound as well. Think about an elephant again but this time in terms of non-visual traits . . .
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 | |
Homologies
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
Parallelism
Convergence
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Australian Tasmanian wolf or tiger | North American wolf |
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
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.
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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 on the right, 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.
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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 interspecies breeding patterns and results. Likewise, we cannot carry out a breeding experiment between ourselves and our ancestors from a million years ago. 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. Higher, more inclusive divisions of the Linnaean system (e.g., phylum and class) are created by including together closely related clusters of the immediately lower divisions. The result is a hierarchical
species | species | species | species | species | species | species | species |
Most researchers today take a cladistics
Copyright � 1998-2012 by Dennis O'Neil. All rights reserved.
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