Classification Biological

Our present system of biological classification grew out of the need to find a way of organizing the growing lists of new plants and animals carried back to Europe during the Age of Discovery. Prior to Linnaeus, organisms were named in a variable manner, usually with a descriptive phrase. The Linnaean system supplanted earlier forms of classification, primarily because it replaced more ponderous ways of naming species with the simple and flexible binomial system. (For example, genus and species: our species is Homo sapiens, with the name of our genus [Homo] always written before our species epitaph [sapiens].) This, coupled with the Linnaean system of ordering hierarchically and showing relative hierarchy through the use of additional categorical ranks, provided subsequent biologists with a flexible and readily modifiable system that was capable of absorbing our ever-expanding knowledge of biological diversity.

Darwin changed forever how we interpret biological classifications. Before the general acceptance of evolution as a process and phy-logenetic descent as a pattern, there was considerable diversity of opinion as to how the form and meaning of biological classifications should be understood. In general, organisms were placed together into larger taxonomic groups using some criterion of similarity. Actual classifications took various forms. For example, the quintarians were firmly convinced that groups occurred in fives and classified accordingly. Religiously minded scientists, such as Louis Agassiz, interpreted truly natural classifications as a plan of how God created. (Thus, taxonomy could almost be thought of as the discovery of how God thinks about things.) One of Darwin's political successes, no doubt, was the fact that one could simply change the way one thought about existing classifications without having to abandon them. One simply switched from interpreting an existing classification as a scheme of similarity to interpreting the classification as a reflection of descent with modification.

Today there are two aspects of biological classification that must be understood; one is conceptual, the other is purely technical. The conceptual issue can be stated directly: what is a classification meant to represent? The technical aspect can also be stated directly: what rules should be adopted so that all can understand biological classifications?

Changing Concepts

Darwin suggested that classifications should portray the genealogical history of organisms. Of course, he also recognized that our knowledge of genealogy was woefully deficient, a situation that still exists. Much research was invested in trying to understand the major line of evolution in plants and animals during the latter part of the nineteenth century and the first half of the twentieth. However, the use of rigorous and programmatic methods to reconstruct phylogenetic history was not widely adopted until the German entomologist Willi Hennig introduced phylogenetic systematics around 1950. Hennig suggested two major reforms. First, he suggested that "relationship" in a rigorous evolutionary sense meant one and only one thing: genealogical relationship. This is the relationship between parent and child or ancestral species and its descendent (daughter) species. Before this refinement, there was a dual concept of the meaning of relationship: relationship of similarity and relationship of genealogical descent. They had coexisted as equal partners in biological classification since the general adoption of the evolutionary paradigm. Prior to Hennig, one could justify a particular classification either by claiming that species should be placed in a genus (or a genus in a family, and so forth) because they were similar to each other, or because they shared a common ancestor. It is important for us to understand that the criterion of grouping because organisms are similar (or the more "modern" refinement of phenetic similarity) is a pre-evolutionary idea that carried over through the Darwinian revolution. Although it may be true that we can expect genealogical relatives to be similar, it is not always the case. For example, crocodiles "look" more like lizards, but they are the closest living genealogical relatives of birds (both are archosaurs, as are dinosaurs). Hennig insisted that genealogy was primary, while similarity was secondary.

The second refinement grew out of the first. Before Hennig's work, systematists recognized two kinds of groups. Monophyletic groups were supposed to correspond to the descendants of a common ancestor, while polyphyletic groups were composed of organisms that did not share a common ancestor included in the group. For example, Mam malia is a monophyletic group, but a group composed of birds and mammals (Home-othermia) is a polyphyletic group, since the last common ancestor of both would be classified as a reptile. Hennig recognized that many "monophyletic" groups were not monophyletic at all. He called these groups "paraphyletic." Paraphyletic groups contain some descendants of a common ancestor but leave some other descendants out, usually because they are not very similar to members of the paraphyletic group. For example, Reptilia are a paraphyletic group because it leaves out mammals (Mammalia) and birds (Aves). Hennig asserted that paraphyletic groups were as unnatural as poly-phyletic groups. This was a major threat to existing classifications, because as the phylo-genies of major groups were refined, systema-tists found that many familiar and widely used groups were paraphyletic. Reptilia is one example (crocodiles and dinosaurs being classified with lizards rather than with birds). An example closer to home is the Pongidae, a group that consists of the great apes but that excludes humans (which are placed in their own family, Hominidae but are actually closely related to gorillas and chimpanzees). Such luminaries as G. G. Simpson, Ernst Mayr, and Steven J. Gould attacked Hennig's assertion that paraphyletic groups should be abandoned in true evolutionary classifications. However, it turns out that Hennig was right. The philosopher David Hull, even before reading Hennig, pointed out that paraphyletic groups were logically inconsistent with the phylogenies they sought to summarize. This places those who wish to continue to use paraphyletic groups in the awkward position of advocating a system of classification that is illogical relative to the phylogenies they accept as good hypotheses.

Adoption of these two refinements allowed Hennig to formulate a consistent method for recovering phylogenetic relationships by using only those homologies that are directly relevant to corroborating or refuting genealogical relationships at a particular level of analysis. His method, and later refinements, form the techniques universally applied by modern sys-tematists to reconstruct the evolution histories of organisms.

To summarize: Hennig wished to reform systematic biology and make it truly Darwinian. Building on the work of earlier German taxonomists, he formulated a consistent and rigorous method of reconstructing evolutionary history. He asserted that genealogical relationships were the primary criterion of relationships. And he uncovered a kind of unnatural group that had previously been recognized as a kind of monophyletic group, the paraphyletic group. With the recognition that paraphyletic groups are illogical relative to evolutionary descent, Hennig's assertion that paraphyletic groups should be abandoned in favor of classifications containing only mono-phyletic groups is growing in acceptance.

What Biological Classification Can and Cannot Do

Scientists as well as teachers and students frequently expect that classifications embody certain kinds of information that they are not capable of containing. This leads to dashed expectations and even problematic science. As a form of hierarchical language, biological classifications are capable of informing a reader about the groupings of organisms. If the reader understands the intention of the person who does the classifying, then the reader understands the criterion of grouping. For example, if you know that I intend to classify using the criterion of grouping by common ancestry, you can interpret the grouping I form as my hypothesis of the genealogical/evolutionary relationships of the organisms I classify. You might then be able to compare my classification with my phylogenetic tree hypothesis and evaluate how well I actually reflected the common ancestry relationships. If, on the other hand, I am working with a poorly known group of obscure marine worms, I may not have a hypothesis of the common ancestry relationships of the worms. I may simply be attempting to organize the diversity as well as I can, using similarity or even intuition until such time as I, or others, might be able to study the phylogeny of the group. The vast majority of all classifications actually fall under the category of this second group: classifications that reflect best guesses of relationships rather than actual phylogenetic hypotheses. As I understand more about the evolution of my group of marine worms, my classifications will change to reflect this increase in knowledge. A classification made on the basis of a first guess looks as well organized and solid as a classification made on the basis of a detailed knowledge of evolutionary relationships. There are no warning flags saying: "This classification is a solid reflection of evolutionary relationships, while that other one is an arbitrary arrangement." Both equally reflect one and only one thing: the group relationships hypothesized by the taxonomist to exist between the organisms classified.

In an earlier and simpler time, biological classifications were thought capable of conveying much more than grouping relationships. It is common to find the following myths, and it is easy to find defenders of these myths even today among practicing taxono-mists:

Myth 1. Classifications can convey some sense of the distinctiveness of a taxon by adjusting the rank of that taxon. It is true that one can raise the rank of a taxon to reflect your idea of its novel and distinctive nature (not that everyone will agree with you!). How ever, you cannot retrieve that information from the classification. For example, asserting that birds are a class of vertebrates does not automatically give a clue as to how distinctive birds are from their closest relatives, crocodiles and dinosaurs. Further, the matter of how distinctive a particular group might be is a matter of opinion and thus a subjective criterion rather than an objective one. Why, for example, are birds a class of vertebrates while bats are simply an order of mammals?

Myth 2. Distantly related taxa ranked at the same categorical level are comparable. Modern phylogenetic classifications use ranking purely to denote subordination (that is, the position of a taxon within a hierarchy). An order or family of insects is not comparable to an order or family of fishes. Indeed, one family of fishes may not be comparable to another family of fishes. Only when these two taxa of fishes are closest relatives ("sister groups") are they directly comparable. The idea that distantly related groups ranked at the same level are biologically comparable is a vestige of the old idea of the scala naturae, the idea that organisms could be arranged in order of increasing perfection.

Myth 3. Classification should remain stable and unchanged. Taxonomic classifications are really hypotheses. In science, hypotheses are tested. If a classification is found deficient, it is replaced by a better hypothesis. To ask a taxonomist not to change a classification when she understands more about the relationships of her organisms is like asking any other scientists not to change their hypotheses with increased knowledge. The effect would be to remove taxonomy from science. Rather than lamenting the fact that we have to learn a new classification, we should be celebrating the progress of scientific understanding. Believe it or not, students actually understand this point.

Myth 4. Textbooks present classifications that are logical relative to known phyloge-nies. I know of no secondary-level textbook and few college-level textbooks that present summary classifications of organisms that contain only monophyletic groups. The continued presentation of paraphyletic groups to students perpetuates the myth that classification is not very relevant to evolution. Alternatively, such classifications actually mislead students who may think that classifications should reflect phylogenies. A classification that groups humans in one family and chimpanzees in a different family creates the impression that humans are no more closely related to chimps than to gibbons. A classification that groups birds as a class and crocodiles and dinosaurs in another class creates the impression that birds are not related to dinosaurs or crocodiles any more than birds are related to lizards and snakes.

See also: Linnaean Hierarchy; Phylogeny; Systematics Bibliography

Hennig, Willi. 1966. Phylogenetic Systematics. Urbana: University of Illinois Press; Kitching, Ian J., et al. 1998. Cladistics: The Theory and Practice of Parsimony Analysis, 2d ed. The Systematics Association Publ. 11. New York: Oxford University Press; Ridley, Mark. 1985. Evolution and Classification: The Reformation of Cladism. New York: Long; Wiley, Edward O. 1981. Phylogenetics: The Principles and Practice of Phylogenetic Systematics. New York: John Wiley.

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