Systematics And Taxonomy

separated on the basis of single characters. As a result, such schemes have extremely re- 95

stricted value and, usually, can be used only for the purpose for which they were initially designed. More importantly, artificial classifications provide no indication of the "true" or "natural" relationships of the constituent species.

Almost all modern classifications are natural, that is, they indicate the affinity (degree of similarity) between the organisms within the classification. Organisms placed in the same taxon (showing the greatest affinity) are said to form a natural group. There is, however, considerable controversy among systematists over the meaning of "degree of similarity," "natural group," and "natural classification." Essentially systematists fall into three major groups, according to their interpretation of the above terms. These are the phyleticists, cladists, and pheneticists. To the cladistic group, led by Hennig (see Hennig, 1965, 1966, 1981), belong those systematists who base classification entirely on genealogy, the recency of common ancestry. Critical to the modus operandi of cladists are the distinction between primitive and advanced homologous characters (so-called "character polarity") and the recognition of sister groups (see below for further discussion of these terms). Among the various ways used by cladists to assign character polarity are paleontology, ontogeny, and outgroup comparison. In theory, the study of fossils should clearly show when a character first appears, making the separation of primitive and advanced characters an easy task. However, the fossil record is typically discontinuous and preservation imperfect so that vital characters are missing. The idea that "ontogeny recapitulates phylogeny," suggested by Haeckel in 1866, proposes that an organism's development will reflect its evolution, giving clues therefore as to which of its features are primitive and which are advanced. Ontogeny has been relatively little used by cladists, however, perhaps because in development evolutionary steps are compressed, omitted, or masked. Outgroup comparison, which is the method most used, is a comparison of character states in the group under study with those in increasingly distant sister groups. The character state common to the largest sister groups is generally taken to be the primitive condition. This method requires, of course, some previous knowledge of a group's phylogeny and has been criticized because of its circularity. As a result of their studies, cladists usually express their results in the form of a cladogram.

Beginning in the 1950s, some taxonomists, dissatisfied with the perceived subjective approach to classification, began to devise schemes based on the number of common characters among organisms, regardless of whether these were primitive or advanced. The pheneticists (originally known as numerical taxonomists), led by Sokal and Sneath (see Sokal and Sneath, 1963; Sneath and Sokal, 1973), have as their major principles: (1) the more characters studied the better; (2) all characters are of equal weight; and (3) the greater the proportion of similar characters, the closer are two groups related. Pheneticists usually present the results of their analyses as phenograms or scatter diagrams.

Phyleticists such as Simpson (1961) and Mayr (1969, 1981) may be considered as forming a "middle-of-the-road" group, employing both cladistic and phenetic information on which to base their classifications. The proportions of cladistic and phenetic information used may vary significantly depending, for example, on the extent of the fossil record; in other words, in contrast to the cladistic and phenetic methods, the phyletic system does not follow a set of carefully established rules.

An implicit point of natural classifications, regardless of how they are derived, is that they are based on genealogy (i.e., relationship by descent). In other words, they show evolutionary relationships among taxa. Thus, the key step in any natural classification is the determination of homology (whether features common to groups were derived from the same feature in the most recent common ancestor of the groups). Similar, but non-homologous,

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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