Life began with a single species, while diversity today is estimated at between 2 and 20 million species. This increase in diversity needs to be quantified and understood. The first of these aims is achieved by classifying organisms, the second by the study of evolution.
The most common unit of classification, or taxonomy, is the species. This is defined as a potentially interbreeding group, whose individuals cannot breed and produce fertile offspring with other organisms outside the group. A species may be considered as the sum of all the genes that could be shared by its members. In practice, species are usually defined by their physical similarity to one another. Modern techniques of DNA and enzyme extraction have allowed some classification using biochemical characters.
Species are grouped together into genera, genera into orders, and so on, until all life is linked within a series of nested sets. The aim of taxonomy is usually to produce a "natural" classification, where these nested sets reflect the history of each species through time, and to display the evolutionary relationships of each species. Sometimes the purpose of classification is to make identification simple, or to produce a working description of diversity in advance of an understanding of the evolution of a group. In this case the classification is useful but artificial (Fig. 2.1). Traditional methods of classification are supplemented by cladistic techniques that produce diagrams
showing the simplest possible relationships between organisms given the characters they share.
Natural classifications reflect the ways in which species have changed with time, and the ways in which larger groups, such as orders, have arisen. Understanding how these events occur is the realm of evolutionary studies, and depends on two vital elements. One is an appreciation of change being driven by natural selection, as suggested by Darwin. The other is an understanding of the genetic basis for such modifications. In addition to investigating the mechanisms of change, it is important to study rates of change. As species generally survive for between 4 and 10 million years, this is something best done by using the fossil record.
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