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What exactly is a species?

Terminal Terra shows when species come and go. But defining what exactly a species is, remains controversial in biology. This is easy to understand since species continuously evolve and diversify into distinct subspecies and ultimately new species. Most widely used is the "biological species concept", which defines species in terms of interbreeding. The influential evolutionary biologist Ernst Mayr described species as "groups of interbreeding natural populations that are reproductively isolated from other such groups". In this view, the degree of morphological difference is not an appropriate criterion. Other species concepts include phenotypic, ecological, molecular or physiological criteria. Increasingly important become evolutionary considerations, reflecting the phylogenetic history of a species as reconstructed from DNA sequence comparisons; species should have unique positions at the terminal branches in the Tree of Life.

But the biological species concept has several limitations. It is often difficult to determine whether separate populations have the intrinsic ability to interbreed under natural conditions. Also for micro-organisms that often reproduce asexually this definition is difficult to apply. In those cases genetic differences can be used as a measure for species demarcation. A complication then is that among bacteria horizontal gene transfer is an abundant feature, i.e., the transfer of pieces of DNA between very dissimilar groups.

For fossils the biological species concept, being dependant on interbreeding potential, is obviously not applicable. Here morphological similarity is the only means for demarcating species. But since differences are often gradual, the actual distinction of species is a difficult and intrinsically subjective task. In palaeontology one therefore often is already content when a fossil can be classified at the genus or family level.

As a consequence of differences in species definition and changing insights in genetic or behavioural properties there are regular changes in species nomenclature. For example, until recently orang-utans were considered as a single species, Pongo pygmaeus, but mainly on basis of DNA sequence differences two species are now recognized, Pongo pygmaeus from Borneo and Pongo abelii from Sumatra [1]. On the other hand, the extinct quagga was once considered a separate species of zebra (Equus quagga), until DNA sequences from museum skins showed the quagga to rather be a subspecies of the plains zebra (Equus burchellii) [2].

In Terminal Terra we adhere to the species names as used in the literature or databases from which the information for a particular species is obtained, as referred to in the Terminal Terra database.

(See References for the numbered references above)