While the typological definition of a species is currently used most in practice, the definition of a biological species is used most often in the theoretical area; this is also sometimes denoted as the isolation definition of a species. According to it, species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Similar to the previous case, this method of distinguishing species can be employed only for sexually reproducing organisms. Although it does not follow directly from the wording of the definition, it is generally tacitly agreed that crossing can occur to a certain degree between individuals that belong to two different species. However, under natural conditions, the frequency of this crossing or the fertility of the crosses is so low that the consequent gene flow between the gene pools of the two species is weak and selection or even genetic drift are capable of maintaining the integrity and mutual differences between the genetic compositions of the gene pools of the two species. The definition of a biological species can be quite readily understood intuitively. On the other hand, it is very difficult to use it in practice to define the boundaries between actual species. This need not be a drawback of the actual definition of a biological species, but can be a simple consequence of the fact that sharp and unambiguously definable boundaries between species do not exist (cf., for example the nominalist definition of species undergoing evolution over time). Because populations and species develop over time, it is apparent that the boundaries between the individual species are also not absolute and invariable and are, at the very least, moving in time.
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