Originally posted by Halitose
If the definition of "kind" is vague, could you offer a difinition of "specie"?
The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist:
* A morphological species is a group of organisms that have a distinctive form: for example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. Although much criticised, the concept of morphological species remains the single most widely used species concept in everyday life, and still retains an important place within the biological sciences, particularly in the case of plants.
* The biological species or isolation species concept identifies a species as a set of actually or potentially interbreeding organisms. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It distinguishes between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations. For example, it is possible to cross a horse with a donkey and produce offspring, however they remain separate species—in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. The key to defining a biological species is that there is no significant cross-flow of genetic material between the two populations.
* A mate-recognition species is defined as a group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually.
* A phylogenetic or evolutionary or Darwinian species is a group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species.
* See also microspecies under apomixis, for species that reproduce without meiosis or mitosis so that each generation is genetically identical to the previous generation.
In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. Given the capricious nature of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition
Macroevolution
16 Jul '05 15:55
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