|MadSci Network: General Biology|
Hi there The standard definition of a species is one that has evolved to the extent that it can't reproduce successfully with other types of organism. For example, if a population of mice were suddenly separated by a mountain range (unlikely I know), they would all be the same species until they had evolved to become sufficiently different that they could no longer interbreed - then they would be two different species. This is called speciation and happens due to changes in body structure, behaviour (in animals) or other factors such as pollen structure and flowering time (in plants). However, some closely related species do seem to be able to interbreed such as lions and tigers (making 'tigons' and 'ligers' depending on which is the mother and which is the father) and more famously horses and donkeys which produce mules. However, in all these cases, the offspring are sterile and therefore are unable to reproduce further. So, the rule remains - separate species can never successfully reproduce and that is how species are defined. You may also read or hear about 'sub-species' - these are populations of a species which are becoming different (maybe they are speciating) but are still able to reproduce, although they may not do so in nature because they have been separated and no longer meet. This is very true of plants as many have complicated mixtures of subspecies and species which can be difficult to tell apart and which can interbreed to form 'hybrids'. I hope that answers your question - and adds a bit more interesting information too - of course there is much more, especially regarding plants! Bye for now Dr Dave Hubble, Hampshire, UK
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