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Organisms that are similar in characteristic, which may or may not exchange genetic material, and which as a group differ in multiple ways from other such groupings.
In general, the amount of genetic diversity found within prokaryotic species is much greater than that found within eukaryotic species. This difference is a consequence of the requirement for sexual exchange of genetic material in obligately sexual eukaryotes.
The result is much greater conservation of sequence within eukaryotic species, not so much as a consequence of extensive genetic recombination but instead because there is only so much diversity between homologues sourced from each parent that meiosis can successfully or consistently handle. That is, natural selection is probably a stronger force for genetic conservatism in obligately sexual eukaryotic species than for the only occasionally gene exchanging prokaryotic species.
An alternative view of why prokaryotic species are so diverse is because, as single-celled organisms, the routes towards horizontal gene transfer can be much more diverse. The result is a potential for movement of genes over much greater phylogenetic distances than is seen with the germ cells of, especially, animals and plants.
On the other hand, with lineages that truly do not participate in gene exchange there is a potential for substantial divergence through mutation alone.
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