> I do not believe any consensus exists among
> scientists as to how much genetic divergence
> is needed to name a new species.
It seems like there is consensus that calling something a different species
hinges on reproductive isolation. The isolation may result from actual
reproductive incompatibility (ie, genetic difference) or spatial-temporal
isolation. The latter reason is why different runs of salmon are considered
different species, for the purpose of the endangered species act. When
populations are reproductively isolated for any reason, they will begin to
diverge genetically. Plant breeding has tended to break down barriers in an
attempt to stir the pot to develop new varieties. For example, wild Solanum
species are crossed with potato to move disease resistance genes from the
wild species into the crop. Sometimes the crosses are not very compatible,
and special techniques are needed to permit crossing.
In any case, the meaning of "species" depends on the context. In the
breeding context, only cross-compatibility is relevant.
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