VII.1.4 Limited gene flow can also occur between different species.

Although it is the isolation of gene pools that, in principle, defines the species, a number of species experience at least limited gene flow from alien gene pools by way of cross-breeding between species (Veen et al. 2001)or, in single-cell organisms in particular, by means of horizontal gene transfer. Interspecific hybrids and their offspring are usually less biologically fit than other members of their population and, as a result, tend to be gradually eliminated from the population by natural selection. Nevertheless, some genes from the alien species will remain in the gene pool and may, in the future, become an important source of genetic polymorphism and hence also of microevolutionary plasticity of the corresponding species.  

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.