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.