<P> As important as identifying an isolation gene is knowing its function . The Hmr gene, linked to the X chromosome and implicated in the viability of male hybrids between D. melanogaster and D. simulans, is a gene from the proto - oncogene family myb, that codes for a transcriptional regulator . Two variants of this gene function perfectly well in each separate species, but in the hybrid they do not function correctly, possibly due to the different genetic background of each species . Examination of the allele sequence of the two species shows that change of direction substitutions are more abundant than synonymous substitutions, suggesting that this gene has been subject to intense natural selection . </P> <P> The Dobzhansky - Muller model proposes that reproductive incompatibilities between species are caused by the interaction of the genes of the respective species . It has been demonstrated recently that Lhr has functionally diverged in D. simulans and will interact with Hmr which, in turn, has functionally diverged in D. melanogaster to cause the lethality of the male hybrids . Lhr is located in a heterochromatic region of the genome and its sequence has diverged between these two species in a manner consistent with the mechanisms of positive selection . An important unanswered question is whether the genes detected correspond to old genes that initiated the speciation favoring hybrid non-viability, or are modern genes that have appeared post-speciation by mutation, that are not shared by the different populations and that suppress the effect of the primitive non-viability genes . The OdsH (abbreviation of Odysseus) gene causes partial sterility in the hybrid between Drosophila simulans and a related species, D. mauritiana, which is only encountered on Mauritius, and is of recent origin . This gene shows monophyly in both species and also has been subject to natural selection . It is thought that it is a gene that intervenes in the initial stages of speciation, while other genes that differentiate the two species show polyphyly . Odsh originated by duplication in the genome of Drosophila and has evolved at very high rates in D. mauritania, while its paralogue, unc - 4, is nearly identical between the species of the group melanogaster . Seemingly, all these cases illustrate the manner in which speciation mechanisms originated in nature, therefore they are collectively known as "speciation genes", or possibly, gene sequences with a normal function within the populations of a species that diverge rapidly in response to positive selection thereby forming reproductive isolation barriers with other species . In general, all these genes have functions in the transcriptional regulation of other genes . </P> <P> The Nup96 gene is another example of the evolution of the genes implicated in post-copulatory isolation . It regulates the production of one of the approximately 30 proteins required to form a nuclear pore . In each of the simulans groups of Drosophila the protein from this gene interacts with the protein from another, as yet undiscovered, gene on the X chromosome in order to form a functioning pore . However, in a hybrid the pore that is formed is defective and causes sterility . The differences in the sequences of Nup96 have been subject to adaptive selection, similar to the other examples of speciation genes described above . </P> <P> Post-copulatory isolation can also arise between chromosomally differentiated populations due to chromosomal translocations and inversions . If, for example, a reciprocal translocation is fixed in a population, the hybrid produced between this population and one that does not carry the translocation will not have a complete meiosis . This will result in the production of unequal gametes containing unequal numbers of chromosomes with a reduced fertility . In certain cases, complete translocations exist that involve more than two chromosomes, so that the meiosis of the hybrids is irregular and their fertility is zero or nearly zero . Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations . An example of chromosomal changes causing sterility in hybrids comes from the study of Drosophila nasuta and D. albomicans which are twin species from the Indo - Pacific region . There is no sexual isolation between them and the F1 hybrid is fertile . However, the F2 hybrids are relatively infertile and leave few descendants which have a skewed ratio of the sexes . The reason is that the X chromosome of albomicans is translocated and linked to an autosome which causes abnormal meiosis in hybrids . Robertsonian translocations are variations in the numbers of chromosomes that arise from either: the fusion of two acrocentric chromosomes into a single chromosome with two arms, causing a reduction in the haploid number, or conversely; or the fission of one chromosome into two acrocentric chromosomes, in this case increasing the haploid number . The hybrids of two populations with differing numbers of chromosomes can experience a certain loss of fertility, and therefore a poor adaptation, because of irregular meiosis . </P>

Reproductive isolation caused by a river is an example of