Before tackling the issue of the level at which speciation occurs as the result of divergence, we must first establish the definition of speciation. In the scientific community, this terminology is associated with the formation of new and distinctive species, from which a single evolutionary line splits into two or more genetically independent ones (Cook 1906). In essence, from one common ancestor derives two descendent species. Taking into consideration the controversial nature of what constitute a species, this is will refer to species as related individuals that resemble one another; they are able to breed with themselves, but not with members of another species (Rosenzweig 1995).
Now that we have established the meaning of
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As a result, reproductive isolation takes place, as a secondary process, in order to reinforce a divergent event and stop the flow of genes (Rosenzweigh 1995). Although there are several models of speciation that enable barriers to gene exchange to evolve, geographic (allopatric) isolation provides the most effective barrier (Hoskin, Higgie, McDonald and Moritz 2005, 1353). We thus consider the geographic speciation model, which states that the separation of two (or more) groups of organisms by geographical features such as a river, mountain range, ocean, or desert gives rise to genotypic and/or phenotypic divergence.
The effect of geographic isolation is when two populations are subjected to different amounts of pressures, causing the condition of the two areas to be different (Hoskin, Higgie, McDonald and Moritz 2005, 1353). Gradually, different alleles will be selected for and genetic differences will accumulate between the populations. In time, enough genetic differences will occur so that the two populations can no longer interbreed, thus by definition they would have become different species. Reproductive isolation has occurred once the members of two populations can no longer interbreed and produce fertile offspring. At this point, the populations have reached the level at which they no longer share the same gene pool, thus, they respond to natural selection as a