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 speciation and species, we can proceed to answering the question of when speciation takes place. Speciation is more probable as a result of geographical isolation, which gives rise to reproductive isolation as a way to reinforce a divergent event that has already occurred. Since it has been established that the simplest form of speciation is lineage splitting, we must thus, understand how the split of one common ancestor can result in the formation of two separate and distinct species. Literature affirms that divergence of species is the outcome of accumulated differences between groups, which leads to the formation of new species (Salzburger et al. 2002). Usually this is a result of diffusion of the same species to different and isolated environments.
The isolation of environment plays the role of blocking the gene flow among the distinct population, allowing differentiated fixation of characteristics through natural selection to take place (Salzburger et al. 2002). Provided that most geographic variation is the result of local adaptation to local environments, genetic divergence alone will not lead to a species divergence as long as genes can flow between members of the species. 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...