Gene flow and range of a species
Different types of organisms are present only in a specific limited area. This area is called the range of the species. In some cases the range is delimited by natural barriers, continental edges, mountain ranges or rivers. Quite often, however, we are not able to discern any natural barriers of this kind – the natural conditions in the given territory change more or less gradually. In species with discontinuous range, conditions in the individual areas of that range are often very different but local populations are able to adapt to these differences through microevolution. It follows from what has been stated above that the geographic delimitation of range is probably the result not of some natural abiotic barriers in the environment but rather of a biological phenomenon. One of the possible reasons was already suggested in the middle of the twentieth century by Haldane (Haldane 1956). According to his hypothesis, spatial delimitation of ranges is the consequence of gene flow. As the natural, for example climatic, conditions gradually change within the range, local populations of the species adapt to these local conditions. The effect of natural selection, which optimizes the composition of the gene pool with respect to local conditions, is, however, simultaneously countered by gene flow, introducing alleles from other populations’ gene pools via migrants. These alleles tip the local population’s gene pool composition off its optimal balance. Considering that populations are more numerous towards the centre of the range and less numerous towards its edges, the impact of migrants on the composition of local populations’ gene pools grows with an increase in the distance from the centre. At a certain distance from the centre, local populations are so sparse that even relatively weak gene flow can prevent their microevolutionary adaptation to local conditions. This is the distance at which the natural limit of the species’ range will be found (Garciaramos & Kirkpatrick 1997).
This model also serves to explain Rapoport’s rule (Case & Taper 2000). According to this empirically derived, biogeographic rule, ranges of species living at low latitudes, i.e. mainly in the tropics, are usually smaller than the ranges of similar species living at higher latitudes. For species living near the equator, as a rule, environmental productivity decreases as we move away from the centre of its range, i.e. from the equator and, consequently, the density of the local populations also decreases. This means that the impact of gene flow on the composition of a local population’s gene pool increases rapidly with increasing distance from the equator, preventing microevolutionary adaptation of these populations to local conditions even at a relatively short distance from the centre of the range. To the contrary, species with ranges centered at high latitudes reach a more productive environment as they penetrate towards the equator and can therefore create more numerous populations in these areas. Consequently, the flow of genes from the centre of the range has a smaller impact on the composition of the local populations’ gene pools and does not hinder their microevolutionary adaptation to local conditions. As a result of their higher microevolutionary plasticity, species at higher latitudes can, on an average, have larger ranges. Naturally, there are other explanations for Rapoport’s rule, such as the selection for broader environmental tolerance in species living in the less stable and rougher conditions of the higher latitudes (Stevens 1989).