Slide 9: Principles: Range edges
Populations do not just cease to exist at the edge of their geographic distributions, but rather taper off gradually. The edges of geographic ranges are thus defined by a change in the local population dynamics, where net gains in population are reduced to levels lower than the net losses.
These population level changes are brought about by:
Changes in abiotic factors (physical barriers, climate factors, absence of essential resources) and biotic factors (impact of competitors, predators or parasites)
Genetic mechanisms that prevent species from becoming more widespread.
Abiotic/biotic factors are only limiting because a species has not evolved the morphological / physiological / ecological means to overcome them. That is, most species have the potential to invade adjacent areas, assuming minor adaptation on the part of the species... it is this genetic drift at the edges of populations that causes evolution, since large-scale transformation within optimal conditions is unlikely.