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| Description: Speciation can be sympatric, allopatric, or parapatric. In all three cases reproductive isolation among subpopulations needs to occur; however, the mechanisms that cause this isolation will differ. In both allopatric and parapatric speciation, the evolution of reproductive isolation could occur through drift or natural selection. Since genetic drift is random, fluctuations in allele frequencies in subpopulations will be independent, and thus loss of genetic diversity through drift could lead to reproductive isolation. Alternatively, many geographic barriers may alter the local environment; or many new habitats within a landscape may contain a new microclimate. In such cases, there could be differential selective pressures on the subpopulations, and genetic differentiation could occur. If this influences a trait related to the production of a viable offspring, reproductive isolation and speciation can occur.
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