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Allopatric speciation vs Sympatric Speciation!
Allopatric speciation occurs when new species arise as a result of geographic isolation from the ancestral species. It involves an extrinsic barrier to gene exchange and can occur by a number of means including subdivision or peripheral division. The key idea in this mode of speciation is that the populations are isolated from one another.
By subdivision: The ancestral population is divided into 2 sub populations, each of which forms a new species. The ancestral population literally divides so that the species in each are no longer able to breed with one another. This leads to reproductive isolation between the 2 populations. By peripheral division: A new species evolves from a small population isolated at the edge of the ancestral species. This occurs over very small spatial and temporal scales. Basically a small number of individuals colonize a unique area on the edge of the parent population. These isolates breed and evolve rapidly and independently of the parent population.
In both cases, secondary contact with the parental population without breeding = speciation.
Sympatric speciation is speciation that occurs within a population. The 2 populations are not geographically isolated; they inhabit the same location. This kind of speciation involves the evolution of intrinsic barriers to gene flow. Essentially a group of individuals adopt a new niche within the range of the parental species. These individuals mate within this new niche and adapt to their niche and you get evolution of reproductive isolation without geographic isolation. There are a few conditions for this kind of speciation and it is somewhat controversial because there are few convincing examples however it must be pretty rapid and may underpin many adaptive radiations.
See image below for easier understanding.
Post by Rhee