Rassenkreis and speciation, Biology

According to the current concept of speciation, a widely distributed species should break up into partially isolated rule species. The different subspecies become differentiated further due to the action of selection and other factors. This would result in a circle or group of races or Rassenkreis. And the terminal members of such a circle will be sufficiently different from others, so that a sterility barrier sets in. We shall try to explain the concept with an example of an amphibian species. In the west coast of USA there are two high mountain ranges, the Coastal Range and the Sierra Nevada. At their northern ends, near the Canadian border they are united. Proceeding south, they are separatcd by a hot and arid desert, Further south, close to the Mexican border, these high mountains meet again. There is an amphibian species Ensatina eschscholtzii at the place marked as A in Fig, It is supposed that this original species split into two populations, B and G, which then moved down the two respective mountain ranges. On one range the populations can be seen to change gradually from B to C to D to E and on to F.

On the other mountain range the population G, like B derived from A, gradualljl moved south changing into H, I, J and K. While these changes were taking place the two series were isolated from each other by the desert, and isolating mechanisms were gradually developing. Since this amphibian Ensatina has poor locomotor powers, it must have taken hundreds of thousands of years for them to proceed from the Canadian border to the Mexican border. It stands to reason that populations B and G will have the least amount of isolating mechanisms. The isolating mechanisms will be more between D and I. They will be greater between E and J. Remember, both the series, BCDE and GHIJ, are derived from a single species, Ensatina eschscholtzii, designated in Fig. as A. But when they reach the Mexican border, the two populations F and K are able to overlap, as seen in the cross-hatched area, without mating. They have passed the test of sympatry. F and K are now two distinct species.

Similar ring species have been shown to occur in sea-gulls forming a circumpolar ring around the world.

Posted Date: 4/5/2013 5:34:56 AM | Location : United States

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