Hey CosmicChimp,
How can the selective pressure be LOW and not HIGH? I assume that the environment is stable and the niches full. I would think that mutations would be fiercely selected against.
What we have are two different parts of the equation: the pressure to survive and the pressure to breed.
In stable ecosystems there is less pressure to survive - the average organism is basically fit to live and breed in that ecosystem - so the main pressure is to breed. I agree with you that selection would tend to favor the existing average organism and cause selection for stasis, more of the same, while eliminating extremes. Think of a bell curve and variation about a mean, with small numbers the more extreme they are. There is little chance that the population as a whole will go extinct however, so this is regarded as low selection pressure.
In a changed ecosystem (or a new one that has been invaded) there is pressure to survive, with the population numbers dropping due to failures to survive depending on the severity of the change. In this instance survival is critical and breeding is less selective, so this will select for more variation among the survivors. In this case you will have selection for punctuational changes to adapt to the new ecosystem. Think of that bell curve again and shift the "ecosystem adapted organisms" point from the center average off to one side, towards those extreme variation individuals in low numbers. In this case there is a chance that the whole population will go extinct if the change is severe enough, hence this is generally regarded as high selection pressure.
Of course those extreme variations at the edges of the bell curves for all the hereditary traits are where the new mutations fit in to the program to introduce new features, so they are not likely to be seen in populations in stasis compared to populations undergoing population survival pressures.
Another way for these "extreme variation" features to become mainstream is for the population to diverge into a second, similar but different ecosystem where those features are beneficial. In this case the earlier diversification of the original population enables members of the following generations to take the opportunity to live in a different ecosystem. You can see this in ring species such as the
asian greenish warbler where small genetic changes from subpopulation to subpopulation result in two subpopulations that do not interbreed even though they occupy the same ecosystem: they no longer recognize the other population as potential mates due to genetic difference for
mating song
coloration
Both these populations evolved from a common ancestral population, as evidenced by their interbreeding with the other neighboring varieties, each a little different from the next, and thus their being different means that one or the other has "new genetic material" ... and no one population contains all the variations seen in each of the subpopulations.
Enjoy.
Edited by RAZD, : added topical comment
Edited by RAZD, : added last
Edited by RAZD, : duplicate word word
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