Stephen E. Jones wrote:
>In fact, small populations are thought now to be how evolutionary change
>happens, because favourable genes get diluted in large populations:
Genes don't get diluted; that is a concept from pre-Mendelian blending
theories of inheritance. Genes are particulate and inviolate, and they
can spread quickly through a population.
[Gould:]
> "All major theories of speciation maintain that splitting takes place
> rapidly in very small populations. The theory of geographic, or
> allopatric, speciation is preferred by most evolutionists for most
> situations (allopatric means "in another place"). A new species can
> arise when a small segment of the ancestral population is isolated at
> the periphery of the ancestral range.
This implies that founder populations are marginalized groups struggling
to adapt to sub-optimal conditions. I think it more likely that speciation
occurs when a population expands rapidly into a new niche that is highly
favorable but which is different from the old environment.
> Large, stable central populations exert a strong homogenizing
> influence. New and favorable mutations are diluted by the sheer bulk
> of the population through which they must spread.
This is nonsense. Favorable mutations spread undiluted; unfavorable
mutations don't spread.
> They may build slowly in
> frequency, but changing environments usually cancel their selective
> value long before they reach fixation.
Changing environments? I thought he was talking about "large, stable
central populations".
> Thus, phyletic transformation
> in large populations should be very rare as the fossil record
> proclaims. But small, peripherally isolated groups are cut off from
> their parental stock. They live as tiny populations in geographic
> corners of the ancestral range. Selective pressures are usually
> intense because peripheries mark the edge of ecological tolerance
> for ancestral forms. Favorable variations spread quickly. Small,
> peripheral isolates are a laboratory of evolutionary change." (Gould
> S.J., "The Episodic Nature of Evolutionary Change," in "The
> Panda's Thumb," 1990, pp.152-153)
Small populations are common among so-called 'living fossils', species
that have undergone relatively little evolutionary change. I guess small
populations are like ID, they can do anything.
>But the take-home point is that when populations get smaller, random
>genetic drift becomes the dominant factor. Natural selection is not as
>strong a force as we intuitively think.
But drift is not directional. Natural selection is. Natural selection picks
out what works, whether drift is going on or not. The implication is wrong,
that small populations should naturally get stranger and stranger, as
random mutations outpace natural selection.
>When you think of it. Even with the strong selection of artificial selection
>(i.e. selective breeding), it is hard to obtain the desired results. That is
>why they are spending so much effort on animal cloning. It is only by cloning
a
>known genotype that they can guarantee the phonotype will be exactly
>what they want.
Breeders like to produce a predictable, uniform product, that's for sure.
But the relevance of this fact is obscure. If the 'desired result' is something
different from what nature has been selecting for over millions of years,
it is not surprising that it is hard to obtain.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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