Re: Survival of the Fittest - Why So Many Varieties?

On Tue, 20 Jun 2006 20:07:07 -0600, dkomo <dkomo871@xxxxxxxxxxx>
wrote:

r norman wrote:

On Mon, 19 Jun 2006 23:19:53 GMT, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:


Ex_OWM wrote:


If 'survival of the fittest' is a major driving force in evolution, why
does it not lead to a reduction in the number of varieties within a species?
For example, I can understand differences between ethnic groups based on
geography, but shouldn't humans in a particular geographic region be more or
less the same size with the same physical attributes?

Several reasons for this.

1. Much variation is neutral, and thus invisible to selection.
2. There is constant gene flow between populations living in different
environments.
3. Some selection actually operates to increase variation;
frequency-dependent selection gives an advantage to rare alleles in
proportion to their rarity.
4. Likewise, different genetic backgrounds (i.e. alleles at other genes
in the same genome) give different selective values to different alleles.
5. There may sometimes be reduced gene flow between two populations
living in the same environment, keeping them a bit different.

That's what I think of immediately. There are plenty more, I'm sure.


On a more humble level, why can I pick from so many varieties of rose for my
garden (ignoring man made hybrids) ?

I think all the varieties are manmade, whether hybrids or just selected.
Different strokes for different folks.


John's answer is a rather thorough review of the general subject in
biology technically referred to as "balanced polymorphism", a very
Googlable term. All it means, of course, is "different varieties
coexisting", except it doesn't include John's category #1. Other
responses include the notion that random or stochastic factors are
involved but, if these have any selective value (that is, not falling
into category #1) then by your reasoning all except the "most fit"
should be eliminated.

John's categories can be recast into three major examples commonly
used in intro biology. First, as many other responses have indicated,
there is enormous variation in the environment so that features that
are "most fit" in one may not be in another. Whether the environment
varies as you move from one place to another or with time, from season
to season and year to year, a population with many varieties will do
better than one that is highly specialized. Of course my explanation
is heavy with "group selection" ideas, but I believe the notion can be
recast in other forms to eliminate that, if it is a problem for you.

The classic example for genetic variability at one gene locus is that
of hybrid vigor: heterozygotes often have higher fitness than either
homozygote alone. That explains why two alternative alleles persist
in a population. The class case is sickle cell anemia where one
homozygote is very strongly selected against so you would expect the
allele to disappear. Since the heterozygote is favored in areas with
high malaria incidence, it persists. Put that together with lots of
gene interaction (John's category #4) and you have a lot of genetic
variability.

The third class of examples used in intro biology classes is John's
category #3, frequency dependent selection. Put most simply (and
therefore not too accurately), if you are "part of the crowd" then you
are all competing for exactly the same resources. If you were
different enough to use the resources in the environment in a slightly
different way, then you have opportunities the others don't are have
an advantage. However, if the population changed so that everyone
else did what you do, then that advantage is lost and it switches to
another variety which is still in the minority. You end up with a
very diverse population that can make full use of the range of
resources available.


Nice summary of the selectionist point of view. You completely left out
Kimura's neutral theory, however. It actually makes a quantitative
prediction of the amount of variation expected in a population. Not
just qualitative hand waving like the selectionists and
panadaptationists engage in, as illustrated in this thread.

"Because of its mathematical simplicity, the neutral theory is able to
make several firm, quantitative predictions about variation: namely,
concerning the extent of variation expected in a population; the rate at
which it accumulates, and the circumstances under which it might be
expected to be maximal...The issue of *rate* of accumulation of genetic
variation is central to the neutral theory. As we will see later, it
leads directly to the notion of a molecular evolutionary clock."

Roger Lewin, _Patterns in Evolution -- the New
Molecular View_, p. 100-101

You are right: I specifically excluded John Harshmans item #1, neutral
variation and said I did so. However neutral changes in the genetic
material tend to be "silent", not terribly visible in the phenotype.
Also I was describing "balanced polymorphism" which in fact is a
stable situation and therefore automatically excludes the unstable
variation of drift (variation in variation, so to speak). And,
finally, I did deliberately want to put in a good word for selection
as producing and maintaining variation to counter all those who
talked only about the stochastic nature of evolution.

Drift and selection are two powerful and important factors in all
evolutionary processes. I don't want to minimize the role of neutral
changes but I don't want to overemphasize it, either. Just because it
may be "simple" and leads to "quantitative predictions" does not make
it the only important factor.


.

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