Re: Alex's gibbon-like CA
Phil Nicholls (pnich@globalone.net)
Fri, 10 Nov 1995 20:26:46 GMT
Paul Crowley <Paul@crowleyp.demon.co.uk> graced us with the following
words:
>In article <47l816$dfo@geraldo.cc.utexas.edu> aduncan@mail.utexas.edu "Alex Duncan" writes:
>> Paul@crowleyp.demon.co.uk writes:
>> >I'm sure this is the "evolution forces" error. A species faced with
>> >a shrinking habitat does not change its diet. It declines or dies.
>>
>> Well, when do taxa change their diet then? (And BTW, I didn't suggest a
>> change in diet, but a change in the proportions of the dietary items
>> eaten. After all, human and chimp diets aren't that different, especially
>> in areas where ranges overlap.)
>>. . . <various snips>
>>
>> As far as your thing about "shrinking habitat" (or changing habitat) and
>> extinction goes -- as far as I can see you've ruled out most of the
>> important evolutionary changes in the history of the earth.
>>
>> I think you make way too much of your cute little "evolution forces" idea.
>It's basic to any serious thinking on evolution. The common failure
>to grasp it is at the root of most bad ideas - like all the standard
>ideas on the origin of bipedalism.
I keep running this phrase over and over in my mind and each time I am
stunned by the sheer chutzpah of this statement.
>A new species arises from the cross-over of a tiny group (relative
>to the whole of the originating species). That tiny group is
>seizing an opportunity presented by some fortuitous circumstances.
>These circumstances *could* arise from some environmental pressure
>operating on the whole population, but that is *most* unlikely.
A species, Paul, is composed of several populations and within any
species there is variation. A new species arises when one population
becomes reproductively isolated from other populations. Gene flow is
disrupted. Variation that appears in the newly isolated population
will not be shared with the rest of the species. Natural selection
and genetic drift cause the isolated population to diverge genetically
until interfertility is lost.
Environmental changes such as the shrinking of forest habitat are
critical periods. Some populations are better able to deal with the
shift in the environment than others. These are periods in which
speciation is more likely to occur. Mammals have a particular edge in
that they have something called neocortex which gives them a degree of
behavioral plasticity which means they can cope with environmental
changes by changing behavior.
>Consider the numbers: a successful species will have a population of
>millions and last millions of years. An environmental change (like
>the decline of forests) will, over the time involved, cause the death
>of billions (of gibbon-like CA's). But if that species was to give
>rise to a new one, at the most a few hundred individuals are involved.
>For example, a group of g-like CA's finds some fortuitous circumstance
>that makes it decide "walkabout" is great idea. The death of
>billions is "forced". The cross-over of the tiny group is NOT.
There is no hard and fast rule as to the size of the population that
can found a new species. All you really need is isolation. A
successful species may consist of a million organisms but those
organisms will not constitute a single population. Rather, they will
form many populations which will be scattered over the species range.
Habitate variation over a species range may be considerable and that
means that natural selection and genetic drift will likely result in
considerable variation. For such a species, environmental change is
likely to result in several new species being formed. Some will
quickly become extinct while others will not.
>We can only guess at the set of fortuitous circumstances that enabled
>a very small group of protohominids to adopt true bipedalism. They
>are most unlikely to have arisen from forces operating on the
>population as a whole. This is where your thinking (and general PA
>thought) is so bad. Does it come from the weakness of the human
>brain at conceiving large numbers? The odds are billions to hundreds
>- or ten million to one.
Part of the problem here comes from your use of the words "species"
and "population" interchangably. Another part of the problem, one
which you share with other aquatic apers is that you confuse the
concept of habitat with that of niche. Finally, like many aquatic
apers you fail to grasp a fundemental truth of evolution.
The major constraints on the direction of evolutionary change are
historical, not environmental.
>Why is this fundamental evolutionary concept so hard to grasp?
What you have presented above is not a fundamental evolutionary
concept.
>Paul.
Phil Nicholls pnich@globalone.net
"To ask a question you must first know most of the answer"
-Robert Sheckley
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