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Re: Dinosaurs to birds



In a message dated 1/24/99 1:44:50 AM EST, jbois@umd5.umd.edu writes:

<< On Sat, 23 Jan 1999 Dinogeorge@aol.com wrote:
 
 > No, speciation occurs randomly. 
 
 _Mutations_ occur randomly.  Individuals are _selected_ to reproduce.  And
 often, if not usually, nature selects not at random but on the basis of
 rules (I mean this in the same sense that mathematical rules exist).  One
 such rule might be: grouping together in flocks makes survival more likely
 for the individual.  There are probably an unlimited number of such
 "rules" to be found--in time.  Are you arguing that no such rules exist? >>

I'm not talking about mutation, which is also random, but speciation.
(Although the randomness of mutations is surely also a factor in the
randomness of speciation.) For example, in the isolation model of speciation,
a subpopulation becomes isolated from the main population's gene pool and
after time evolves into a separate species. The isolating event is essentially
random. You can't even be sure that an isolating event will produce a new
species; sometimes the isolated population persists unchanged, or changes very
little. Sometimes it simply becomes extinct.

New species appear randomly, or what is effectively the same thing,
unpredictably, because the events that produce new species are contingent on a
huge number of variables. Certainly the organisms in a daughter species will
resemble the parent organisms, but >how< they will differ, or >whether< a new
species will appear at all, is unknowable in advance.

You may imagine you have identified a trend or a rule or a tendency of
evolution, but such rules are much like loading dice--one may increase the
odds that certain combinations will be thrown, but for any particular throw
one still can't be sure of the outcome. This is the nature of natural
selection. This is also, for example, the essence of something like Cope's
rule for tetrapods. There seem to be more selection pressures to evolve larger
size than smaller size, but every so often things work out such that a smaller
organism evolves from a larger one; and most often there is no significant
size change one way or another. If there's any true or obvious "trend" or
"rule" in evolution, it must simply be evolutionary stasis from generation to
generation.

Lots of scenarios have been advanced over the years to account for the
differential survival of one species (particularly _Homo sapiens_!) over
another, but there is really no way to confirm whether any of those scenarios
holds water. Some sound quite plausible, others do not. We need to observe
populations of organisms in detail for periods of time measured in hundreds of
thousands of years to see whether evolution really does take place according
to the scenarios we have put forth, and whether new species do indeed appear.