Date: Fri, 26 Apr 1996 19:35:19 -0500
From: rahul-AT-peaches.ph.utexas.edu (Rahul Mahajan)
Subject: Re: Punctuated equilibria -Reply



>RM:  What seems new to me is [snip] that our picture that gradual
>change within species occurs over millions of years (in addition to
>whatever more rapid change/speciation may occur) is either wrong or
>much less universal than we supposed. In effect, they posit a
>stability of species to small deformations and present a picture of
>long periods of almost complete genetic stasis (in the adaptive
>sense, of course; nonadaptive genetic drift continues at a
>near-constant rate).
>
>Lisa:  I'm not recognizing this picture.  You seem to be
>distinguishing between speciation as species-splitting, and
>speciation as changes in a single line.  If so, I think I disagree
>with that view.  Who posits such stability?  I'd only expect it if
>the sum of [possibly counter-balancing] selective forces is about
>zero.  Also drift is not expected to be continuous or constant.  It's
>effects are much greater in very small populations, and negligible in
>very large ones, no?

Last I'd heard, the evidence was that the allopatric mode of speciation is
at least the vastly dominant mode, if not perhaps the only one. I don't
know if this has changed recently, though. Certainly the punctuated
equilibria model is more compatible with allopatric than with sympatric
speciation. However, that's not really what I'm addressing here.

The Gould-Eldredge model, at least according to Gould's explanation,
definitely does seem to posit the existence of species for millions of
years with little or no discernible change, and in fact, suggests that at
least the vast majority of evolutionary change occurs in identifiable
events (presumably of speciation) rather than gradually over time. Of
course, we all know of episodes of simple change within a single line, like
the white moths turning black in England. So, really, I would like to find
out from someone with a much wider source of examples whether the picture
of, essentially, creation of species which remain as largely static objects
until the next sudden episode applies to most cases. Perhaps G and E are
just wrong about the facts.

The rate of adaptively neutral genetic change is generally postulated to be
near constant, thus all the recent molecular biology dating of events of
human evolution, like when the orangutans broke off, when the chimps broke
off, etc. I think what you're saying is that separated populations will
tend to drift away from each other, but I can't think of any reason why a
smaller population would drift away from its starting point faster than a
larger one.

>RM:  What would be especially interesting would be to look at the
>persistence and prevalence of stasis broken down by complexity of
>organism. We already know that the more complex an organism is, the
>less potential for radical change it has (viz. the evidence of the
>Burgess Shale and other post-Cambrian Explosion sites).
>
>LR:  Tell me more.  I'm a little familiar with the Burgess, but not
>aware of this implication.  How does complexity break down stasis?
>What causes stasis?

I was simply referring to the evidence that the range of variation of these
simple life forms was so much greater than that of multicellular life
today. The number of phyla which don't exist any more, the number of extinc
classes of arthropods, etc. All of this vast variation was produced in one
spasm (10 million years long?), whereas the entire evolution of the 600
million years since then has produced not one single new phylum. Clearly
it's impossible that such a highly complex creature as, say, a mammal,
could ever be the ancestor of a line that results in a nonvertebrate
descendant. Similarly, everything descended from bees will be not only
arthropods, but insects. This is not about rate of change but about the
limits of variation. When I said "broken down", I didn't mean complexity
breaks down stasis, but rather that I would like to see a table, organized
by complexity of organism, of the persistence and prevalence of stasis. For
example, are new mammalian species more likely to spend almost their entire
existence without change than, say, new annelid or arthropod species?

Rahul




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