Text 990, 72 rader
Skriven 2004-12-12 06:22:00 av Wirt Atmar (1:278/230)
Ärende: Re: The "fuel" of evoluti
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Someone wrote (I lost track in all of the quoting):

>Why is this? Assuming populaton size to be constant, a single
>parthenogentic type will, after predation etc have taken their toll,
>produce 2 p-type offspring, whereas 2 sexual types, M&F, are required
>to generate 2 offspring s-types. As Catherine Woodgold states, this
>will lead to an exponential increase of the population size of
>p-types, with corresponding decline and ultimately elimination of the
>s-type. 

This statement can be found commonly in the biological literature as well --
and demonstrates more than anything else the relatively low utility of
theoretical musings in biology (sometimes called "armchair biology"). You can
tell in an instant it's not true. There are relatively few fully
parthenogenetic species in existence. Clearly, they're non-competitive on the
long-term.

[btw, I consider myself primarily a theoretical biologist. And I unfortunately
hardly ever get out of my chair].

The vertebrate parthenoforms offer up the easiest tests of the ideas in the
quoted text. The first parthenogenetic, all-female populations of vertebrates
were discovered in the 1950's, in the lizard genera, Cnemidophorus and Lacerta,
first at White Sands Missile Range, New Mexico, just a very little distance
from where I am, and in central Asia. We now know of a fair number of other
reptilian genera that are also fully parthenogenetic, and more are being
discovered every year or so.

Following their discovery in the 1950's, for the next 25 years, all sorts of
evolutionary advantages were attributed to the evolution of these populations,
but in the end they were found not to have any sort of advantage at all, but
were instead the result of reproductive incompetancies caused by hybridization
events. The lifetimes of the hybridized clonal populations tends to be very
short, perhaps only a few hundred years, and they would not persist if there
were not constant re-hybridization events occurring between congener fully
sexual parental species somewhere in their range.

What then of the various ecological advantages ascribed to parthenogenetic
species? Where I am, we have the great fortune to have two large Cnemidophorus
species, one a parthenoform and the other one of its parental species, living
on top of one another. The animals are very similar in size and behavior,
virtually the ecological twins of one another.

To test the validity of the various advantages that had been attributed to
parthenogenesis, twenty years ago Andy Price staked out large plots and
selectively removed either all of the parthenoforms from one plot or the sexual
species from the other in an attempt to measure the level of competition
between the two (and to see if one population or the other would "take off" in
the absence of its congeneric competitor).

The results were as expected. The presence or absence of the sexual species
made no difference to the parthenogen. Being a hybrid, it was not only
reproductively incompetent, it was ecologically and behaviorally incompetent as
well. Its population levels just bumbled a long at something only slightly
greater than minimal survival.

Andy's dissertation text is available at:

     http://aics-research.com/research/whiptail.pdf

if you care to read it.

Wirt Atmar
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