Wirt,
I will certainly disagree here! Friendly disagreement, of course.
In my continuing bid to become the group's curmudgeon,
Since there are no other candidates, I guess you will be elected! :-)
Natural selection judges only whatever advantages it finds in populations in
the moment. What it truly never does is assess the long-term consequences of
its preferences.
Natural selection is not a judge. It is only differential
representation of genes in subsequent generations, in which more
successful genes become more common from one generation to the next.
Sure, natural selection can be occurring while extinction is taking
place, but the extinction is NOT the result of natural selection.
For example, we could say that natural selection is favoring longer
bills, while habitat loss is eliminating the species. That is, those
birds with longer bills leave relatively more descendents, but, it is a
moot point because it was habitat loss that eliminated the species.
The first is the reversion of a sexual lineage back to parthenogenesis.
Parthenogensis is unaffected by natural selection, because one of the
premises of natural selection is genetic variability among the
population. And, the accident of becoming parthenogenetic also was
NOT the result of natural selection, but rather a point event.
Doing this offers the lineage a number of hypothetical advantages, most
especially
freeing itself from the burden of maintaining males
Males are not a burden. Species do not suffer ecological costs,
individuals do. And, you would not say often find a situation in which
males compete with females, and both lose future reproductive success
due to this competition.
A population free of males is also capable of rapid expansions into recently
vacated territories.
But, not for reasons of natural selection.
It can also survive in extremely adverse situations where a sexual population
would go extinct, simply due to low population numbers and the difficulty in
finding a mate.
Often, species that reproduce both sexually and asexually do the sexual
part exactly WHEN the conditions are adverse, presumably because it is
precisely those conditions that favor genetic (and phenotypic)
variability. Rotifers and aphids, for example.
The second condition is the evolution of high-order polyploidy.
High-order polyploidy is also the result of point changes in a
population, not natural selection. This does not result from a gradual
change, nor a genetic tendency? That is, adults do NOT reproduce a
variable set of offspring, some polyploid and the rest normal, that
after the fact leave a variable number of offspring There is no
EVOLUTION for polyploidy, it happens by accident. After that, the
polyploid often becomes genetically isolated from its ancestors, and
then perhaps natural selection acts on it, and all its polyploid
descendents, based on their phenotypic (and underlying genotypic)
variability.
High-order polyploidy seems on the surface to be an excellent
information-assurance
mechanism, mitigating the informational corruption of any body of information
that is
replicated generation after generation indefinitely.
Accidents are not adaptations.
While we find both types of populations in nature, their rarity is prima
facie evidence that they are not strategies that are successful on the
long-term,
That is a circular argument. Also, there are examples of both that have
probably been around since the cambrian. So, duration is also evidence
of success.
However, the phenomenon is unknown in mammals, and I have long attributed the
evolution of differential imprinting of the chromosomes that pass through
either maternal or paternal gametogenesis to be an evolutionary brake that
prevents a reversion to parthenogenesis in mammals.
You cannot call parthenogenesis a reversion, since ancestral vertebrates
were probably not parthenogenetic. As accidents, there is nothing to
explain. Mammals are just unable to have these kinds of accidents.
Hybridizing lizards MAY become parthenogenic (Cnemidophorus), but they
don't have to.
There are no evolutionary brakes, as that implies planned evolution, and
by your own accounting, evolution does not plan...nor does natural
selection.
Mutations are not planned, they just happen.
Cheers,
Jim
--
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James J. Roper, Ph.D.
Universidade Federal do Paraná
Depto. de Zoologia
Caixa Postal 19020
81531-990 Curitiba, Paraná, Brasil
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E-mail: [EMAIL PROTECTED]
Phone/Fone/Teléfono: 55 41 33611764
celular: 55 41 99870543
e-fax:1-206-202-0173 (in the USA)
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Zoologia na UFPR
http://www.bio.ufpr.br/zoologia/
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http://www.bio.ufpr.br/ecologia/
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http://jjroper.sites.uol.com.br
