Hi, Thilina, et al.,
I don't think there is any single or simple answer to this question.
First of all, as Wirt Atmar's answer point's out, historical factors are
certainly very important. Some evolutionary "decisions" were made in the
Carboniferous, and we are still living with the consequences. The reasons
why temperature-based vs chromosomally based sex determination won out in
certain clades at that time are rooted in the ecological conditions and the
mating systems of one or a few species who were slugging it out *at that
time*, and those particular conditions obviously have changed. Maybe
temperature-dependent determination would be better for some mammals
nowadays too, given their mating systems and ecology, but how are they going
to go back? There's is probably no easy genetic raw material just waiting
to be selected for under conditions when it would be favorable, just as
there are presumably no genetic complexes that could revert mammals to egg
laying or developing gills, even though there might be situations when these
changes would seem desirable. (How inconvenient for porpoises or sea turtles
that they have to come up breathe, but millions of years of evolution has
never provided a way; it has merely fine-tuned metabolism for getting the
most out of a lungful of air, and I would guess at a high cost to other
areas of metabolic function.)
One thing about chromosomal sex determination is that it seems well
designed to produce a fifty-fifty sex ratio, but depending on mating
systems, sexual dimorphism, and various ecological factors, a fifty-fifty
ratio may not be at all optimal. In fact, genetic recombination itself is
not always favorable, which probably accounts for the success of
parthenogenic clones in lizards and fish. George Williams has speculated
that many mammalian lines might revert to parthenogenesis or some form of
asexual reproduction, if only they could, but they're stuck with sex, just
like they're stuck with gilllessness (how's that for a word?). So anyway,
maybe a good thing about temperature-dependent sex determination is that it
is an easy way to get around the fifty-fifty sex-ratio limitation. Somebody
who knows about crocodile ecology and mating systems might be able to figure
out what factors determine optimal sex ratios for parent crocodiles to
produce in order to leave the most progeny, and might further be able to
show whether the observed temperature-dependent ratios are in the right
direction or not. But don't look at me; I have a strict policy of avoiding
crocodiles.
Martin Meiss
2008/10/27 Thilina Surasinghe <[EMAIL PROTECTED]>
> Hi all
>
> I have a question on the temperature dependant sex determination of
> reptiles. What is the evolutionary or ecological advantage of such a
> phenomena, especially in reptiles when the females are produced under both
> low and high temperature extremities?
>
>
> --
> Thilina Dilan Surasinghe (BS in Zoology Colombo, Sri Lanka)
> Graduate Teaching Assistant
> Dept of Biological Sciences
> 132 Long Hall
> Clemson University
> Clemson, SC 29634-0314, USA
>
