Re: Interrelation between Sex and Death
- From: "whitesickle@xxxxxxx" <whitesickle@xxxxxxx>
- Date: Tue, 29 Nov 2005 13:18:46 -0500 (EST)
You know I thought about how variation equates to individuals but I
also thought of how much like identical clones we are. Natural
selection seems to operate on one level as an impediment to genetic
variation. For example, a culture of Drosophila set up with equal
numbers of red-eyed and white-eyed flies of both sexes will, after 25
generations or so, end up having only red-eyed (the "normal") flies in
it. This, despite the fact that white-eyed flies are just as healthy
and live just as long as red-eyed flies, i.e., they are equal in terms
of survival. But, as it turns out, not only do red-eyed females prefer
red-eyed males, but white-eyed females do also. This natural
selection/sexual selection has nothing to apparently do with survival
yet the flies choose the red eyed ones. How many possible traits of
humans continue to be naturally selected/sexually selected which have
no survival value or indeed might be counter productive to survival? I
imagine quite a few. No wonder evolution takes millions of years to
occur considering such clumsy selection and pressures against increased
genetic variation.
Michael Ragland
Sexual Selection
In sexual selection, one sex - usually the female - chooses among
the available males. Any inherited trait that improves the mating
success of certain individuals will become more pronounced in
succeeding generations. Some examples:
When ready to mate, female three-spined sticklebacks (fish) choose
males with many Class II MHC alleles over males with fewer alleles.
Class II alleles encode the proteins that present antigens to the
immune system. Presumably, the more of them you have, the greater the
diversity of parasite antigens your immune system can recognize and
defend against. Link to discussion of how polymorphism of MHC alleles
protects against parasites.
The females distinguish between the males by soluble molecules
("odors") the males release into the water. How these "odors" are
controlled by the MHC alleles is not known.
A culture of Drosophila set up with equal numbers of red-eyed and
white-eyed flies of both sexes will, after 25 generations or so, end up
having only red-eyed (the "normal") flies in it. This, despite the fact
that white-eyed flies are just as healthy and live just as long as
red-eyed flies, i.e., they are equal in terms of survival. But, as it
turns out, not only do red-eyed females prefer red-eyed males, but
white-eyed females do also.
In other cases of sexual selection, one phenotype prefers to mate with
others of the same phenotype. This is called assortative mating.
Fecundity Selection
The production of a large number of mature offspring is a measure of
fitness. I stress mature because only they can pass these traits on to
another generation. Some ways to do this:
Earlier breeding. If some females become sexually mature earlier than
others, their chances of leaving offspring are enhanced.
For some species (e.g., fish, oysters), which provide little or no care
for their young, fitness is measured by the number of fertilized eggs
they produce.
For species (such as ourselves) that take care of their young;
selection acts to reduce family size (to a point).
Are Humans Exempt from Natural Selection?
It has been argued that advances in medicine, sanitation, etc. have
removed humans from the rigors of natural selection. There is probably
some truth to this, but consider:
of all the human eggs that are fertilized, only one-third of so will
ever reproduce themselves.
The others are eliminated as follows:
Mortality selection
Some 30% of pregnancies end by spontaneous abortion of embryos and
fetuses.
5% by stillbirths and infant deaths.
3% by childhood deaths.
Sexual selection
Another 20% will survive to adulthood but never marry.
Fecundity selection
Of those that do marry, 10% will have no children.
.
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- Interrelation between Sex and Death
- From: whitesickle@xxxxxxx
- Interrelation between Sex and Death
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