sexual selection...

Frode Skarstein frodes at ibg.uit.no
Tue Jun 6 04:26:50 EST 1995


Greetings!

Inspired by the openness and interest in functional explanations revealed
in this group during the thread "The real function of the immune system",
I would like to present a theory that has gained interest with behavioral
ecologists, and which I believe would reward from communication with
immunologists:

I work in the field of sexual selection, which has as a main goal in
understanding the evolution of conspicuous sexual ornaments, like the
peacock's tail. One explanation for these traits is that they reveal "good
genes" (i.e., genes that give fitness advantages), and that females
choosing mates based on such traits will have fitter offspring, since the
offspring will inherit these genetic advantages. However, one main
criticism that has haunted "good gene" believers, is that females choosing
only genetically superior mates will exhaust the population of genetic
variability. After sending all the genetically inferior males to
reproductive death by refusing to mate with them, no more variability for
the ornaments will be left in the population. In other words, all males in
the population will express ornaments to the same degree, and females will
have nothing more to choose from putting  sexual selection to a grinding
halt.

Happily for me, this is not what happens. Take a look outside and you'll
see enormous between-male variability in the expression of sexual
ornaments. Some males are more colorful, have longer tails, more elaborate
songs and so on. But then, if the "good genes" model is correct, how can
this be? Over generations, what keeps females from removing all the 
"not-so-good genes" from the population, why don't all males end up as
supermales? 

The solution is this: What if the quality desired by the females is
constantly changing? As with resistance against parasites. As
immunologists (should) know, parasites co-evolve with their hosts,
resistance "today" could be worthless after a few generations. This
evolutionary race between parasite and host forces the host to come up
with new variations to keep the parasite in check. Thus, if females choose
males based on ornaments revealing genetic resistance against parasites,
the variation in ornament expression will not be exhausted, since
resistance genes "today" may not provide resistance the "next day".
Theoretically this sounds neat. But what is really "heritable parasite
resistance"?

This is where you immunology people come in. I would love to hear how you
guys define "genetic resistance". What heritable aspects of the immune
defence can enable some host genotypes to deal with a given pathogen while
other host genotypes succumb? 

Most aspects of the immune system probably is probably under some degree
of genetic influence, and could thus be lumped onto the pile "genetic
resistance". However, I am interested in the aspects of the immune
function that could potentially create cyclic co-evolution between host
and pathogen, and for which variation between individuals exists.

First out here would be the ability to produce antibodies against the
pathogen in question. However, if any animal can produce antibodies
against nearly any imaginable antigen, how can this have a genetic
component? Could the initial gene sequences used by a B-cell when
developing its unique antibody confer advantages over other sequences, or
may any gene sequence equally well produce any given antibody? Am I
rambling blindly along here?

Whether or not your definition of genetic resistance is what you believe
would suffice the criteria outlined above, let me know your feelings on
the matter. I believe that if the parasite resistance version of the "good
gene" idea is to progress beyond its current point, behavioral ecologists
need to understand the immunology involved. To solve the puzzle of
elaborate sexual ornaments, we need to understand the shape and function
of all the jigsaw pieces involved.

So if you feel like it, join in and give your impression on how females
can ensure parasite resistance in their offspring by choosing healthy
males. What is "genetic resistance"? As one immunologist friend of mine
said:"Genetic resistance is a can of worms." Maybe you guys can help sort
them out?

Sincerely,
Frode Skarstein



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