Validity of the theory of sex in relation to parasites.

Xia xia at darwin.genetics.washington.edu
Sat Dec 5 14:57:11 EST 1992

>     Hi.  I'm looking for some information into the validity of the theory that
>sexual reproduction has evolved as a defence against the actions of parasites.
>Since most parasitic invaders reproduce assexually and may have many generations
>in the course of the host organisms's life, one would expect that natural
>selection would play a more important role in the parasite population.
>Therefore, evolution would proceed more quickly in parasite populations. Hence,
>the development of sexual reproduction as a means evening the odds.
>Its a tempting theory. Does it make sense to anyone else?
>                                               John Antonioni
>                                               Sudbury, Canada

No, it does not make sense, because it applies only to specialised
host-parasite pairs. When the host is harbouring many different species
of parasites, a recombination to avoid one species of parasite would 
send the host to the mercy of other species of paes. Also, the
theory would require sex to evolve independently many times. Moreover,
the theory requires the diversified recombinants to be realized in
progeny. For this reason it does not apply to species with low fecundity.

I have written a paper on the issue, and 4 months has passed since
I submitted it to Am Nat.

Xuhua Xia
Department of Genetics
U of Washington

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