Perplexed in Peoria  wrote or quoted:
> "Donald Forsdyke"  wrote in message

> >         If a horse that is potentially a member of an incipient
> > species crosses with another member of the same incipient species then
> > there is no reprotypic counterselection (i.e. hybrid sterility). The
> > offspring are abundant and fertile. As Romanes noted in 1886, finding
> > a reprotypically compatible mate is likely to be a rare event, but
> > then, speciation itself is a rare event. For more on this see section
> > 7 of my new JTB paper [http://post.queensu.ca/~forsdyke/speciat2.htm]
> 
> Allow me to put this into my own words, which you may then
> critique:
> 
> The sterility of the mule (and the inviability of her brother)
> cannot be seen as adaptive for the mule individual.  Nor can
> her sterility be seen as adaptive for her parents - an individual
> horse and ass.  However, the hybrid insterility CAN be seen
> as an adaptation of the horse SPECIES (or the ass species, or
> both).  The small loss of fitness that this entails for the
> mule's individual parents can be described as a kind of "tough
> love" that the species imposes on its members.  This is
> (incipient) species-level selection.

What *would* be adaptive would be if the hybrid was *so* sterile
that it got aborted before birth (though such selection doesn't
have to be pre-zygotic).

Simple selection at the individual level would favour this.
It would be a form of divergent selection.

Arguments about group selection favouring sterility should bear in mind 
the sterile hybrid is not common.  Most likely evolution doesn't care 
much about their fates.  Most likely it's a non-adaptive entity - the 
natural and inevitable result of two gene pools drifting apart.
-- 
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