"crognum"  wrote in message
>
> I am interested in learning of any occurences in nature where two
> related species (or subspecies) both derived from the same parent
> species, display a difference in their propensity for parasitism.
>
I can't think of a good example of two closely related species where one is
clearly parasitic whilst another is free-living.
However you might want to look at the yellow-billed oxpecker, Buphagus
africanus, and its relative the red-billed oxpecker, Buphagus
erythrorhynchus. These birds clean ticks from large mammals, but also take
blood form wounds, so are part way to parasitism. B. erythrorhynchus seems
to benefit its host less, so it might be the more parasitic of the two - I
haven't researched this properly but you will find plenty of material on the
web.
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