I read in Scientific American (Jan 92) about the parasitic worm
Leucochloridium paradoxum, which spends part of its life cycle in a snail
and part in a bird. To move from the snail to the bird, large populations
of these worms migrate to the snail's eyestalks, where they cause the
eyestalks to swell and change in coloration. Such changes result in the
snail's coming to resemble caterpillars, which the birds then eat. The
worms subsequently lay their eggs in the bird's digestive track. It takes
millions of worms to effect the change in the snail's eyestalks. But can
entire populations of individuals undergo the same genetic mutation at the
same time and thus secure an advantage? If not, why would such a
behavioral mechanism evolve, since the behavior could not confer any
advantages on a single individual?
Any instruction on this point would be greatly appreciated.