By Susan E. Barker
For a closer look at the group selection debate, David Sloan Wilson suggests
we con sider the strange life cycle of the trematode parasite Dicrocoelium
dendri-ticum, a parasite that lives in adult form in the liver and bile
ducts of sheep, cows, deer and wild rab bits.
The adult parasite lays eggs, which are passed in the host's feces.
The eggs are eaten by snails.
While in the snails, the eggs undergo an asexual multiplication process
before they are excreted by the snails as "mucus capsules."
The capsules are eaten by ants.
Once the parasites are in the ant, one out of the many forms a thin-walled
cyst and burrows into the brain of the ant becoming a "brainworm."
(The other parasites form thick-walled cysts that will protect them from
digestive fluids if the ant is eaten.)
The brainworm emits chemicals that alter the behavior of the ant causing
the ant to spend inordinate amounts of time hanging out on the tips of grass
where it is exceed ingly more likely to be eaten by a grazing sheep, cow,
deer or rabbit.
Once the ant is eaten, the brainworm is digested and dies along with the
ant. The remaining parasites are released into the host where they mature
and lay eggs, begin ning the cycle anew.
Wilson's group selection interpretation: To think about this situation
in familiar terms, Wilson suggests you imagine that you are in a group of
people in which a danger ous activity that will ultimately benefit the group
must be performed by a single person.
Some of you decide to draw straws to see who will perform the dangerous
act. Oth ers in the group say, "Great idea! Go for it!" but choose
not to participate in the drawing. It's obvious that those who take the
risk of drawing straws are at a disadvantage, com pared to those who don't.
In just the same way, parasites that are prone to become the brainworm are
at a disadvantage compared to other parasites in the same ant that get the
benefits of the brainworm's behavior without taking the risk.
Natural selection within a single group acts against the brainworm genes.
On the other hand, groups in which one parasite becomes a brainworm outcompete
groups with out brainworms in their passage to the next host in the life
cycle.
Brainworm genes are, therefore, selected against (as opposed to selecting
for a par ticular trait) within single groups but favored by between-group
selection. Seen in this light, the brainworm is also appropriately labeled
an altruist since it risks itself for the ultimate good of the group.
A selfish gene interpretation: Since some percentage of the group
members share the genes of the brainworm, it is giving its life not to benefit
the group, but to selfishly assure the propagation of its own genes. Any
benefit to non-related genes is incidental.
Wilson's reply:"The selfish gene explanation doesn't change
any facts provided by the group selection explanation but only describes
them in another way. In general, self ish gene theory labels any gene that
evolves 'selfish' compared to the genes that don't
evolve. However, the group selection explanation of the brainworm never
contested the fact that the behavior is caused by genes and that the genes
evolve. The question is whether the gene evolves by natural selection within
groups or by between-group selec tion."
Ultimately, both group selectionists and selfish gene proponents say the
gene for "brainworm" behavior evolves, and both are right, Wilson
said.
"But, if you ask the next question, how does it evolve, the selfish
gene theorists say, 'Not by group selection,' and they are wrong,"
Wilson added.