Adaptationist Lessons I: Snail shells & the assumption of drift

Bryant (mycol1@unm.edu)
29 Aug 1996 21:20:33 -0600

I. Meaningless shell bands?

Lenny and others here have alluded to non-natural selective forces of
evolution as viable alternatives to adaptationist explanations for the
origins of apparently "functional" traits. "Drift," or chance events
affecting the frequency and distribution of alleles in a population, seem
to be what they are alluding to, specifically.

Gould & Lewontin (1979, ref below) likewise emphasize non-selective forces
and attack adaptationists for paying too much attention to natural selection
and not enough attention to phyletic or developmental constraints,
pleiotropic side-effects, and the like. [Pleiotropy is when a gene has
more than one set of phenotypic effects.]

(Interestingly, though, the Gould/Lewontin do say: "Although Darwin regarded
selection as the most important of evolutionary mechanisms *(as do we)*, no
argument from opponents angered him more than the common attempt to
caricature and trivialize his theory by stating that it relied
exclusively upon natural selection.") Odd thing is, people attacking
evolutionary theory, not imaginary hyperadaptationists, were who angered
Darwin thusly. Gould & Lewontin characterize adaptationism in just the
way that they quote Darwin objecting to.

I want to illustrate why adaptationists agree with Cronin when she says:

"Finally, the adaptationist is purportedly a dogmatist. He shows an
'unwillingness to consider alternatives to adaptive stories' (Gould &
Lewontin 1979, p. 581). Why, critics complain, won't he ever give up?
...This is sheer dogmatism. It is sterile and it blinds Darwininians to
the factors that are really at work.
"Many adaptationists would not deny the charge of
dogmatism--though they may prefer to call it, say, 'tenacity' or
'perserverance'. But they do deny, most emphatically, the charge of
sterility. On the contrary, they assert, their approach has proved highly
fruitful. Their 'dogmatism' has been vindicated by history."

Through the 1940s, Mayr (1982, ref below) complains, any puzzling
evolutionary phenomenon was ascribed to genetic drift (chance). No
rigorous tests of adaptationist alternatives were undertaken.

In their 1979 attack on adaptationists, Gould & Lewontin continue
this tradition of declaring a trait functionless when they ridicule
Wallace for his suggestion that land snails' colored shells (polymorphic
within populations) hold some adaptationist lesson even though we don't
know what that lesson is, yet. They use it to illustrate the
adaptationists' "rule" that one should "attribute failure to imperfect
understanding of where an organism lives and what it does." [How this is
inferior to their proposed alternative of declaring it a result of drift,
as was so often done in the 40s and 50s, is not explained.]

Wallace, it turns out, was correct. The shell band polymorphisms
apparently afford snails some measure of protection from predators'
"search images" and, perhaps more interestingly, because it was Wallace's
original guess for snail color function, snails create their own thermal
microclimates by spending different amounts of time in the sunlight,
depending on their coloration (Jones 1982, ref below).

This does not mean that drift was unimportant in the process of the
evolution of snail shell bands, necessarily, although measuring that
effect is a rather daunting, if not impossible, task. The example,
however, *does* illustrate, I think, why adaptationist analysis should come
before dismissal and surrender to drift.

It's a nice, ironic example of why Gould & Lewontin's charactiture of
adaptationists as Panglossian story tellers, and their championing of
Bauplanism and evolutionary constraints, have little changed evolutionary
biology in practice.

If Gould & Lewontin's thoughts on the matter had been heeded, we would
still be assuming without evidence that snail bands are functionally
neutral. Only by testing an adaptationist hypothesis did Jones provide an
argument for natural selection's power in the shaping of things and
clarify the functional significance of snail shell color bands.

Finally, the side-effects or "Spandrels" that Gould & Lewontin point to as
alternatives to adaptationist explanations are really confirmation of the
power of selection, because selection for pleiotropically linked traits
is what maintains such side-effects in the face of mutation accumulation.

In the next, much shorter, post, I will dust off another example of a
supposed "constraint"'s triviality as a rival of selection: "Allometry."

Cheers, Bryant

***References***

Cronin, 1991. The Ant and the Peacock, Cambridge Univ. Press.

Gould & Lewontin, 1979. Spandrels of San Marco and the Panglossian
Paradigm. Proc. R. Soc. London, series B, 205: 581-598.

Jones, 1982. Genetic differences in individual behavior associated with
shell polymorphism in the snail Cepea nemoralis. Nature, 298: 749-750.

Mayr, 1983. How to carry out the adaptationist programme? American
Naturalist, 121, 324-334.