|
|
Exaptation and The Adaptationist Stance
Jonathan A. Smith (jasmith@U.WASHINGTON.EDU)
Sun, 21 May 1995 11:31:35 -0700
Danny Yee wrote:
> One of the reasons I dislike the term "preadaptation" is that it
> conveys entirely the wrong idea to those without a background
> in evolutionary biology ("exaptation" is the preferred term).
> There's no conflict between "natural selection" and the existence
> of "exaptations" -- the latter are purely fortuitous, after all!
> Exaptations are only an argument against *adaptionism*. Once again,
> let me recommend:
>
> R. Lewontin & S.J. Gould
> "The Spandrels of San Marco and the Panglossian Paradigm:
> A Critique of the Adaptionist Programme". Proc. R. Soc.
> London, 1978, 205: 581-598.
I agree with Dany Yee about the status of natural selection, it is very
much alive and well. And I also very much enjoy S. J. Gould's writing,
however in this case I consider Gould and Lewontin to be
very much mistaken. Not only do Lewontin and Gould fail to make a good
case against adaptationist thought, they in fact make use of
adaptationist arguments in their own work. They could hardly fail to do
so. I would suggest that if you do read Lewontin and Gould's article it
might also be a good idea to read the response from evolutionary
biologist Ernst Mayer (an important figure in the creation of the modern
synthesis) and philosopher of science Daniel Dennett:
Mayer, Ernst
1988 "How to carry out the adaptationist program?" Toward a New
Philosophy of Biology, essay 9 pp. 148-159, Cambridge: Harvard University
Press.
Dennett, Daniel C.
1990 "Intentional Systems in Cognitive Ethology: The 'Panglossian
Paradigm' Defended." The Intentional Stance, ch. 6 pp. 236-268.
Cambridge: MIT Press.
Dennett, Daniel C.
1990 "Reflections: Interpreting Monkeys, Theorists and Genes" The
Intentional Stance, ch. 6 pp. 270-286. Cambridge: MIT Press.
Dennett, Daniel C.
1995 Darwin's Dangerous Idea: Evolution and the Meanings of Life.
Dennett points out, rightly, that Lewontin and Gould's program is
analogous to the narrow behaviorist program in psychology (Dennett 1990
261-265). As the behaviorist program demanded that we expunge all
language that refers to human intention and belief; Lewontin & Gould heap
scorn on all language related to adaptive function in evolution. Very
much like putting a ball and chain on one's own (and science's) feet.
Adaptivist claims are sometimes testable, that is they have consequences
that we can observe. As Mayer points out:
"As a consequence of the adaptationist dilemma, when one selectionist
explanation of a feature has been discredited, the evolutionist must test
other possible adaptationist solutions before he can resign and say: this
phenomena must be a product of chance. Gould and Lewontin ridicule the
research strategy: "if one adaptive argument fails try another one." Yet
the strategy to try another hypothesis when the first fails is a
traditional methodology is all branches of science. It is the standard
in physics, chemistry, physiology, and archeology." (Mayer 1988 p. 151)
A instance of exaptation provides a feature -- and that feature
will survive only if it just happens to be useful, or at least does not
decrease the fitness of the organisms that inherit it. In some cases it
is just a part of some large complex of adaptive features and did not
evolve independently, but all features either contribute to or detract
from fitness when compared to alternative solutions. There are just
about always alternative solutions. Dennett makes this point well in
"The Spandrel's Thumb" (Dennett 1994 pp. 267-282). In many important
cases if the feature survives it is because it increases fitness. Once
the feature increases fitness its survival can be understood using the
adaptationist strategy, no matter how that feature originated. The
adaptationist approach does not always work (some features are neutral
and sometimes maladaptive features have not yet been removed from the
population) but it does work in many cases and is worth trying.
Note that Lewontin & Gould are also proposing that certain
features have no independent function, that they cannot be replaced by
alternatives that make a difference in fitness. How do you go about
testing a hypothesis of 'no function'? How do you test a spandrel?
If we give up the adaptivist strategy in science the only stories
that we can tell about the past will be to list is laborious detail the
individual instances of selection and genetic change. Why does the
butterfly have eye-spots? Without adaptationism we can only list the
long sequences of ancestors who managed to survive being eaten by
individual animals and particular chance genetic changes and instances of
exaptation. No, what we accept as an explanation, and what we should
accept as an explanation is that eye spots increase fitness by serving as
camouflage. Why call some substance food (for some particular species)
and not list the physical object's exact chemical makeup and physical
structure and the particular instances of it ending up inside particular
organisms? Once again we hypothesize that this class of substances serve
particular (ultimately adaptive) functions that are similar for just
about all members of the species. Dennett writes in response to Gould:
"As his own account shows, one has not yet answered the 'why' question
posed when one has abstemiously set out the long and (and in fact largely
inaccessible) history of mutation, predation, reproduction, selection --
with no adaptationist gloss. Without the adaptationist gloss we wont know
why." (Dennett 1990 p. 267)
The point I want to make is that scientific knowledge is created
for human use and shorthand terminology like 'food' or "camouflage' helps
us write stories about our world. These are not 'just so' stories
because they are practical stories -- we make decisions and take actions
based on them and these actions have consequences. Sometimes the
observed consequences are not consistent with our stories so we have to
go back and revise, adjust, or refine them. The category 'food' is not
itself a spandrel, it has taken part in a very very large body of useful
scientific stories. Why should we give up this concept for Gould and
Lewontin's sake?
|