A curious young elephant got its nose stretched by a crocodile, with
the result that elephants everywhere now carry a trunk. What this
story tells us is that Rudyard Kipling was a thoroughgoing
Lamarckian—a believer in the inheritance of acquired
characteristics. As it happens, Lamarckian ideas were already in
disrepute when Kipling wrote his "Just So" stories. The
German biologist August Weismann, in a remarkably Kiplingesque
experiment, had shown that chopping off a rat's tail did not lead to
the birth of tailless ratlets. Experimental protocols have gotten
more sophisticated since then, but the verdict is the same: There's
no sign of Lamarckian inheritance anywhere in the kingdoms of life.
But why not? A few years ago Colin McGinn wrote (in a review of a
book by Daniel Dennett): "Why have Lamarckian organisms never
evolved? Surely a mutation which made the genes responsive to
changes of phenotype ('learning') would have selectional advantage,
and there seems to be no physical impossibility in such a
set-up. Wouldn't natural selection favour a physiological mechanism
that allowed learned characteristics to be passed genetically to offspring?"
These are good questions. One way of answering them is to note that
the molecular pathways needed for Lamarckian inheritance just don't
exist. Within the context of life-as-we-know-it, there's no way for
the elephant's nose to talk to the elephant's genes—especially
the germ-line genes. The "central dogma" of molecular
biology says that information flows from DNA to RNA to protein, not
the other way around. A Lamarckian feedback loop would seem to
require some mechanism by which the proteins of the phenotype could
alter the DNA of the genotype.
The trouble with such an answer is that it invites a further
annoying question: Why is it that such feedback loops have never
evolved? Given all that has evolved in the way of genetic
detours and shortcuts—plasmids, transposons, retroviruses,
prions—it seems a bit arbitrary to declare this one pathway
out of bounds. The case of retroviruses is particularly provocative,
since they produce an enzyme (reverse transcriptase) that violates
the central dogma, copying information from RNA back into DNA.
Here's another possible reason for the absence of Lamarckian
inheritance in nature: Maybe it's just not worth the bother. Many
authors seem to take for granted that a genetic means of passing on
learned traits would be beneficial if it could exist. They assume
Lamarckism would make for a smoother and quicker kind of evolution
than Darwin's blindfolded selection of random variations. But what
are the true costs and benefits of Lamarckism? Perhaps the reason we
see no Lamarckian organisms is not that nature cannot invent the
necessary apparatus but rather that the result is maladaptive.
Lamarckism could be a trick that nature has tried and discarded.
I have attempted to investigate this issue through some simple
computer simulations. Specifically, I've addressed the following
question: If you were offered a Lamarckian capability, how much
should you be willing to pay for it, when the price is exacted in
the form of some compensating detriment to fitness? My experiments
in free-market genetics are too crude to yield a definitive answer,
but I can report that within the rather narrow bounds of this one
model, I've been unable to find any situation where the benefits of
Lamarckism would justify paying more than a small price.
» Post Comment