Darwin's Dangerous Idea - Evolution and the Meaning of Life

100 THE TREE OF LIFE
which it followed that they were—as they later turn out to be—the founders
of a new species. We can imagine, if we want, an extreme (and improbable)
case in which a single mutation guarantees reproductive isolation in a single
generation, but, of course, whether or not the individual who has that
mutation counts as a species-founder or simply as a freak of nature depends
on nothing in its individual makeup or biography, but on what happens to
subsequent generations—if any—of its offspring.
Darwin was not able to present a single instance of speciation by natural
selection in Origin of Species. His strategy in that book was to develop in
detail the evidence that artificial selection by dog- and pigeon-breeders could
build up large differences by a series of gradual changes. He then pointed out
that deliberate choice by title animals' keepers was inessential; the runts of
the litter tended not to be valued, and hence tended not to reproduce as much
as their more valued siblings, so, without any conscious policy of breeding,
human animal-keepers presided unwittingly over a steady process of design
revision. He offered the nice example of the King Charles spaniel, "which
has been unconsciously modified to a large extent since the time of that
monarch" (Origin, p. 35)—as can be confirmed by a careful examination of
the dogs in various portraits of King Charles. He called such cases
"unconscious selection" by human domesticators, and he used it as a
persuasive bridge to get his readers to the hypothesis of even more
unconscious selection by the impersonal environment. But he had to admit,
when challenged, that he could provide no cases of animal-breeders'
producing a new species. Such breeding had definitely produced different
varieties, but not a single new species. Dachshund and St. Bernard were not
different species, however different in appearance. Darwin admitted as much,
but he might quite correctly have gone on to point out that it was simply too
early to tell whether he had given any examples of speciation accomplished
by artificial selection. Any lady's lapdog could at some future date be
discovered to have been the founding member of a species that split off from
Canis familiaris.
The same moral applies to the creation of new genera, families, and even
kingdoms, of course. The major branching that we would retrospectively
crown as the parting of the plants from the animals began as a segregation of
two gene pools every bit as inscrutable and unremarkable at the time as any
other temporary drifting apart of members of a single population.
4. PATTERNS, OVERSIMPLIFICATION, AND EXPLANATION
Much more interesting than the question of how to draw the species boundary
are all the questions about the shapes of the branches—and even more
interesting, the shapes of the empty spaces between the branches. What
Patterns, Oversimplification, and Explanation 101
trends, forces, principles—or historical events—have influenced these
shapes or made them possible? Eyes have evolved independently in dozens
of lineages, but feathers probably only once. As John Maynard Smith observes,
mammals go in for horns but birds do not. "Why should the pattern
of variation be limited in this way? The short answer is that we do not know"
(Maynard Smith 1986, p. 41).
We can't rewind the tope of life and replay it to see what happens next
time, alas, so the only way to answer questions about such huge and experimentally
inaccessible patterns is to leap boldly into the void with the
risky tactic of deliberate oversimplification. This tactic has a long and distinguished
history in science, but it tends to provoke controversy, since
scientists have different thresholds at which they get nervous about playing
fast and loose with the recalcitrant details. Newtonian physics was overthrown
by Einstein, but it is still a good approximation for almost all purposes.
No physicist objects when NASA uses Newtonian physics to calculate
the forces at liftoff and the orbital trajectory of the space shuttle, but, strictly
speaking, this is a deliberate use of a false theory in order to make calculation
feasible. In the same spirit, physiologists studying, say, mechanisms for
changing the rate of metabolism try in general to avoid the bizarre complexities
of subatomic quantum physics, hoping that any quantum effects will
cancel out or in other ways be beneath the threshold of their models. In
general, the tactic pays off handsomely, but one can never be sure when one
scientist's grubby complication will be elevated into another scientist's Key to
the Mystery. And it can just as well work the other way around: the Key is
often discovered by climbing out of the trenches and going for the panoramic
view.
I once got in a debate with Francis Crick about the virtues and vices of
Connectionism—the movement in cognitive science that models psychological
phenomena by building up patterns in the connection-strengths
between the nodes in very unrealistic and oversimplified "neural nets" simulated
on computers. "These people may be good engineers," Crick averred
(as best I recall), "but what they are doing is terrible science! These people
willfully turn their backs on what we already know about how neurons
interact, so their models are utterly useless as models of brain function." This
criticism somewhat surprised me, for Crick is famous for his own brilliant
opportunism in uncovering the structure of DNA; while others struggled up
the straight and narrow path of strict construction from the evidence, he and
Watson took a few daring and optimistic sidesteps, with gratifying results.
But in any case, I was curious to know how widely he would cast his
denunciation. Would he say the same thing about population geneticists? The
derogatory term for some of their models is "bean-bag genetics," for they
pretend that genes for this and that are like so many color-coded beads on a
string. What they call a gene (or an allele at a locus)