Darwin's Dangerous Idea (7 page)

be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have In fact, Darwin saw his idea explaining both great sources of wonder in a occurred useful to man. But if variations useful to any organic being do single stroke. The generation of adaptations and the generation of diversity occur, assuredly individuals thus characterized will have the best chance were different aspects of a single complex phenomenon, and the unifying of being preserved in the struggle for life; and from the strong principle of insight, he claimed, was the principle of natural selection.

inheritance they will tend to produce offspring similarly characterized.

Natural selection would inevitably produce
adaptation,
as the summary This principle of preservation, I have called, for the sake of brevity, Natural makes clear, and under the right circumstances, he argued, accumulated Selection.
[Origin,
p. 127.]

adaptation would create speciation. Darwin knew full well that explaining variation is not explaining speciation. The animal-breeders he pumped so vigorously for their lore knew about how to breed
variety
within a single This was Darwin's great idea, not the idea of evolution, but the idea of species, but had apparently never created a new
species,
and scoffed at the evolution
by natural selection,
an idea he himself could never formulate with idea that their particular different breeds might have a common ancestor.

sufficient rigor and detail to prove, though he presented a brilliant case for it.

"Ask, as 1 have asked, a celebrated raiser of Hereford cattle, whether his The next two sections will concentrate on curious and crucial features of this cattle might not have descended from longhorns, and he will laugh you to summary statement of Darwin's.

scorn." Why? Because "though they well know that each race varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all general arguments and refuse to sum up in their minds slight differences 3. DID DARWIN EXPLAIN THE ORIGIN OF SPECIES?

accumulated during many successive generations"
(Origin,
p. 29).

The further diversification into species would occur, Darwin argued, because if there was a variety of heritable skills or equipment in a population
Darwin did wrestle brilliantly and triumphantly with the problem of
(of a single species), these different skills or equipment would tend to have
adaptation, but he had limited success with the issue of diversity
—
even
different payoffs for different subgroups of the population, and hence these
though he titled his book with reference to his relative failure: the origin
subpopulations would tend to diverge, each one pursuing its favored sort of
of species.

excellence, until eventually there would be a complete parting of the ways.

—STEPHEN JAY GOULD 1992a, p. 54

Why, Darwin asked himself, would this divergence lead to separation or clumping of the variations instead of remaining a more or less continuous
Thus die grand fact in natural history of the subordination of group
fan-out of slight differences? Simple geographical isolation was part of his
under group, which, from its familiarity, does not always sufficiently
answer; when a population got split by a major geological or climatic event,
strike us, is in my judgment fully explained.

or by haphazard emigration to an isolated range such as an island, this

—CHARLES DARWIN,
Origin,
p. 413

discontinuity in the environment ought to become mirrored eventually in a discontinuity in the useful variations observable in the two populations. And Notice that Darwin's summary does not mention speciation at all. It is en-once discontinuity got a foothold, it would be self-reinforcing, all the way to tirely about the adaptation of organisms, the
excellence
of their design, not separation into distinct species. Another, rather different, idea of his was that the diversity. Moreover, on the face of it, this summary takes the diversity of in intraspecific infighting, a "winner take all" principle would tend to species
as an assumption:
"the infinite [sic] complexity of the relations of all operate:

organic beings to each other and to their conditions of existence." What makes for this stupendous (if not actually infinite ) complexity is the presence For it should be remembered that the competition will generally be most at one and the same time (and competing for the same living space) of so severe between those forms which are most nearly related to each other many different life forms, with so many different needs and strategies. Darwin inhabits, constitution and structure. Hence all the intermediate forms 44 AN IDEA IS BORN

Did Darwin Explain the Origin of Species?
45

between the earlier and later states, that is between die less and more ing that reunites the splitting groups, mixing their genes and "frustrating" the improved state of a species, as well as the original parent-species itself, will process of speciation. It is not that anything
wants
speciation to happen, of generally tend to become extinct.
[Origin,
p. 121.]

course (Dawkins 1986a, p. 237), but if the irreversible divorce that marks speciation is to happen, it must be preceded by a sort of trial separation He formulated a variety of other ingenious and plausible speculations on period in which interbreeding ceases for one reason or another, so that the how and why the relentless culling of natural selection would actually create parting groups can move further apart. The criterion of reproductive isolation species boundaries, but they remain speculations to this day. It has taken a is vague at the edges. Do organisms belong to different species when they century of further work to replace Darwin's brilliant but inconclusive
can't
interbreed, or when they just
don't
interbreed? Wolves and coyotes and musings on the mechanisms of speciation with accounts that are to some dogs are considered to be different species, and yet interbreeding does occur, degree demonstrable. Controversy about the mechanisms and principles of and—unlike mules, the offspring of horse and donkey—their offspring are not speciation still persists, so in one sense neither Darwin nor any subsequent in general sterile. Dachshunds and Irish wolfhounds are deemed to be of the Darwinian has explained the origin of species. As the geneticist Steve Jones same species, but unless their owners provide some distinctly unnatural (1993) has remarked, had Darwin published his masterpiece under its arrangements, they are about as reproductively isolated as bats are from existing title today, "he would have been in trouble with the Trades dolphins. The white-tailed deer in Maine don't in fact interbreed with the Description Act because if there is one thing which
Origin of Species
is not white-tailed deer in Massachusetts, since they don't travel that far, but they about, it is the origin of species. Darwin knew nothing about genetics. Now surely could if transported, and naturally they count as of the same species.

we know a great deal, and although the way in which species begin is still a And finally—a true-life example seemingly made to order for philoso-mystery, it is one with the details filled in."

phers—consider the herring gulls that live in the Northern Hemisphere, their But the fact of speciation itself is incontestable, as Darwin showed, build-range forming a broad ring around the North Pole.

ing an irresistible case out of literally hundreds of carefully studied and closely argued instances. That is how species originate: by "descent with As we look at the herring gull, moving westwards from Great Britain to North America, we see gulls that are recognizably herring gulls, although modification" from earlier species—not by Special Creation. So in another they are a little different from the British form. We can follow them, as sense Darwin undeniably did explain the origin of species. Whatever the their appearance gradually changes, as far as Siberia. At about this point in mechanisms are that operate, they manifestly begin with the emergence of the continuum, the gull looks more like the form that in Great Britain is variety within a species, and end, after modifications have accumulated, with called the lesser black-backed gull. From Siberia, across Russia, to northern the birth of a new, descendant species. What start as "well-marked varieties"

Europe, the gull gradually changes to look more and more like the British turn gradually into "the doubtful category of subspecies; but we have only to lesser black-backed gull. Finally, in Europe, the ring is complete; the two suppose the steps in the process of modification to be more numerous or geographically extreme forms meet, to form two perfectly good species: greater in amount, to convert these... forms into well-defined species"

die herring and lesser black-backed gull can be both distinguished by their
(Origin,
p. 120).

appearance and do not naturally interbreed. [Mark Ridley 1985, p. 5]

Notice that Darwin is careful to describe the eventual outcome as the creation of "well-defined" species. Eventually, he is saying, the divergence

"Well-defined" species certainly do exist—it is the purpose of Darwin's becomes so great that there is just no reason to deny that what we have are book to explain their origin—but he discourages us from trying to find a two different species, not merely two different varieties. But he declines to

"principled" definition of the concept of a species. Varieties, Darwin keeps play the traditional game of declaring what the "essential" difference is: insisting, are just "incipient species," and what normally turns two varieties into two species is not the
presence
of something (a new essence for each

... it will be seen that I look at the term species, as one arbitrarily given for group, for instance ) but the
absence
of something: the intermediate cases, the sake of convenience to a set of individuals closely resembling each which used to be there—which were necessary stepping-stones, you might other, and that it does not essentially differ from the term variety, which is say—but have eventually gone extinct, leaving two groups that are
in fact
given to less distinct and more fluctuating forms.
[Origin,
p. 52.]

reproductively isolated as well as different in their characteristics.

Origin of Species
presents an overwhelmingly persuasive case for Dar-One of the standard marks of species difference, as Darwin fully recog-win's first thesis—the historical fact of evolution as the cause of the origin nized, is reproductive isolation—there is no interbreeding. It is interbreed-46
AN IDEA IS BORN

Did Darwin Explain the Origin of Species?
47

of species—and a tantalizing case in favor of his second thesis—that the another process that might achieve these effects? What
else
could account fundamental mechanism responsible for "descent with modification" was for evolution, if not the mechanism he had described?

natural selection.4 Levelheaded readers of the book simply could no longer This challenge effectively turned Hume's predicament inside out. Hume doubt that species had evolved over the eons, as Darwin said they had, but caved in because he could not imagine how anything other than an Intelligent scrupulous skepticism about the power of his proposed mechanism of natural Artificer could be the cause of the adaptations that anyone could observe. Or, selection was harder to overcome. Intervening years have raised the more accurately, Hume's Philo imagined several different alternatives,
but
confidence level for both theses, but not erased the difference (Ellegard
Hume had no way of taking these imaginings seriously.
Darwin described

[1958] provides a valuable account of this history). The evidence for evo-how a Nonintelligent Artificer could produce those adaptations over vast lution pours in, not only from geology, paleontology, biogeography, and amounts of time, and proved that many of the intermediate stages that would anatomy (Darwin's chief sources), but of course from molecular biology and be needed by that proposed process had indeed occurred. Now the challenge every other branch of the life sciences. To put it bluntly but fairly, anyone to imagination was reversed: given all the telltale signs of the historical today who doubts that the variety of life on this planet was produced by a process that Darwin uncovered—all the brush-marks of the artist, you might process of evolution is simply ignorant—inexcusably ignorant, in a world say—could anyone imagine how any process
other
than natural selection where three out of four people have learned to read and write. Doubts about could have produced all these effects? So complete has this reversal of the the power of Darwin's idea of natural selection to explain this evolutionary burden of proof been that scientists often find themselves in something like process are still intellectually respectable, however, although the burden of the mirror image of Hume's predicament. When they are confronted with a proof for such skepticism has become immense, as we shall see.

prima facie
powerful and undismissable objection to natural selection (we So, although Darwin depended on his idea of the mechanism of natural will consider the strongest cases in due course), they are driven to reason as selection to inspire and guide his research on evolution, the end result follows: I cannot (yet) see how to refute this objection, or overcome this reversed the order of dependence: he showed so convincingly that species difficulty, but since I cannot imagine how anything other than natural
had
to have evolved that he could then turn around and use this fact to selection could be the cause of the effects, I will have to assume that the support his more radical idea, natural selection. He had described a mech-objection is spurious;
somehow
natural selection must be sufficient to explain anism or process that, according to his arguments,
could
have produced all the effects.