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

94 THE TREE OF LIFE Colorcoding a Species on the Tree 95
FIGURE 4.6
arrangement" (of descent), we could not put them into a "natural classification"—we
need the biggish gaps between extant forms to form the
"boundaries" of any such classes.
The theoretical concept of species that predates Darwin's theory had two
fundamental ideas: that species members have different essences, and that
"therefore" they don't/can't interbreed. What we have subsequently figured
out is that in principle there could be two subpopulations that were different
only in that their pairings were sterile due to a tiny genetic incompatibility.
Would these be different species? They could look alike, feed alike, live
together in the same niche, and be genetically very, very similar, yet
reproductively isolated. They would not be different enough to count as
salient varieties, but they would satisfy the primary condition for being two
different species. In fact, there are cases of "cryptic sibling species" that
approximate this extreme. As we already noted, at the other extreme we have
the dogs, readily distinguished into morphological types by the naked eye,
adapted to vastly different environments, but not reproductively iso-
lated. Where should we draw the line? Darwin shows that we don't need to
draw the line in an essentialist way in order to get on with our science. We
have the best of reasons to realize that these extremes are improbable: in
general, where there is genetic speciation there is marked morphological
difference, or marked difference in geographical distribution, or (most likely)
both. If this generalization weren't largely true, the concept of species would
not be important, but we need not ask exactly how much difference (in
addition to reproductive isolation) is essential for a case of real speciesdifference.
3
Darwin shows us that questions like "What is the difference between a
variety and a species?" are like the question "What is the difference between
a peninsula and an island?" 4 Suppose you see an island half a mile offshore
at high tide. If you can walk to it at low tide without getting your feet wet, is
it still an island? If you build a bridge to it, does it cease to be an island?
What if you build a solid causeway? If you cut a canal across a peninsula
(like the Cape Cod Canal), do you turn it into an island? What if a hurricane
does the excavation work? This sort of inquiry is familiar to philosophers. It
is the Socratic activity of definition-mongering or essence-hunting: looking
for the "necessary and sufficient conditions" for being-an-X. Sometimes almost
everyone can see the pointlessness of the quest—islands obviously
don't have real essences, but only nominal essences at best. But at other
times there can still seem to be a serious scientific question that needs
answering.
More than a century after Darwin, there are still serious debates among
biologists (and even more so among philosophers of biology ) about how to
define species. Shouldn't scientists define their terms? Yes, of course, but
only up to a point. It turns out that there are different species concepts with
different uses in biology—what works for paleontologists is not much use to
ecologists, for instance—and no clean way of uniting them or putting them
in an order of importance that would crown one of them (the most important
one) as the concept of species. So I am inclined to interpret the persisting
debates as more a matter of vestigial Aristotelian tidiness than a useful
disciplinary trait. (This is all controversial, but see Kitcher 1984 and G. C.
Williams 1992 for further support and concurring arguments, and the recent
anthology on the topic, Ereshefsky 1992, and Sterelny 1994, an insightful
review essay on that anthology.)
3. The issues are further complicated by the existence of hybridization—in which members
of two different species do have fertile offspring—a phenomenon that raises interesting
issues that are off the track we are exploring.
4. The evolutionary epistemologist and psychologist Donald Campbell has been the most
vigorous developer of the implications of this side of Darwin's legacy.