Darwin's Dangerous Idea (17 page)

obvious. There are no secret facts that could tell us which choice is right, which choice carves nature at the joints, for we are looking right at the places where the joints would have to be, and there aren't any. There is nothing The same sort of quandary faces us when we try to complete the task of more to being a species than being one of these branches of interbreeding color-coding the whole species by carrying our red ink down the Tree to organisms, and nothing more to being the
conspecific
of some other organism include all Lulu's ancestors. We will encounter no gaps or joints on this (contemporary or not) than being part of the same branch. The choice we downward path, which will take us all the way to the prokaryotes at the base make will then have to depend on pragmatic or aesthetic considerations: Is it of the Tree if we persist. But if we also color
sideways
as we go down, filling
ungainly
to keep the same label for this branch as for its parent trunk? Would in the cousins, aunts, and uncles of Lulu and her ancestors, and then color up it be
misleading
for one reason or another to say the branch on the right from these sideways spreaders, we will eventually fill in a whole branch on rather than the branch on the left was the new species?2

which Lulu resides down to the point where coloring any lower ( earlier ) nodes (for instance, at A in figure 4.6) causes "leakage" of red into neighboring branches that clearly belong to other species.

If we stop there, we can be sure that
only
members of Lulu's species have been colored red. It will be arguable that we have left out some that deserve 2. The cladists (whose views will be briefly discussed later) are a school of taxonomists to be colored, but only arguable, for there are, again, no hidden facts, no that reject, for various reasons, the concept of a "parent" species' persisting. Every essences that could settle the issue. As Darwin pointed out, if it weren't for speciation event, in their terms, results in a pair of daughter species and the extinction the separations that time and the extinction of the intermediate stepping-of their common parent, no matter how closely one surviving branch resembles the parent, compared with the other branch.

stones has created, although we could put the life forms into a "natural

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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
species-difference.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 FIGURE 4.6

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 arrangement" (of descent), we could not put them into a "natural classifi-ecologists, for instance—and no clean way of uniting them or putting them cation"—we need the biggish gaps between
extant
forms to form the in an order of importance that would crown one of them (the most important

"boundaries" of any such classes.

one) as
the
concept of species. So I am inclined to interpret the persisting The theoretical concept of species that predates Darwin's theory had two debates as more a matter of vestigial Aristotelian tidiness than a useful fundamental ideas: that species members have different essences, and that disciplinary trait. (This is all controversial, but see Kitcher 1984 and G. C.

"therefore" they don't/can't interbreed. What we have subsequently figured Williams 1992 for further support and concurring arguments, and the recent out is that in principle there could be two subpopulations that were different anthology on the topic, Ereshefsky 1992, and Sterelny 1994, an insightful
only
in that their pairings were sterile due to a tiny genetic incompatibility.

review essay on that anthology.)

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 3. The issues are further complicated by the existence of hybridization—in which mem-different
species.
In fact, there are cases of "cryptic sibling species" that bers of two different species
do
have fertile offspring—a phenomenon that raises inter-approximate this extreme. As we already noted, at the other extreme we have esting issues that are off the track we are exploring.

the dogs, readily distinguished into morphological types by the naked eye, 4. The evolutionary epistemologist and psychologist Donald Campbell has been the most adapted to vastly different environments, but not reproductively iso-vigorous developer of the implications of this side of Darwin's legacy.

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97

saliencies—weddings, rings, certificates—by which it can be observed. We 3. RETROSPECTIVE CORONATIONS: MITOCHONDRIAL EVE AND

can see this feature of speciation in a better light by looking first at another INVISIBLE BEGINNINGS

instance of retrospective crowning, the conferring of the title of Mitochondrial Eve.

When we tried to see whether Lulu's descendants split into more than one Mitochondrial Eve is the woman who is the most recent direct ancestor, in species, we had to look ahead to see if any large branches appeared, and then the female line, of every human being alive today. People have a hard time
back up
if we deemed that somewhere along the line a speciation event must thinking about this individual woman, so let's just review the reasoning.

have happened. We never addressed the presumably important question of Consider the set
A,
of all human beings alive today. Each was born of one
exactly
when speciation should be said to occur. Speciation can now be seen and only one mother, so consider next the set,
B,
of all the mothers of those to be a phenomenon in nature that has a curious property: you can't tell that it alive today.
B
is of necessity smaller than
A,
since no one has more than one is occurring at the time it occurs! You can only tell much later that it has mother, and some mothers have more than one child. Continue with the set occurred, retrospectively crowning an event when you discover that its C, of mothers of all those mothers in set
B.
It is smaller still. Continue on sequels have a certain property. This is not a point about our epistemic with sets
D
and
E
and so forth. The sets must contract as we go back each limitations—as if we
would
be able to tell when speciation occurs if only we generation. Notice that as we move back through the years, we exclude many had better microscopes, or even if we could get in a time machine and go women who were contemporaries of those in our set. Among these excluded back in time to observe the appropriate moments. This is a point about the women are those who either lived and died childless or whose female objective property of being a speciation event. It is not a property that an progeny did. Eventually, this set must funnel down to one— the woman who event has simply by virtue of its spatio-temporally local properties.

is the closest direct female ancestor of everybody alive on earth today. She is Other concepts exhibit similar curiosities. I once read about a comically Mitochondrial Eve, so named (by Cann et al. 1987) because since the bad historical novel in which a French doctor came home to supper one mitochondria in our cells are passed through the maternal line alone, all the evening in 1802 and said to his wife-. "Guess what / did today! I assisted at mitochondria in all the cells in all the people alive today are direct the birth of Victor Hugo!" What is wrong with that story? Or consider the descendants of the mitochondria in her cells!

property of being a widow. A woman in New York City may suddenly The same logical argument establishes that there is—must be—an Adam acquire that property by virtue of the effects that a bullet has just had on some as well: the closest direct male ancestor of everybody alive today. We could man's brain in Dodge City, over a thousand miles away. (In the days of the call him F-Chromosome Adam, since all our F-chromosomes pass down Wild West, there was a revolver nicknamed the Widowmaker. Whether a through the paternal line just the way our mitochondria pass through the particular revolver lived up to its nickname on a particular occasion might be maternal line.5 Was F-Chromosome Adam the husband or lover of Mito-a fact that could not be settled by any spatio-temporally local examination of chondrial Eve? Almost certainly not. There is only a tiny probability that its effects.) This case gets its curious capacity to leap through space and time these two individuals were alive at the same time. (Paternity being a much from the conventional nature of the relation of marriage, in which a past less time-and-energy-consuming business than maternity, what is
logically
historical event, a wedding, is deemed to create a permanent relation—
a
possible is that F-Chromosome Adam lived very recently, and was very, very
formal
relation—of interest in spite of subsequent wanderings and concrete busy in the bedroom—leaving Errol Flynn in his, um, dust. He could, in misfortunes (the accidental loss of a ring, or the destruction of the marriage principle, be the great-grandfather of us all. This is about as unlikely as the certificate, for instance.)

case in which F-Chromosome Adam and Mitochondrial Eve were a couple.) The systematicity of genetic reproduction is not conventional but natural, Mitochondrial Eve has been in the news recently because the scientists but that very systematicity permits us to think
formally
about causal chains who christened her think they can analyze the patterns in the mitochondrial extending over millions of years, causal chains that would otherwise be virtually impossible to designate or refer to or track. This permits us to become interested in, and reason rigorously about, even more distant and locally invisible relationships than the formal relationship of marriage. Spe-5. Note one important difference between the legacies of Mitochondrial Eve and Yciation is, like marriage, a concept anchored within a tight, formally defin-Chromosome Adam: we all, male and female, have mitochondria in our cells, but they able system of thought, but, unlike marriage, it has no conventional all come from our mothers; if you are male, you have a V-chromosome and got it from your father, but most—virtually all, but not quite all—females have no Y-chromosome at all.

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DNA of the different people alive today and deduce from that how recently existed! If Amy's granddaughters had all starved to death in infancy—as so Mitochondrial Eve lived, and even where she lived. According to their many infants did in those days—the same oblivion would be ours.

original calculations, Mitochondrial Eve lived in Africa, very, very recently—

The curious invisibility of the crown of Mitochondrial Eve in her own less than three hundred thousand years ago, and maybe less than half that.

lifetime is easier to understand and accept than the near-invisibility of what These methods of analysis are controversial, however, and the African Eve every species must have: a beginning. If species aren't eternal, then all of hypothesis may be fatally flawed. Deducing
where
and
when
is a far trickier time can be divided, somehow, into the times before the existence of species task than deducing
that
there was a Mitochondrial Eve, something that
x,
and all subsequent times. But what must have happened at the interface? It nobody denies. Consider a few of the things we already know about Mito-may help if we think of a similar puzzle that has baffled many people. Have chondrial Eve, setting aside the recent controversies. We know that she had you ever wondered, when hearing a new joke, where it came from? If you are at least two daughters who had surviving children. (If she had just one like almost everybody else I have ever known or heard of, you never make up daughter, her daughter would wear the crown of Mitochondrial Eve.) To jokes; you pass on, perhaps with "improvements," something you heard from distinguish her title from her proper name, let's call her Amy. Amy bears the someone who heard it from someone, who... Now, we know the process title of Mitochondrial Eve; that is, she just happens to have been the maternal cannot go on forever. A joke about President Clinton, for instance, cannot be founder of today's line of people.6 It is important to remind ourselves that
in
more than a year or so old. So who makes up the jokes? Joke-authors (as
all other regards,
there was probably nothing remarkable or special about contrasted with joke-purveyors) are invisible.7 Nobody ever seems to catch Mitochondrial Eve; she was certainly not the First Woman, or the founder of them in the act of authorship. There is even folklore—an "urban legend"—to the species
Homo sapiens.
Many earlier women were unquestionably of our the effect that these jokes are all created in prison, by prisoners, those species, but happen not to have any direct female lines of descendants dangerous and unnatural folks, so unlike the rest of us, and with nothing leading to people living today. It is also true that Mitochondrial Eve was better to do with their time than to fashion jokes in their secret underground probably no stronger, faster, more beautiful, or more fecund than the other joke-workshops. Nonsense. It is hard to believe— but it must be true—that women of her day.