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

92 THE TREE OF LIFE
that reproduce asexually, groupings of one sort or another may interest us for
various good reasons—groupings of shared morphology or behavior or of
genetic similarity, for instance—and we might choose to call the resulting
group a species, but there may very well be no theoretically important sharp
edges that would delimit such a species. So let us concentrate on sexually
reproducing species, all of which are to be found up in the multicellular
fronds of the Tree. How might we go about coloring all the life-lines of a
single such species red? We could start by looking at individuals at random
until we found one with lots of descendants. Call her Lulu, and color her red.
(Red is represented by the thick lines in figure 4.5.) Now move stepwise up
the Tree, coloring all Lulu's descendants red; these will all be members of
one species unless we find our red ink spreading into two distinct higher
branches, none of whose members form junctions across the void. If that
happens, we know there has been speciation, and we will have to back up and
make several decisions. We must first choose whether to keep one of the
branches red (the "parent" species continues red and the other branch is
considered the new daughter species ) or to stop the red ink altogether as
soon as the branching happens (the "parent" species has gone extinct,
fissioning into two daughter species).
If the organisms in the branch on the left are all pretty much the same in
appearance, equipment, and habits as Lulu's contemporaries, while the organisms
in the right branch almost all sport novel horns, or webbed feet, or
stripes, then it is pretty obvious that we should label the left branch as the
continuing, parent species, and the right branch the new offshoot. If both
branches soon show major changes, our color-coding decision is not so
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
more to being a species than being one of these branches of interbreeding
organisms, and nothing more to being the conspecific of some other organism
(contemporary or not) than being part of the same branch. The choice we
make will then have to depend on pragmatic or aesthetic considerations: Is it
ungainly to keep the same label for this branch as for its parent trunk? Would
it be misleading for one reason or another to say the branch on the right
rather than the branch on the left was the new species? 2
2. The cladists (whose views will be briefly discussed later) are a school of taxonomists
that reject, for various reasons, the concept of a "parent" species' persisting. Every
speciation event, in their terms, results in a pair of daughter species and the extinction
of their common parent, no matter how closely one surviving branch resembles the
parent, compared with the other branch.
Color-coding a Species on the Tree 93
FIGURE 4.5
The same sort of quandary faces us when we try to complete the task of
color-coding the whole species by carrying our red ink down the Tree to
include all Lulu's ancestors. We will encounter no gaps or joints on this
downward path, which will take us all the way to the prokaryotes at the base
of the Tree if we persist. But if we also color sideways as we go down, filling
in the cousins, aunts, and uncles of Lulu and her ancestors, and then color up
from these sideways spreaders, we will eventually fill in a whole branch on
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
to be colored, but only arguable, for there are, again, no hidden facts, no
essences that could settle the issue. As Darwin pointed out, if it weren't for
the separations that time and the extinction of the intermediate steppingstones
has created, although we could put the life forms into a "natural