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

102 THE TREE OF LIFE
bears only a passing resemblance to the intricate machinery of the codon
sequences on DNA molecules. But thanks to these deliberate simplifications,
their models are computationally tractable, enabling them to discover and
confirm many large-scale patterns in gene flow that would otherwise be
utterly invisible. Adding complications would tend to bring their research to
a grinding halt. But is their research good science? Crick replied that he had
himself thought about the comparison, and had to say that population
genetics wasn't science either!
My tastes in science are more indulgent, as perhaps you would expect
from a philosopher, but I do have my reasons: I think the case is strong that
not only do "over"-simplified models often actually explain just what needs
explaining, but no more complicated model could do the job. When what
provokes our curiosity are the large patterns in phenomena, we need an
explanation at the right level. In many instances this is obvious. If you want
to know why traffic jams tend to happen at a certain hour every day, you will
still be baffled after you have painstakingly reconstructed the steering, braking,
and accelerating processes of the thousands of drivers whose various
trajectories have summed to create those traffic jams.
Or imagine tracing all the electrons through a hand calculator as it multiplies
two numbers together and gets the correct answer. You could be 100
percent sure you understood each of the millions of causal microsteps in the
process and yet still be utterly baffled about why or even how it always got
the right answer to the questions you posed it. If this is not obvious, imagine
that somebody made—as a sort of expensive prank—a hand calculator that
usually gave the wrong answers! It would obey exactly the same physical
laws as the good calculator, and would cycle through the same sorts of
microprocesses. You could have perfect explanations of how both calculators
worked at the electronic level, and still be utterly unable to explain the
intensely interesting fact that one of them got the answers right and the other
got them wrong. This is the sort of case that shows what would be silly about
the preposterous forms of reductionism; of course you can't explain all the
patterns that interest us at the level of physics (or chemistry, or any one low
level). This is undeniably true of such mundane and unperplexing
phenomena as traffic jams and pocket calculators; we should expect it to be
true of biological phenomena as well. (For more on this topic, see Dennett
1991b.)
Now consider a parallel question in biology, a textbook standard: why do
giraffes have long necks? There is one answer that could in principle be
"read off" the total Tree of Life, if we had it to look at: Each giraffe has a neck
of the length it has because its parents had necks of the lengths they had, and
so forth back through the generations. If you check them off one by one, you
will see that the long neck of each living giraffe has been traced back
through long-necked ancestors all the way back... to ancestors who didn't
Patterns, Oversimplification, and Explanation 103
even have necks. So that's how come giraffes have long necks. End of explanation.
(And if that doesn't satisfy you, note that you will be even less
satisfied if the answer throws in all the details about the individual developmental
and nutritional history of each giraffe in the lineage.)
Any acceptable explanation of the patterns we observe in the Tree of Life
must be contrastive: why do we see this actual pattern rattier than that one—
or no pattern at all? What are the nonactualized alternatives that need to be
considered, and how are they organized? To answer such questions, we need
to be able to talk about what is possible in addition to what is actual.
CHAPTER 4: There are patterns in the unimaginably detailed Tree of Life,
highlighting crucial events that made the later flourishing of the Tree possible.
The eukaryotic revolution and the multicellular revolution are the most
important, followed by the speciation events, invisible at the time, but later
seen to mark even such major divisions as those between plants and animals.
If science is to explain the patterns discernible in all this complexity, it must
rise above the microscopic view to other levels, taking on idealizations when
necessary so we can see the woods for die trees.
CHAPTER 5: The contrast between the actual and the possible is fundamental
to all explanation in biology. It seems we need to distinguish different grades
of possibility, and Darwin provides a framework for a unified treatment of
biological possibility in terms of accessibility in "the Library of Mendel," the
space of all genomes. In order to construct this useful idealization, we must
acknowledge and then set aside certain complications in the relations
between a genome and a viable organism.