More Articles on Evolution
Feeling for the Organism
Are living things nothing more than the sum of their gradually-evolved
parts?
Robert C. Berwick
Climbing Mount Improbable
Richard Dawkins
Norton, $25
There is a strong urge to begin and end a review of Richard Dawkins's
most recent book about evolution at its title: Climbing Mount Improbable.
Dawkins's metaphorical mountain provides a handy picture postcard for how he
believes evolution by natural selection works, and also locates his gene-centered
pulpit (he calls his third chapter "The Message from the Mountain").
Regrettably though, Climbing Mount Improbable does triple duty, characterizing
the plausibility of Dawkins's own ultra-Darwinian outlook. Evolution by natural
selection is, to be sure, a firmly established part of contemporary science-like
continental drift in geology, it's the explanatory bedrock for an entire field.
But Dawkins believes, further, that "all questions about life have the
same answer-natural selection." Such fundamentalist faith oversimplifies
the biological world and obscures important questions about the forces driving
evolution.
Adaptationism: The Uphill Climb
Dawkins sets out on seemingly safe ground. We get the stock
recipe: one part charmingly-written natural history; one part evolution as
the sole explanation for the "goodness of apparent design"; and
a dash of selfish genes and "biomorph programs"-computer algorithms
simulating the evolution of forms from spider webs to centipede segments-thrown
in for spice. At heart, though, Dawkins's book rests on mid-century alpine
imagery borrowed from evolutionary theorist Sewall Wright. Wright proposed
a now-classic picture of evolution as organisms scrambling uphill in an "adaptive
landscape." The terrain's height corresponds to fitness: organisms
reach for better vision, fleeter feet, or, from a gene's-eye view, simply
increased frequency of that particular gene over time. Dawkins adds to this
picture assumptions about the precise lay of the land and how organisms scale
slopes-intellectual inheritance from Darwin himself and the evolutionary theorist
R. A. Fisher's Genetical Theory of Natural Selection (1930)-to
arrive at the "the main lesson" of his book: that "the evolutionary
high ground cannot be approached hastily. Even the most difficult problems
can be solved, and even the most precipitous heights can be scaled, if only
a slow, gradual, step-by-step pathway can be found." Though the exquisite
designs of stems, pistils, eyes, jaws, wings, gills, and tentacles may all
seem improbable, we can understand them once we see how they emerged from
a long series of tiny improvements. Broken down into three main mountaineering
propositions, we have:
1. "[T]here can be no sudden leaps upward-no precipitous
increases in ordered complexity" (insect wings can't jump from stubs
to full-length flappers overnight).
2. There's no going downhill (species can't get worse as
a prelude to getting better).
3. There may be "more than one peak-more than one way
of solving the same problem" (eyes, Dawkins explains, or at least eye
lens and "camera body," have evolved independently 40 to 60 times).
A world of creatures all driven by selectional pressures, inching
upwards to their adaptive peaks-what's wrong with this picture? It's not immediately
obvious. Although the metaphor of evolution-as-gradual-mountain-climbing is
not fresh, it has endured. The notion of minute, step-by-step improvement
descends directly from the famous Linnaean dictum Natura non facit saltum
and Darwin's On the Origin of Species: "Extremely slight modifications
in the structure or habits of one inhabitant would often give it an advantage
over others." Evolutionary gradualism was further bolstered by R. A.
Fisher's mathematical marriage of Mendelism to natural selection-the so-called
"neo-Darwinian Modern Synthesis." Fisher showed that, under some
(strong) assumptions about population size and the underlying mechanism of
inheritance, even slight selective advantages could be sifted by natural selection
and accumulate over time to weave the "tangled bank" of complex
adaptations we see. Ever since, Wright's adaptive landscape has been a staple
of evolutionary texts, from Theodosius Dobzhansky's Genetics and the Origin
of Species (1937). Local hill-climbing does the trick-or so it seems.
The problem, then, isn't vintage-it is fundamentalism. Perhaps
like all fundamentalists, Dawkins over-simplifies: For all his talk of life's
complexity, he makes the biological world out to be much simpler than it actually
is. Left unchecked, this ultra-Darwinist faith in natural selection dissolves
into a doctrinal irrationality that rivals that of creationism, with a ready
answer for all logically possible outcomes.
The Constraining Terrain
One problem with paying exclusive attention to natural selection
is a corresponding inattention to physical constraints. If an insect needs
to clamber over an adaptive landscape, it's good to know what possible next
steps it can take, and what the terrain just ahead looks like-the "physical
channel". For example, many biological reactions take place on cell membranes.
Why? Because evolution by natural selection made it that way? That sounds
like creationist litany, and for good reason. The real explanation probably
follows from what Nobel laureate Manfred Eigen dubs the "coffee pot"
theorem: Membranes are required for the same reason that people set up coffee
pot stations-people cluster more readily around them. Specifically, the limiting
probability for two molecules to meet in three dimensions is vanishingly small,
but on a flat sheet-a membrane-the probability approaches near certainty.
Thus life lingers at a coffee break.
Similarly, why do poliovirus shells resemble geodesic domes?
The answer isn't "because evolution by natural selection made them that
way." The explanation, as molecular and developmental biologist Sydney
Brenner wrote recently, is fundamentally geometric. There are only a handful
of physically possible, symmetrical, space-enclosing shapes: pyramids, cubes
and octahedrons, the pentagonal- faced dodecahedron (12 pentagons glued together),
and the pentagon's dual cousin, the icosahedron (20 equilateral triangles
glued together). Plunk down a molecule on the corner of each triangle, and
one gets virus shells chunked into three-times-20 or 60-unit multiples. From
this inescapable fact of the physical world-its geometry, not the particular
environment in which polioviruses evolved-James Watson and Francis Crick long
ago predicted (correctly) that most spherical virus shells would come in 60-unit
packages.
Of course it may be that natural selection plays some role
in shaping spherical virus shells. Icosahedrons approximate spheres more closely
than pyramids, so selectional factors like "most nearly spherical shape"
or "optimal packing density" might enter in. Possibly this is the
sense in which virus forms are "explained by" natural selection,
if in fact they are. But even if so, it's a credit allocation question. Dawkins
gives DNA pride of place because it alone stores and passes on the informational
know-how to make membranes rather than something else. That's partly true-but
only partly. It's also true that evolution, the "blind watchmaker,"
stumbled onto membranes and icosahedrons because the physical world's regularity
constrains the landscape's "search space," and through this regularity
the physical world itself contributes to the information encoded in DNA.
Now, it would be a foolish biologist indeed who did not view
adaptive evolution by natural selection as the unifying theory lurking behind
the contrasting shape and texture of pine needles and oak leaves, as well
as the peacock's tail. But it would be equally foolish to deny the constraints
of living in the physical world. A decade ago, Dawkins devoted an entire chapter
of his book The Extended Phenotype to "constraints on perfection"-engineering
design tradeoffs that Climbing Mount Improbable still mentions but
largely sets to one side. In fact, all serious biologists, from Darwin to
Dawkins, have agreed that factors beyond natural selection play a central
role in evolution. Just how central? Turn to the last line of the article
"Evolution" in the latest edition of the Encyclopedia Britannica,
written by the population geneticist Francisco J. Ayala: "as a point
of departure" and for good or for ill, today's working evolutionary biologists
start with the "null hypothesis" that natural selection has
not occurred.
All Mutations Great and Small
So why the renewed fundamentalism? Perhaps the answer is
that Dawkins has now swallowed Darwin's and Fisher's gradualist assumptions
whole: add one extra layer of light-sensitive membrane, so the argument goes,
and an eye's photon-trapping improves by a fractional percent-a smooth incline
with no jumps or "surprises."
Though not literally true, this picture of a smoothly additive
world might be a good enough idealization-sufficiently good that evolutionary
biologists could dismiss deviations as noise. But is the biological world
really so simple? An alternative picture-a nonlinear world-seems to
hold considerable promise. Though "nonlinear" is now a fashionable
by-word, popping up in all those books about chaos and the stock market, it
also marks out a serious approach to a wide range of natural phenomena. Certainly,
when it comes to the electrical engineering side of my own profession, nonlinear
circuits have replaced linear ones, and nonlinear dynamics is the framework
of choice for describing natural systems of all kinds-from water disappearing
down a drain, to cirrus cloud formation, to grouses growing.
What does nonlinearity imply for hill climbing? The linear
alpine metaphor suggests that an insect scaling Mount Improbable can attain
optimal insecthood by independently pondering each factor that makes it a
functioning, adaptive whole, even if the insect does this by sending out multiple
"search teams" across hill and down dale in parallel. But the success
of this piece-wise strategy of self-improvement depends on a particularly
simple connection between changes in individual traits and improvements in
fitness-essentially a noninteractive, nonecological world.
Dawkins assumes just such a topography. His evolutionary hills
have gentle slopes, so that inching uphill always works. That follows Fisher
chapter and verse: picture each gene that contributes to better eyesight as
if it were one of millions upon millions of fine sand grains. Piling up all
that sand automatically produces a neatly conical sand pile with just one
peak, a smooth mound to climb. In this way, complex adaptations such as the
eye can always come about via a sequence of extremely small, additive changes
to their individual parts, each change selectively advantageous and so seized
on by natural selection.
The key question is whether the biological world really works
that way, or rather, how often it works that way. And that question
divides into two parts. Theoretically speaking: what works better as the raw
material or "step size" for adaptation-countless genes each contributing
a tiny effect, or a handful of genes of intermediate or large effect? Empirically
speaking: how does adaptation really play out in the biological world? Are
large mutations really always harmful, as Fisher argued? Do organisms usually
tiptoe in the adaptive landscape or take larger strides? Are adaptive landscapes
usually smooth sand piles, jagged alpine ranges, or something in between?
Fisher addressed the theoretical question via a mathematical
version of the "monkey wrench" argument: A large mutation would
be much more likely than a small one to gum up the works of a complex, finely-constructed
instrument like a microscope. It's not hard to see why. Once one is at a mountain
top, a large step is much more likely to lead to free-fall disaster. But the
microscope analogy can easily mislead. Fisher's example considers a mutation's
potential benefits in a particularly simple setting-precisely where there
is just one mountain top, and in an infinite population. But if I'm astride
K90 with Mt. Everest just off to the left, then a large step might do better
to carry me towards the higher peak than a small one. The more an adaptive
landscape resembles the Himalayas, with peaks crowded together-a likely consequence
of developmental interactions, which crumple the adaptive landscape, as we'll
see-the worse for Fisher's analogy. Small wonder then that Dawkins's topographic
maps and the gradual evolutionary computer simulations he invokes constantly
alter how mountain heights get measured, resorting to a single factor-first
for eyes, it's visual resolution; next, for spider webs, it's insect-trapping
effectiveness; then, for insect wings, it's aerodynamic lift or temperature-regulating
ability. An appropriate move, since hill-climbing is guaranteed to work only
if there's exactly one peak and one proxy for fitness that can be optimized,
one dimension at a time.
Even assuming a single adaptive peak, Fisher's microscope analogy
focuses on only half the evolutionary equation-variation in individuals, essentially
the jet fuel that evolution burns-and not the other half-the selective engine
that sifts variations and determines which remain written in the book of life.
Some 50 years after Fisher, the population biologist Motoo Kimura noted that
most mutations of small effect do not last: Because small changes are only
slightly selectively advantageous, they tend to peter out within a few generations
(ten or so). Indeed, most mutations, great or small, advantageous or not,
go extinct-a fact often brushed aside by selectional enthusiasts. Kimura calculated
that the rate at which a mutation gains a foothold and then sweeps through
a population is directly proportional to the joint effect of the probability
that the mutation is advantageous and the mutation's size.2
The upshot is that medium-scale mutations are much more likely to take hold
than minuscule Fisherian sand grains. Moreover, even if medium-scale changes
were less likely to fix in a population than "micromutations," by
definition a larger change will contribute correspondingly more to an organism's
overall response to natural selection than a small one and, as we will see,
there's real evidence from fruitflies that this happens.3
What then of the empirical issue? Four years ago, the evolutionary
biologists Allen Orr and Jerry Coyne found that the genetic evidence for the
role of micromutations as the source of adaptive differences between species,
such as color differences in fruitflies in desert environments, was surprisingly
thin-in the handful of verifiable examples (eight) drawn from the 1940s on,
which easily fit into a single half-page table in their original paper, four
were due to essentially one gene.4 More recently,
biologists have gathered evidence that mutations with medium- to large-scale
effects occur and, far from always being harmful as Fisher asserted, can even
play an important, beneficial role. For instance, fruitfly resistance to certain
insecticides seems to be caused by the alteration of a single "letter"
in the DNA sequence of a single gene. Even for more quantitative or countable
traits that have often been taken as the natural province of Fisher's additive-type
model-lots of genes with small effects piling up-recent evidence suggests
the contrary. Consider the bristle hairs on a fruitfly's abdomen-a fairly
sophisticated part of the fly's sensory system, and often taken as a "classic"
example of a quantitative trait. In 1995, Anthony Long, Susan Mulaney, Trudy
Mackay, and their colleagues showed that the number of abdominal hairs is
largely determined by just one to three genes, not dozens or hundreds.5
Moreover, the effects don't simply add up: if one factor contributes an average
of 2 bristle hairs by itself, and another pitches in two more, an additive
model would predict four hairs on average, but the two factors together produce
roughly six.
Interactions
All this is not to say that such intricate and highly functional
organs as eyes could emerge in one giant mutational leap, like Athena springing
forth from Zeus's forehead on the slopes of Mount Olympus. That seems exceedingly
unlikely. But the troubles for small mutations indicate one major stumbling
block for Dawkins's hill-climbing metaphor. So, too, do interactions among
traits.
We all know how hard it can be to solve a problem that depends
on lots of interacting parts: Imagine trying to tune a television picture
by simultaneously twiddling a million knobs at once. Evolution's in the same
boat. A trait may appear to have an intermediate optimum because it's correlated
with other traits that affect fitness in opposite directions, as in the classic
example of body size: A bigger body yields more offspring, but makes it harder
to escape predators. Tradeoffs again-but how to "solve" them?
Worse for aspiring alpinists, the biological world might not
be pleasantly additive. Suppose ecological interaction rules: more anteaters
mean fewer ants. Then trying to improve the whole organism by improving one
trait at a time can grind to a halt and the organism's fitness may not be
maximized. Stumbles become inevitable, as Dawkins rightly stresses: "ideal
outcomes are not the only possibility." But the situation is worse than
that. Fitness can even be minimized by natural selection-as our crumbling
spines attest. Evolution by natural selection in a finite population can result
in a decreased growth rate, and in some ecological settings lead to
a higher probability of extinction-about as nonadaptive as one could
imagine.
History and Evolutionary Tides
Bad backs are not, then, simply some quirky evolutionary
offshoot. Rather, in the real world-with natural selection and physical constraints,
in which large mutations sometimes dominate small, and improvements depend
on lots of interacting factors-nonadaptation itself is a central ingredient
in the evolution's ebb and flow.
It is all too easy to fall under the sway of natural selection
as Supreme Engineer, because it is a retrospective tautology that "the
mechanisms of evolution have, indeed, produced every result that has appeared
in evolution."6 Evidently, even Darwin was
susceptible. As many others have observed, far from purging the last vestige
of the anthropocentric Great Chain of Being, Darwin can be read as retaining
(a perhaps Victorian) "progressive," perfectionist" residue,
as he reveals in his autobiography: "Believing as I do that man in the
distant future will be a more perfect creature than he is now."7
So the perfectionist pyramid lives on-not only in those who
perpetually appeal to natural selection as Supreme Engineer, but also in those
who believe that, yes, if we rewound the evolutionary tape and played it all
over again, we would end up about where we are now, with intelligent creatures
like us to boot. Such unwavering faith reveals a misunderstanding about the
essentially stochastic and historical nature of evolutionary change. For evolution,
small and medium-size numbers matter-because of the slings and arrows of outrageous
sampling. By Mendel's "laws," the genes for, say, completely dominant
brown (B) and recessive blue eyes (b) should segregate out into exactly four
offspring as three brown (1 BB, 2 Bb) and one blue (bb). But as all parents
know, it's a pure stroke of luck for that to happen with just four children.
(Even Mendel's 929 pea plants had 705 purple and 224 white flowers, a 3.15:1
ratio, not the exact 3:1 ratio predicted by theory.) Because organisms often
have this kind of detailed structure, with differing groups of grouses at
different ages, fluctuations in age-specific birth and death rates lead to
enough variation in population numbers so that the likelihood of new mutations
taking over varies in a probabilistic way. The bottom line is that if we run
the evolutionary tape again, we aren't going to get the same "perfection"
we see now-that is, not unless one adopts very strong constraints on the space
of possible animals that, as far as we know from this book, Dawkins explicitly
disavows.
So historical contingency matters-we've got four limbs because
we're descended from four-lobed Crossopterygiian fish and evolution can select
only the better of possible alternatives-no silk purses from sow's ears. Evolution
is much like a chess game where the next move depends on the possible legal
next moves-the biological and physical constraints like icosahedron-as well
as the position one is at right now, summarizing the moves made up to the
present. But evolution is in worse shape than a chess player, because the
"search strategy" for the best next move is local hill climbing
without a goal-we don't have a teleological target like checkmate. Because
gradual hill-climbing evolution by natural selection can ascend only to local
peaks, and since evolution can't see ahead, and according to Dawkins, evolution
can't ever climb down again, our ant might climb up on a foothill and stay
stuck. Indeed, if evolution were really just one smooth, additive hike to
a single fitness peak, we might expect, echoing the Cole Porter song "You're
the Top," to see just one organism stand supreme as the Tower of Pisa.
We do not. Rather, evolution's more like an ant trundling over a crumpled
piece of paper, with the nooks and crannies revealing where the possible animals
can be.
The bottom line is that mathematical evolutionary biologists
as yet don't have any good general solutions to such nonlinear problems-and
they unabashedly say so. According to Alan Hastings and Gordon Fox, "the
equations of population genetics are complicated nonlinear equations, and
therefore general solutions, particularly of dynamic behavior [of evolution]
have not been found." Dawkins provides no such cautionary road signs.
We are treated to much less catholicism and certainly much less of the "controversy
and uncertainty" that ought to figure, as Dawkins's mentor John Maynard
Smith has written, in the best science writing. Instead Dawkins's faith in
simple hill-climbing seems boundless-as it must if one embraces adaptation-as-problem
solving and incremental hill-climbing as the only means to tackle evolutionary
design questions.
Genes or Organisms?
The same lack of sophistication-ultimately fundamentalism-infects
Dawkins's (in)famous selfish-gene conceit: that genes "manipulate the
world for their replication." For Dawkins, genes drive the explanatory
show. He even calls organisms "vehicles"-an old conceit that leads
us straight back to origins of 20th century genetics. By 1926, the geneticist
H. J. Muller could write that the rest of the cell was simply a "by-product"
of gene action: "its 'function' (its survival value) lies only in fostering
the genes."8 This conceit also simplifies
the evolutionary climb. The more direct the connection between gene and organism,
the easier for the gene to "drive" the vehicle-the organism.
Here again Dawkins oversimplifies. Not that his view is completely
off. Usually, one can think about evolution "one gene at a time,"
as John Maynard Smith, following R. A. Fisher, has written. A gene only
has to worry about how it contributes to "average fitness"-its own
frequency-and can ignore its neighbors'. And averages are additive. But it
doesn't follow that we can talk as if organisms weren't there, and as if all
genes were individually selfish. Genes' causal fingers touch the world only
indirectly through organisms' walls. The further we move out into the world
of interacting organisms, the more our adaptive explanations get couched as
differences among organisms, not genes. True, genes benefit and get implicated
in organisms' success, but genes don't necessarily figure in our explanation
of why things are they way they are.
Take one of Dawkins's own Mount Improbable success stories:
insect wing evolution. Do we need mention "gene" to explain why
stubby-winged insects produced more offspring than wingless varieties? No.
Only the ecological description of organism and environment is required. The
wing stubs were adaptations for the good of the insect, the genes benefited
indirectly. Of course DNA is necessary for evolution by natural selection;
it's just not always an equal partner in explanation. Dawkins recounts how
Joel Kingsolver and Mimi Koehl built hypothetical insect models to test whether
and when wing stubs could develop enough lift to get off the ground, but genes
were not part of this picture and not part of their counterfactual predictions
about the improvements in flight that would result from 1 millimeter changes
in wing size. You can apply the test yourself when you read Dawkins's book:
he opens and closes with the ever-fascinating story of the co-evolution of
figs and fig wasps, adapted from evolutionary biologist W. D. Hamilton's
1975-76 field work in Ribeirao Preto, Brazil. The figs "yearn" for
pollination; the fig wasps oblige, and themselves go through an elaborate
competition to see which male wasps will get there "fastest with the
mostest" to mate inside the fig-killing already-arrived males and mated
females if they can and pollinating the fig as a side-effect. To be sure,
the end result promotes both a fig and fig wasp's genome. But, if the explanations
and predictions about how figs and wasps have adapted rely on features of
whole figs and wasps, then the organisms are the players; if, on the other
hand, the story is told in terms of genes that happen to coexist alongside
other genes in figs and wasps, then we don't need the organisms for the explanation
and the explanatory game goes the other way.
Of course in some environments DNA does get directly selected
without an intervening body getting in the way-most obviously in the environments
of other genes. Not surprisingly, that's precisely where we do see evidence
of "selfish DNA" that, like a virus, says only "copy me,"
and does not benefit the organism itself.
As to whether DNA drives the biological show-as opposed to
the entire cell with its internal "skeleton" that serves as scaffolding
and meeting place, organelles, and detailed exterior "cortex"-history
again has some lessons for us. Dawkins's position belongs to a long tradition,
extending from the 1920s-era enthusiasm for genes as ultimate choreographers
and active "agents," to physicist Erin Schröedinger's famous
1944 "What Is Life?" proclamation that the chromosome contains "architect's
plan and builder's craft in one," to molecular biology's cybernetic lingo
and triumph in the 1960s, to the 1980s view of the gene as "computer
program" (or, as David Baltimore put it in 1984, the "cell's brain").
But much has changed over the past ten years. The deeper the biologists plumb
DNA and replace the tell-tale words "gene action" with "gene
activity," the further the image of DNA-as-agent seems to recede-so much
so that by 1991, even Scientific American announced the "news"
that "organisms control most of their genes."9
Thus "vulgar biology" appears to be turned on its
head: DNA is not self-replicating; only cells are properly self-replicating.
As Richard Lewontin has pointed out, newly-minted DNA is "a copy of the
old . . . but we do not describe the Eastman Kodak factory as a place of self-reproduction
[of photographs]."10 DNA doesn't produce
proteins; proteins produce DNA. And the complete DNA in the nucleus of a cell,
its genome, isn't a program for "computing the organism." Some now
dub DNA simply the "data" that the cell uses. For even a computer
program needs a computer to run it, but unlike a computer program, a genome
doesn't contain all the information about the required sequence and timing
of steps. Moreover, even a computer program requires particular hardware and
software to interpret what the program code means, and that means supplying
extra information. And what might that interpreter be? The most recent edition
of the popular Alberts-Watson textbook Molecular Biology of the Cell
names-you guessed it-the heroine of its title, the cell, as the computer.
If you still believe that DNA carries all the requisite instructions
to build organisms, consider Gunter Stent's gedanken experiment: Ship a cat's
DNA to the planet supposedly circling Pegasi 51 and see whether the creatures
on the other end can grind out a Felix. Not a chance. Or, to take a real example,
consider biologist Frank Solomon's discovery of "mitotic sisters":
Take two "mother" developing neural cells sitting next to each other,
each looking very much the same. Both cells obviously have identical genes,
and are sitting in virtually the identical external soup. The first cell divides,
yielding two sisters with a particular shape-a long extension fiber down and
then a short twist to the right. The second mother cell divides and its offspring
look very different from the first two, with short nerve fibers. Each pair
of sisters is shaped completely differently from its genetically identical
neighbors. Genes don't fix the "surface" traits of organisms-except
in conjunction with a complex, nonlinear waltz of external and internal cellular
positions, chemical gradients, and signaling environments. These
nonlinearities in moving from DNA sequences to organisms wrinkle the evolutionary
landscape even more. As Howard Patee memorably remarked, "life loiters
over two . . . spaces, the first alphabetic [DNA, the genetic code], the second,
zoological." We don't know-yet-what this mapping from genes to bodies
looks like. We are just beginning to tease apart the cascaded genetic control
sequences and feedback loops needed to assemble a fruitfly's eye. Yet it is
absolutely crucial for evolutionary theory to understand the possible range
of organisms that can spring from the platform of a developing egg. If turning
the genetic steering wheel one degree left can jerk the vehicle into a new
pothole, the evolutionary process becomes even more nonadditive.
To appreciate the complexity of moving from genes to organisms,
consider first the space of possible genes. Genes are DNA sequences, and DNA
uses a finite coding "alphabet" of just four letters (amino acids)
with the tags A, T, G, C, and a small number of words with exactly 3 letters
each (for example, ATG or AGC). DNA sequences are just strings of such words.
This space is effectively infinite, then, because one could simply go on forever,
building longer and longer "sentences"-bigger and bigger genes-though
physical limits intervene to bar indefinitely long sequences. Still, the set
of possible DNA sequences is countable: We can pair each sequence with a unique
integer: 1, 2, 3, etc.
We know far less about the space of possible organisms-what
Dawkins calls "Animal Space." But if we take literally Darwin's
claim that variation "extends continuously [my emphasis] in all
directions," then this space is infinite too, but bigger than the merely
countable infinity of possible genes. For every inch long worm, there can
be one half as long, another half again as long, and so on, with gradations
as fine as we wish. But that means the space of possible organisms may not
be computable-that is, a computer program might not be able to calculate it.
A fortiori, the function from genes to organisms that represents developmental
transformation-what biologists call "epigenesis"-may not be computable.
There may be no algorithm to characterize epigenesis. Statements trumpeting
the self-evidence of computability-Dawkins says "we are von Neumann machines"-are
sheer bluff. We might be. We might not. It is an open question.
Dawkins does take a step in the right (nonlinear) direction
in this respect. In the chapter "Kaleidoscopic Embryos," he opts
for a gene-to-organism mapping that multiplies "gene" values together-reaching,
as he says, for a model that works like a kaleidoscope. Gene effects can twist
about like bits of glass and then suddenly jump together into a novel pattern,
like an eye cup bending in instead of out. This is probably closer to the
real picture than the "additive" model. But once we allow for such
"jumps," we leave smooth hill climbing behind. Indeed, we know that
kaleidoscopic jumps must happen, because very small differences between organisms'
genes can get amplified into very large differences in what comes out of the
embryological oven. Chimps, for example, differ from people by a very tiny
amount of DNA, say 1 percent, and in surprising direction: chimps have forty-eight
chromosomes and we have forty-six. But only you and I can read this issue
of Boston Review.11 Having
embraced the kaleidoscope, does Dawkins now agree with Richard Lewontin that
"context and interaction are of the essence"?
Yes, But . . .
In the end, it is hard to tell where Dawkins comes out.
Once all the complications are at hand, Dawkins begins to resemble Captain
Corcoran in H.M.S. Pinafore, singing, "What never, well œ hardly ever."
One by one he backs off from his original alpinist assumptions.
Can there be only infinitesimally graded intermediates? Perhaps
not. We also have kaleidoscopic" embryos. But especially not if physical
possibilities are discontinuous, like the geometric shapes available to spherical
virus shells.
Does evolution never step downhill? Well, hardly ever. Dawkins
tinkers with another of Sewall Wright's innovations, the famous "shifting
balance" model, which one might caricature as clumping uphill with one's
weight first on the left foot and then rocking back onto the right foot. The
two feet correspond to two local, but distinct, populations, what Wright called
"demes." If one gets stuck on a local foothill with the right foot,
then it might be possible to rock back onto the left foot and continue uphill.
Can evolution of traits stall at the top of local foothills?
Sometimes. So Dawkins adopts "preadaptation." The insect wing models
suggest that the first insect wing stubs probably served as body temperature
regulators, rather than flying wings, given the insects' smaller size; only
later, after a burst in overall body size, could the wings develop enough
lift. Dawkins concludes this "teaches us a subtle new way, a kind of
sideways diversion, by which paths up Mount Improbable may be found."
In other words, the familiar story that an organ developed for one purpose
may later serve another.
Were eyes independently invented more than 40 times? Possibly
only once. The larger number tallies distinct lens-and-retina shapes, not
the basic light-processing machinery-a big difference, like counting the camera
lens and body but not the film. For as Dawkins himself writes, Walter Gerhing
has shown that eye photoprocessing apparently evolved just once and stayed
that way, right down to the finest molecular detail-the "film" biochemistry,
the structural and regulatory genes, even the molecular "chaperones"
that escort other proteins to erect the right "external scaffolding"
to build the photoreceptive machinery. From owls to the single-celled Euglena
gracillis with a carrot-colored eye spot: All this has been conserved,
seemingly back to the very first single-celled organisms that could see the
light. If so, then eye evolution seems more like a automotive redesign that
changes the chrome on the body, while leaving the engine untouched.
Indeed, it has become increasingly apparent that all organisms
also come equipped with roughly the same developmental tool kit that literally
builds us from stem to stern and front to back. This control system, a sequence
of genes that activates in a precise linear order, fixes the head-to-tail
orientation of growing embryos, and (like the eye developmental system) probably
evolved exactly once, prior to the last common ancestor of flies and vertebrates,
a half-billion years ago or more, and then stayed that way. Same for front
to back: When the 19th-century biologist Etienne Geoffrey-Saint Hillaire flipped
over a crayfish and dissected it, he discovered that its nerve cord, muscle,
digestive system, and heart were in the same top-to-bottom order as a human
being's. Thus we have the same body plan as a crayfish, but inverted: one
vertebrate's ceiling is another invertebrate's floor, and the reason why is
a common genetic developmental system. A great puzzle of modern biology is
how to reconcile the remarkable diversity of organisms we see with the equally
astonishing conservatism of the genetic developmental toolbox: as if there
really were only one organism, as if little kids were really made of snakes
and snails and puppy dog tails. The oft-castigated Bäuplane theorists
of the past century perhaps guessed rightly-but in a far more sophisticated
way, within the context of modern molecular biology.
With all these qualifications in mind, ponder Dawkins's central
evolutionary pledge and judge whether it reduces to a truism: "[I]f an
engineer looks at an animal or organ and sees that it is well designed to
perform some task, then I will stand up and assert that natural selection
is responsible for the goodness of apparent design. 'Magnets' or 'attractors'
in Animal Space cannot, unaided by selection [my emphasis], achieve
good functional design."
Even if we put to one side the tangles we've seen with "responsible
for" and "good functional design," with "unaided by selection"
as an escape clause, it is hard to see what this pledge amounts to. Along
with any other rational person, I assume that no living thing has come into
the world "unaided by selection." I can't understand a word that
you say "unaided by" my inner ear bones, which evolved from a fish
gill arch to a reptilian jaw residue. But that doesn't mean my inner ear bones
are "responsible for" sentence understanding.
What, then, of Dawkins and his met-aphorical mountain? The
scope and operation of evolution by natural selection remains a matter of
controversy. And when controversy emerges, Dawkins frustrates. As Dawkins
himself notes in his earlier work The Extended Phenotype, "I myself
admit to being irritated by a book that provokes me into muttering 'Yes but
. . . ' on every page, when the author could easily have forestalled my worry
by a little considerate explanation early on." In Climbing Mount Improbable
Dawkins seems to have abandoned his own advice. The "yes-buttery"
that he had so artfully condemned slips in at every page.
So read Climbing Mount Improbable for charming natural
history, and an introduction to evolutionary landscapes. But beware the slippery
slopes. "Charm," as Anthony Blanche in Brideshead Revisited
reminds us, "is the great English blight." The question is whether
Dawkins, like the narrator Charles Ryder in Brideshead, will forever
remain repainting pictures of English village architecture and South American
birds, or will go on to do something much more-feeling for the organism. n
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