Evolution: The Pleasures of Pluralism
by Stephen Jay Gould
harles Darwin
began the last paragraph of The Origin of Species (1859) with a famous
metaphor about life's diversity and ecological complexity:
It is interesting to contemplate an entangled bank,
clothed with many plants of many kinds, with birds singing on the bushes, with
various insects flitting about, and with worms crawling through the damp earth, and
to reflect that these elaborately constructed forms, so different from each other,
and dependent on each other in so complex a manner, have all been produced by laws
acting around us.
He then begins the final sentence of the book with an equally
famous statement: "There is grandeur in this view of life…."
For Darwin, as for any scientist, a kind of ultimate
satisfaction (Darwin's "grandeur") must reside in the prospect that so much variety
and complexity might be generated from natural regularities—the "laws acting
around us"—accessible to our intellect and empirical probing. But what is the
proper relationship between underlying laws and explicit results? The
"fundamentalists" among evolutionary theorists revel in the belief that one
overarching law—Darwin's central principle of natural selection—can
render the full complexity of outcomes (by working in conjunction with auxiliary
principles, like sexual reproduction, that enhance its rate and power).
The "pluralists," on the other hand—a long line of thinkers
including Darwin himself, however ironic this may seem since the fundamentalists
use the cloak of his name for their distortion of his position—accept natural
selection as a paramount principle (truly primus inter pares), but then
argue that a set of additional laws, as well as a large role for history's
unpredictable contingencies, must also be invoked to explain the basic patterns
and regularities of the evolutionary pathways of life. Both sides locate the
"grandeur" of "this view of life" in the explanation of complex and particular
outcomes by general principles, but ultra-Darwinian fundamentalists pursue one true
way, while pluralists seek to identify a set of interacting explanatory modes, all
fully intelligible, although not reducible to a single grand principle like natural
selection.
The first part
of this article outlined the general fallacies of ultra-Darwinian fundamentalism,
especially in the light of new theories and discoveries in the core disciplines of
developmental biology, paleontology, and population genetics.[1]
In this second and concluding part, I shall analyze a prominent philosopher's
influential but misguided ultra-Darwinian
manifesto—Darwin's
Dangerous Idea, by Daniel
Dennett. I shall also take up the methodology of
so-called "evolutionary psychology"—a field now in vogue as a marketplace for
ultra-Darwinian explanatory doctrine. Evolutionary psychology could, in my view,
become a fruitful science by replacing its current penchant for narrow, and often
barren, speculation with respect for the pluralistic range of available alternatives
that are just as evolutionary in status, more probable in actual occurrence, and not
limited to the blinkered view that evolutionary explanations must identify
adaptations produced by natural selection.
Daniel Dennett devotes the longest chapter in Darwin's Dangerous
Idea to an excoriating caricature of my ideas, all in order to bolster his defense
of Darwinian fundamentalism. If an argued case can be discerned at all amid the slurs
and sneers, it would have to be described as an effort to claim that I have, thanks
to some literary skill, tried to raise a few piddling, insignificant, and basically
conventional ideas to "revolutionary" status, challenging what he takes to be the
true Darwinian scripture. Dennett claims that I have promulgated three "false alarms"
as supposed revolutions against the version of Darwinism that he and his fellow
defenders of evolutionary orthodoxy continue to espouse.
Dennett first attacks my view that punctuated equilibrium is the
dominant pattern of evolutionary change in the history of living organisms. This
theory, formulated by Niles Eldredge and me in 1972, proposes that the two most
general observations made by paleontologists form a genuine and primary pattern of
evolution, and do not arise as artifacts of an imperfect fossil record. The first
observation notes that most new species originate in a geological "moment." The
second holds that species generally do not change in any substantial or directional
way during their geological lifetimes—usually a long period averaging five to
ten million years for fossil invertebrate species. Punctuated equilibrium does not
challenge accepted genetic ideas about the rates at which species emerge (for the
geological "moment" of a single rock layer may represent many thousand years of
accumulation). But the theory does contravene conventional Darwinian expectations
for gradual change over geological periods, and does suggest a substantial revision
of standard views about the causes of long-term evolutionary trends. For such
trends must now be explained by the higher rates at which some species branch off
from others, and the greater durations of some stable species as distinguished from
others, and not as the slow and continuous transformation of single populations.
In
his second attack, Dennett denigrates the importance of nonadaptive side consequences
("spandrels"
in my terminology) as sources for later and fruitful reuse. In principle, spandrels
define the major category of important evolutionary features that do not arise as
adaptations. Since organisms are complex and highly integrated entities, any adaptive
change must automatically "throw off" a series of structural byproducts—like the
mold marks on an old bottle or, in the case of an architectural spandrel itself, the
triangular space "left over" between a rounded arch and the rectangular frame of wall
and ceiling. Such byproducts may later be co-opted for useful purposes, but they
didn't arise as adaptations. Reading and writing are now highly adaptive for humans,
but the mental machinery for these crucial capacities must have originated as spandrels
that were co-opted later, for the brain reached its current size and conformation tens
of thousands of years before any human invented reading or writing.
Third, and finally, Dennett denies theoretical importance to the
roles of contingency and chance in the history of life, a history that has few
predictable particulars and no inherent directionality, especially given the
persistence of bacteria as the most
common and dominant form of life on Earth ever since their origin as the first
fossilized creatures some 3.5 billion years ago.[2] Bacteria
are biochemically more diverse, and live in a wider range of environments (including
near-boiling waters, and pore spaces in rocks up to two miles beneath the earth's
surface), than all other living things combined. The number of E. coli cells
in the gut of each human being exceeds the total number of human beings that have
ever lived. Moreover, if recent reports of Martian fossil bacteria are true, then
bacterial domination may be interplanetary or universal, and not merely earthly.
These three concepts work as pluralistic correctives to both the
poverty and limited explanatory power of the ultra-Darwinian research program.
Punctuated equilibrium requires that substantial evolutionary trends over geological
time, the primary phenomenon of macroevolution, be explained by the greater
long-term success of some species versus others within a group of species
descended from a common ancestor. Such trends cannot be explained, as Darwinian
fundamentalists would prefer, as the adaptive success of individual organisms in
conventional competition, extrapolated through geological time as the slow and
steady transformation of populations by natural selection. The principle of
spandrels, discussed at greater length later in this article, stresses the role
that nonadaptive side consequences play in structuring the directions and potentials
of future evolutionary change. Taken together, punctuated equilibrium and spandrels
invoke the operation of several important principles in addition (and sometimes
even opposed) to conventional natural selection working in the engineering mode
that Dennett sees as the only valid mechanism of evolution.
My third pluralistic corrective to traditional theory does not
invoke other principles in addition to natural selection, but rather stresses the
limits faced by any set of general principles in our quest to explain the
actual patterns of life's history. Crank your algorithm of natural selection to
your heart's content, and you cannot grind out the contingent patterns built
during the earth's geological history. You will get predictable pieces here and
there (convergent evolution of wings in flying creatures), but you will also
encounter too much randomness from a plethora of sources, too many additional
principles from within biological theory, and too many unpredictable impacts from
environmental histories beyond biology (including those occasional meteors)—all
showing that the theory of natural selection must work in concert with several
other principles of change to explain the observed pattern of evolution.
Since Dennett shows so little understanding of evolutionary
theory beyond natural selection, his critique of my work amounts to little more
than sniping at false targets of his own construction. He never deals with my
ideas as such, but proceeds by hint, innuendo, false attribution, and error. I
will cite concrete examples in four categories:
1. False assimilation to statements made by other authors.
Since Dennett can't nail me on his desired charges (for I never said the things he
wishes to lay upon me), he often invents a ridiculous interpretation, which he
attributes to others and not to me, and then hopes out loud that I never meant such
a thing. For example, Dennett invents and attributes to "some" writers an absurd
mischaracterization of my views on the Cambrian explosion, the short episode
(535-530 million years ago) when nearly all the major groups of animals make their
first appearance in the fossil record, including the creatures preserved in the
Burgess Shale:
Some say this misses Gould's point: "What is special
about the spectacular diversity of the Burgess Shale fauna is that these weren't
just new species, but whole new phyla! These were radically
novel designs!" I trust this was never Gould's point, because if it was, it was an
embarrassing fallacy of retrospective coronation.
It would be—but since I have never even hinted at such a
silly view, why bother to point out how stupid I would be if I ever had?
In an even more unfair example, Dennett conjectures about what
I might believe (but I don't, and he cites nothing to support his supposition),
and then seems to pretend that I hold such a view by attacking someone else who
truly does:
Is it likely that Gould could be so confused about
the nature of algorithms? As we shall see in chapter 15, Roger Penrose, one of
the world's most distinguished mathematicians, wrote a major
book (1989) on
Turing machines, algorithms, and the impossibility of Artificial Intelligence,
and his whole book is based on that confusion. This is not really such an
implausible error, on either thinker's part [I have now become an explicit supporter
of the idea]. A person who really doesn't like Darwin's dangerous idea often
finds it hard to get the idea in focus.
2. False characterization. Dennett depicts me as
constantly and explicitly claiming to invent one scientific "revolution" after
another. No characterization appears more frequently throughout the chapter, and
none could be so false. "Gould," he states, "has gone from revolution to revolution.
So far, his declarations of revolution have all been false alarms." We then learn
that "the spandrel revolution (against panadaptationism) and the exaptation
revolution (against preadaptationism) evaporate on closer inspection," and that
"Gould's attempted revolution against gradualism was actually his first." In a final
summary, we learn that "Gould's self-styled revolutions…all evaporate."
Of course, I am never quoted directly as making any personal
claim about a "revolution," self-styled or otherwise—and for a good reason: I
have, quite consciously and on principle, never used the word to describe my work.
I have too much admiration for my dear friend, the late Thomas Kuhn, and I have
also too often watched foolish and ambitious colleagues trying to wrap themselves in
a caricature of his "paradigm" in order to proclaim their own revolutions, ever to
engage in such a farce myself.
Dennett says that I started all this revolution-mongering in my
first paper on punctuated equilibrium, published in 1972. In that work, by contrast,
I paid homage to Kuhn's influence, and explicitly disavowed any such fatuous
intention: "We have no desire," I wrote (with Niles Eldredge) in 1972,
to enter the tedious debate over what is, or is
not,…a paradigm [in Kuhn's terminology]. In using the neutral word picture, we
trust that readers will understand our concern with alternate ways of seeing the
world that render the same facts in different ways.
The details of Dennett's discussions also rest largely on
ridicule and mischaracterization. For example, he denigrates the principle of
spandrels by mockery, not argument.
Gould wants to convince us that adaptation is not
"pervasive," so he needs to have a term for the (presumably many) biological
features that are not adaptations. They are to be called "spandrels." Spandrels
are, um, things that aren't adaptations, whatever they are.
But I have always defined spandrels precisely as one particular
and eminently testable kind of nonadaptive structure—an architectural
byproduct, or what Darwin called, in his own favorite and pluralistic example of
nonadaptation, a "correlation of growth."
Dennett's account of punctuated equilibrium is a farrago of false
charges, finally capped on the last page by a grudging glimmer of understanding that
the theory might be trying to say something interesting and new after all. We first
learn that "Gould has several times changed his mind about just what he and Eldredge
were claiming." We have, of course, often altered and expanded the theory in
recognizing further implications and dropping untenable corollaries—as any
active and interesting formulation of a scientific theory must continually do. The
basic principles have, however, remained intact and have, I believe, gained strength.
But Dennett cares little about this method of scientific work; he is only interested
in charging me with flimflam and backpedaling.
In particular, he claims that we turned the theory into a phony
revolutionary claim for abrupt, or "saltational," non-Darwinian change, i.e., the
sudden transformation of one ancestral species into another of markedly different
form. Of course, Dennett cannot quote us on this—because we never said such a
thing. He acknowledges my frequently published complaint that this charge is a
standard mischaracterization, and he quotes several of our rebuttals. But, for
Dennett, the situation remains entirely our fault: "Confusion on this score still
abounds, however, and Gould has had to keep issuing his disclaimers."
Dennett can't support his charge of revolution-mongering with
any quotation from me, but since he remains convinced of his claim, he quotes me in
supposed support, when I am obviously making an almost opposite point—a clear
illustration of the power of an idée fixe to trigger a misreading.
Dennett begins with the standard canard: "For a while, Gould was proposing that the
first step in the establishment of any (my emphasis) new species was a
doozy—a non-Darwinian saltation." To illustrate this claim, Dennett
quotes from a 1980 paper, in which I wrote:
Speciation is not always an extension of gradual,
adaptive allelic substitution to greater effect, but may represent, as Goldschmidt
argued, a different style of genetic change—rapid reorganization of the
genome, perhaps nonadaptive.
This statement still strikes me as entirely
reasonable, and I see no reason to modify it today. I said that some instances of
speciation—probably only a small percentage—proceed in this rapid mode.
"Not always" surely means "not all the time, but usually." It cannot mean
"never"—as Dennett charges in claiming that I attribute "any" speciation event
to the opposite mode of saltation. In other words, I am trying to carve out a modest
space for an unconventional mechanism at low frequency—and Dennett charges me
with advocating a revolution based on universal occurrence for this unorthodox
mode.
3. High density of error. A fair test can be made for a
nonprofessional's grasp of scientific material by noting the frequency of factual
errors in his descriptions of technical work. I do not claim that any of these
minor mistakes produces great distortion, but Dennett's high density of errors, on
easy points that only require accurate reading or copying, indicates an apparent
indifference to the vital details that build the history of life.
Dennett's account of my book Wonderful Life includes the
following errors in only four pages. He misstates the date of the Cambrian
explosion by 70 million years—"a time around six hundred million years ago
when the multicellular organisms really took off." The actual date is 530 million
years ago. He then states (a serious error this time) that C.D. Walcott based his
original Burgess Shale work on "literal dissection of some of the fossils"—when
I emphasize in my book (as a major theme of my narrative) that Walcott failed
precisely because he did not dissect the specimens, which he incorrectly
interpreted as squashed absolutely flat. Dennett then says twice that most of the
Burgess Shale species perished as rapidly as they arose—"most of them vanished
just as suddenly"—when I clearly state that we know nothing at all about the
manner of their dying, because they left no fossil record after the Burgess beds.
Finally, Dennett lists eight of the wonderful Burgess creatures in a single
sentence—and he spells three of their names wrong. I don't wish to harp on
trivialities but Dennett could legitimately accuse me of disrespectful inattention
if I listed Denet, Dawkuns, and Maynid Smith as the apostles of ultra-Darwinism.
4. Gratuitous speculation about motives. In Dennett's last
few pages he apparently feels he must figure out why I could be so obtuse (since he
doesn't regard me as stupid). "It might seem disingenuous," he writes, "for me not
even to mention the obvious 'rival' explanations crying to be considered: politics
and religion." Dennett has no clue about my political or religious views, and he
has never bothered to ask me—but did lack of data ever derail the
ultra-Darwinian game of adaptationist storytelling? So we first hear a little
red-baiting. Then, in a surprise turnaround, I become a closet theist who secretly
hates Darwinism because "Humanity cannot be special enough to matter if it is the
product of merely algorithmic processes." As evidence, Dennett writes:
Gould often quotes the Bible in his monthly columns,
and sometimes the rhetorical effect is striking. Surely, one thinks, an article
with this opening sentence has to have been written by a religious man: "Just as
the Lord holds the whole world in his hands, how we long to enfold an entire subject
into a witty epigram."
I do often quote the Bible as great literature, but this time I
was only citing a famous African-American folk song.
The fallacy of Dennett's argument also undermines his other
imperialist hope—that the universal acid of natural selection might reduce
human cultural change to the Darwinian algorithm as well. Dennett, following Dawkins
once again, tries to identify human thoughts and actions as "memes," thus viewing
them as units that are subject to a form of selection analogous to natural selection
of genes. Cultural change, working by memetic selection, then becomes as algorithmic
as biological change operating by natural selection on genes—thus uniting the
evolution of organisms and thoughts under a single ultra-Darwinian rubric:
According to Darwin's dangerous idea…not only
all your children and your children's children, but all your brainchildren and your
brainchildren's brainchildren must grow from the common stock of Design elements,
genes and memes…. Life and all its glories are thus united under a single
perspective.
But, as Dennett himself correctly and repeatedly emphasizes, the
generality of an algorithm depends upon "substrate neutrality." That is, the various
materials (substrates) subject to the mechanism (natural selection in this case)
must all permit the mechanism to work in the same effective manner. If one kind of
substrate tweaks the mechanism to operate differently (or, even worse, not to work
at all), then the algorithm fails. To choose a somewhat silly example that actually
played an important role in recent American foreign policy, the cold war "domino
theory" held that communism must be stopped everywhere because if one country turned
red, then others would do so as well, for countries are like dominos standing on
their ends and placed one behind the other—so that the toppling of one must
propagate down the entire line to topple all. Now if you devised a general formula
(an algorithm) to describe the necessary propagation of such toppling, and wanted
to cite the algorithm as a general rule for all systems made of a series of separate
objects, then the generality of your algorithm would depend upon substrate
neutrality—that is, upon the algorithm's common working, regardless of substrate
(similarly for dominos and nations in this case). The domino theory failed because
differences in substrate affect the outcome, and such differences can even derail
the operation of the algorithm. Dominoes must topple, but the second nation in a line
might brace itself, stay upright upon impact, and therefore fail to propagate the
collapse.
Natural selection does not enjoy this necessary substrate
neutrality. As the great evolutionist R.A. Fisher showed many years ago in the
founding document of modern Darwinism (The Genetical Theory of Natural
Selection, l930), natural selection requires Mendelian inheritance to be
effective. Genetic evolution works upon such a substrate and can therefore be
Darwinian. Cultural (or memetic) change manifestly operates on the radically
different substrate of Lamarckian inheritance, or the passage of acquired characters
to subsequent generations. Whatever we invent in our lifetimes, we can pass on to
our children by our writing and teaching. Evolutionists have long understood that
Darwinism cannot operate effectively in systems of Lamarckian inheritance—for
Lamarckian change has such a clear direction, and permits evolution to proceed so
rapidly, that the much slower process of natural selection shrinks to
insignificance before the Lamarckian juggernaut.
This crucial difference between biological and cultural evolution
also undermines the self-proclaimed revolutionary pretensions of a much-publicized
doctrine—"evolutionary psychology"—that could be quite useful if
proponents would trade their propensity for cultism and ultra-Darwinian fealty for
a healthy dose of modesty.[3]
Drawing on my preceding discussion, may I propose a good rule of
thumb in judging public announcements by scientists: always be especially suspicious
of claims for revolutionary status, especially since Thomas Kuhn reformulated the
history of science to make "revolution" the explicit object of every ambitious
scientist's fancy. Carol K. Yoon, a perceptive science writer for The New York
Times, cut through the rhetoric to the weak foundation below when she wrote:
"'It's a scientific revolution,' [a prominent supporter of evolutionary psychology]
said of this newly named science, which sometimes seems an uncomfortable mixture of
scientific method and cocktail party conversation."
Humans are animals and the mind evolved; therefore, all curious
people must support the quest for an evolutionary psychology. But the movement that
has commandeered this name adopts a fatally restrictive view of the meaning and
range of evolutionary explanation. "Evolutionary psychology" has, in short, fallen
into the same ultra-Darwinian trap that ensnared Daniel Dennett and his
confrères—for disciples of this new art confine evolutionary accounts to the
workings of natural selection and consequent adaptation for personal reproductive
success.
Evolutionary psychology, as a putative science of human behavior,
itself evolved by "descent with modification" from 1970s-style sociobiology. But the
new species, like many children striving for independence, shuns its actual ancestry
by taking a new name and exaggerating some genuine differences while ignoring the
much larger amount of shared doctrine—all done, I assume, to avoid the odor of
sociobiology's dubious political
implications and speculative failures (amid some solid successes when based on
interesting theory and firm data, mostly from nonhuman species).
Three major claims define the core commitments of evolutionary
psychology; each embodies a considerable strength and a serious (in one case, fatal)
weakness:
l. Modularity. Human behavior and mental operations can be
divided into a relatively discrete set of items, or mental organs. (In one prominent
study, for example, authors designate a "cheater detector" as a mental organ, since
the ability to discern infidelity and other forms of prevarication can be so vital
to Darwinian success—the adaptationist rationale.) The argument for modularity
flows, in part, from exciting work in neurobiology and cognitive science on
localization of function within the brain—as shown, for example, in the precise
mapping, to different areas of the cerebral cortex, of mental operations formerly
regarded as only arbitrarily divisible by social convention (production of vowels
and consonants, for example, or the naming of animals and tools).
Ironically, though, neurobiology and evolutionary psychology
employ the concept of modularity for opposite theoretical purposes. Neurobiologists
do so to stress the complexity of an integrated organ. Evolutionary psychology uses
modularity to atomize behavior into a priori, subjectively defined, and poorly
separated items (not known modules empirically demonstrated by neurological study),
so that selective value and adaptive significance can be postulated for individual
items, as the ultra-Darwinian approach requires.
2. Universality. Evolutionary psychologists generally
restrict their study to universal aspects of human behavior and mentality, thereby
explicitly avoiding the study of differences among individuals or groups. They
argue that variations among individuals, and such groups as races and social
classes, only reflect the influence of diverse environments upon a common
biological heritage. In this sense, they argue, evolutionary psychology adopts a
"liberal" position in contrast with the conservative implications of most previous
evolutionary arguments about behavior, which viewed variation among individuals and
groups as results of different, and largely unalterable, genetic constitutions.
I welcome much of this change; but, in one important respect,
this new approach to universals and differences continues to follow the old
strategy of finding an adaptationist narrative (often in the purely speculative or
storytelling mode) to account for genetic differences built by natural selection.
For the most-publicized work in evolutionary psychology has centered on the
universality in all human societies of a particular kind of difference: the
putative evolutionary reasons for supposedly universal behavioral differences
between males and females.
3. Adaptation. Evolutionary psychologists claim that
they have reformed the old adaptationism of sociobiology into a new and exciting
approach. They will no longer just assume, they now say, that all prominent and
universal behaviors must, ipso facto, be adaptive to modern humans in
boosting reproductive success. They recognize, instead, that many such behaviors
may be tragically out of whack with the needs of modern life, and may even lead
to our destruction—aggressivity in a nuclear age, for example.
Again, I applaud this development. If this principle were
advanced in conjunction with the recognition that a putative evolutionary origin
does not necessarily imply an adaptive value at all, then evolutionary psychology
could make a substantial advance in applying Darwinian theory to human behavior.
But the advocates of evolutionary psychology proceed in the opposite direction by
twisting the observation that the behavior of modern humans may not necessarily
have adaptive value into an even more dogmatic, and even less scientifically
testable, panadaptationist claim. Evolutionary universals may not be adaptive
now, they say, but such behaviors must have arisen as adaptations in the
different ancestral environment of life as small bands of hunter-gatherers on the
African savannas—for evolutionary theory "means" a search for adaptive
origins.
The task of evolutionary psychology then turns into a
speculative search for reasons why a behavior that may harm us now must once have
originated for adaptive purposes. To take an illustration proposed seriously by
Robert Wright in The Moral Animal, a sweet tooth leads to unhealthy
obesity today but must have arisen as an adaptation. Wright therefore states:
The classic example of an adaptation that
has outlived its logic is the sweet tooth. Our fondness for sweetness was designed for
an environment in which fruit existed but candy didn't.
This ranks as pure guesswork in the cocktail party mode;
Wright presents no neurological evidence of a brain module for sweetness, and no
paleontological data about ancestral feeding. This "just-so story" therefore
cannot stand as a "classic example of an adaptation" in any sense deserving the
name of science.
Much of evolutionary psychology therefore devolves into a search
for the so-called EEA, or "environment of evolutionary adaptation" that allegedly
prevailed in prehistoric times. Evolutionary psychologists have gained some
sophistication in recognizing that they need not postulate current utility to
advance a Darwinian argument; but they have made their enterprise even more fatuous
by placing their central postulate outside the primary definition of science—for
claims about an EEA usually cannot be tested in principle but only subjected to
speculation. At least an argument about modern utility can be tested by studying the
current impact of a given feature upon reproductive success. Indeed, the disproof
of many key sociobiological speculations about current utility pushed
evolutionary psychology to the revised tactic of searching for an EEA instead.
But how can we possibly know in detail what small bands of
hunter-gatherers did in Africa two million years ago? These ancestors left some
tools and bones, and paleoanthropologists can make some ingenious inferences from
such evidence. But how can we possibly obtain the key information that would be
required to show the validity of adaptive tales about an EEA: relations of kinship,
social structures and sizes of groups, different activities of males and females,
the roles of religion, symbolizing, storytelling, and a hundred other central
aspects of human life that cannot be traced in fossils? We do not even know the
original environment of our ancestors—did ancestral humans stay in one region
or move about? How did environments vary through years and centuries?
In short, evolutionary psychology is as ultra-Darwinian as any
previous behavioral theory in insisting upon adaptive reasons for origin as the
key desideratum of the enterprise. But the chief strategy proposed by
evolutionary psychologists for identifying adaptation is untestable, and therefore
unscientific. This central problem does not restrain leading disciples from
indulging in reveries about the ubiquity of original adaptation as the source of
revolutionary power for the putative new science. I detect not a shred of caution
in this proclamation by Wright—embodying the three principal claims of
evolutionary psychology as listed above:
The thousands and thousands of genes that influence
human behavior—genes that build the brain and govern neurotransmitters and
other hormones, thus defining our "mental organs" [note the modularity
claim]—are here for a reason. And the reason is that they goaded our ancestors
into getting their genes into the next generation [the claim for adaptation in the
EEA]. If the theory of natural selection is correct, then essentially everything
about the human mind should be intelligible in these terms [the ultra-Darwinian
faith in adaptationism]. The basic ways we feel about each other, the basic kinds
of things we think about each other and say to each other [note the claim for
universality], are with us today by virtue of their past contribution to genetic
fitness.
Wright's closing sermon is more suitable to a Sunday pulpit than
a work of science:
The theory of natural selection is so elegant and
powerful as to inspire a kind of faith in it—not blind faith,
really…. But faith nonetheless; there is a point after which one no longer
entertains the possibility of encountering some fact that would call the whole
theory into question. I must admit to having reached this point. Natural selection
has now been shown to plausibly account for so much about life in general and the
human mind in particular that I have little doubt that it can account for the
rest.
This adaptationist premise is the fatal flaw of evolutionary
psychology in its current form. The premise also seriously compromises—by
turning a useful principle into a central dogma with asserted powers for nearly
universal explanation—the most promising theory of evolutionary psychology:
the recognition that differing Darwinian requirements for males and females imply
distinct adaptive behaviors centered upon male advantage in spreading sperm as
widely as possible (since a male need invest no energy in reproduction beyond a
single ejaculation) and female strategies for extracting additional time and
attention from males (in the form of parental care or supply of provisions, etc.).
(In most sexually reproducing species, males generate large numbers of "cheap"
sperm, while females make relatively few, "energetically expensive" eggs, and then
must invest much time and many resources in nurturing the next generation.)
This principle of differential "parental investment" makes
Darwinian sense and probably does underlie some different, and broadly general,
emotional propensities of human males and females. But contrary to claims in a
recent deluge of magazine articles, parental investment will not explain the
full panoply of supposed sexual differences so dear to pop psychology. For
example, I do not believe that members of my gender are willing to rear babies
only because clever females beguile us. A man may feel love for a baby because
the infant looks so darling and dependent, and because a father sees a bit of
himself in his progeny. This feeling need not arise as a specifically selected
Darwinian adaptation for my reproductive success, or as the result of a female
ruse, culturally imposed. Direct adaptation is only one mode of evolutionary
origin. After all, I also have nipples not because I need them, but because
women do, and all humans share the same basic pathways of embryological
development.
If evolutionary psychologists continue to push the theory of
parental investment as a central dogma, they will eventually suffer the fate of
the Freudians, who also had some good insights but failed spectacularly, and
with serious harm imposed upon millions of people (women, for example, who were
labeled as "frigid" when they couldn't make an impossible physiological
transition from clitoral to vaginal orgasm), because they elevated a limited
guide into a rigid creed that became more of an untestable and unchangeable
religion than a science.
Exclusive adaptationism suffers fatally from two broad classes
of error, one external to Darwinian theory, the other internal. The external
error arises from fundamental differences in principle and mechanism between, on
the one hand, genetic Darwinian evolution and, on the other, human cultural
change, which cannot be basically Darwinian at all. Since every participant in
these debates, including Dennett and the evolutionary psychologists, agrees that
much of human behavior arises by culturally induced rather than genetically
coded change, giving total authority to Darwinian explanation requires that
culture also work in a Darwinian manner. (Dennett, as discussed earlier, makes
such a claim for cultural change in arguing for the "substrate neutrality" of
natural selection.) But for two fundamental reasons (and a host of other factors),
cultural change unfolds virtually in antithesis to Darwinian requirements.
First, topological: As the common metaphor proclaims,
biological evolution builds a tree of life—a system based upon continuous
diversification and separation. A lineage, after branching off from ancestors as
a new species, attains an entirely independent evolutionary fate. Nature cannot
make a new mammalian species by mixing 20 percent dugong with 30 percent rat and
50 percent aardvark. But cultural change works largely by an opposite process of
joining, or interconnection, of lineages. Marco Polo visits China and returns
with many of the customs and skills that later distinguish Italian culture. I
speak English because my grandparents migrated to America. Moreover, this
interdigitation implies that human cultural change needn't even follow genealogical
lines—the most basic requirement of a Darwinian evolutionary process—for
even the most distant cultural lineages can borrow from each other with ease. If
we want a biological metaphor for cultural change, we should probably invoke
infection rather than evolution.
Second, causal: As argued above, human cultural change
operates fundamentally in the Lamarckian mode, while genetic evolution remains
firmly Darwinian. Lamarckian processes are so labile, so directional, and so
rapid that they overwhelm Darwinian rates of change. Since Lamarckian and
Darwinian systems work so differently, cultural change will receive only limited
(and metaphorical) illumination from Darwinism.
The internal error of adaptationism arises from a failure to
recognize that even the strictest operation of pure Darwinism builds organisms
full of nonadaptive parts and behaviors. Nonadaptations arise for many reasons
in Darwinian systems, but consider only my favorite principle of "spandrels."
All organisms evolve as complex and interconnected wholes, not
as loose alliances of separate parts, each independently optimized by natural
selection. Any adaptive change must also generate, in addition, a set of
spandrels, or nonadaptive byproducts. These spandrels may later be "co-opted" for
a secondary use. But we would make an egregious logical error if we argued that
these secondary uses explain the existence of a spandrel. I may realize someday
that my favorite boomerang fits beautifully into the arched space of my living
room spandrel, but you would think me pretty silly if I argued that the spandrel
exists to house the boomerang. Similarly, snails build their shells by winding a
tube around an axis of coiling. This geometric process leaves an empty cylindrical
space, called an umbilicus, along the axis. A few species of snails use the
umbilicus as a brooding chamber for storing eggs. But the umbilicus arose as a
nonadaptive spandrel, not as an adaptation for reproduction. The overwhelming
majority of snails do not use their umbilicifor brooding, or for much of
anything.
If any organ is, prima facie, replete with spandrels,
the human brain must be our finest candidate—thus making adaptationism a
particularly dubious approach to human behavior. I can adopt (indeed I do) the
most conventional Darwinian argument for why the human brain evolved to large
size—and the nonadaptationist principle of spandrels may still dominate
human nature. I am content to believe that the human brain became large by
natural selection, and for adaptive reasons—that is, for some set of
activities that our savanna ancestors could only perform with bigger brains.
Does this argument imply that all genetically and biologically
based attributes of our universal human nature must therefore be adaptations? Of
course not. Many, if not most, universal behaviors are probably spandrels, often
co-opted later in human history for important secondary functions. The human
brain is the most complicated device for reasoning and calculating, and for
expressing emotion, ever evolved on earth. Natural selection made the human brain
big, but most of our mental properties and potentials may be spandrels—that
is, nonadaptive side consequences of building a device with such structural
complexity. If I put a small computer (no match for a brain) in my factory, my
adaptive reasons for so doing (to keep accounts and issue paychecks) represent a
tiny subset of what the computer, by virtue of inherent structure, can do
(factor-analyze my data on land snails, beat or tie anyone perpetually in
tic-tac-toe). In pure numbers, the spandrels overwhelm the adaptations.
The human brain must be bursting with spandrels that are
essential to human nature and vital to our self-understanding but that arose as
nonadaptations, and are therefore outside the compass of evolutionary psychology,
or any other ultra-Darwinian theory. The brain did not enlarge by natural
selection so that we would be able to read or write. Even such an eminently
functional and universal institution as religion arose largely as a spandrel if
we accept Freud's old and sensible argument that humans invented religious belief
largely to accommodate the most terrifying fact that our large brains forced us
to acknowledge: the inevitability of personal mortality. We can scarcely argue
that the brain got large so that we would know we must die!
In summary, Darwin cut to the heart of nature by insisting so
forcefully that "natural selection has been the main, but not the exclusive means
of modification"—and that hard-line adaptationism could only represent a
simplistic caricature and distortion of his theory. We live in a world of
enormous complexity in organic design and diversity—a world where some
features of organisms evolved by an algorithmic form of natural selection, some
by an equally algorithmic theory of unselected neutrality, some by the vagaries
of history's contingency, and some as byproducts of other processes. Why should
such a complex and various world yield to one narrowly construed cause? Let us
have a cast of cranes, some more important and general, others for particular
things—but all subject to scientific understanding, and all working together
in a comprehensible way. Why not admit for theory the same delight that Robert
Louis Stevenson expressed for objects in his "Happy Thought":
The world is so full of a number of things,
I'm sure we should all be as happy as kings.
Notes
- "Darwinian
Fundamentalism," The New York Review, June 12, 1997, pp. 34-37.
- See my recent book
Full
House (Crown, 1996) for an account of this.
- See such technical works as J. Barkow, L.
Cosmides, and J. Tooby, The Adapted Mind: Evolutionary Psychology and the Generation of
Culture (Oxford University Press, 1992); and D.M. Buss,
The Evolution of
Desire (Basic Books, 1994); and, especially, for its impact by good writing
and egregiously simplistic argument, the popular book of R. Wright,
The Moral Animal:
Why We Are the Way We Are: The New Science of Evolutionary Psychology
(Random House, 1994).