I had grown more interested in theory by the late 1950s, but down deep I was still possessed by an elemental self-image: hunter in the magical forest, searching not just for animals now but also for ideas to bring home as trophies. A naturalist, real and then more metaphorical, a civilized hunter, I was destined to be more of an opportunist than a problem solver. The boy inside still made my career decisions: I just wanted to be the first to find something, anything, the more important the better, but something as often as possible, to own it a little while before relinquishing it to others. I confess that to the degree I was insecure, I was also ambitious. I hungered for the recognition and support that discovery in science brings. To make this admission does not embarrass me now as it would have when I was young. All the scientists I know share a desire for fair recognition of their work. Acknowledgment is their silver and gold, and why they are usually very careful to grant deserved priority to others while so jealously guarding their own. New knowledge is not science until it is made social. The scientific culture can be defined as new verifiable knowledge secured and distributed with fair credit meticulously given.
Scientists, I believe, are divided into two categories: those who do science in order to be a success in life, and those who become a success in life in order to do science. It is the latter who stay active in research for a lifetime. I am one of them, and I suspect that all my fellow devotees in this category are also driven by a childhood dream of one kind or another, possibly closer to my own than I have guessed, for evolutionary biology has become the last refuge of the explorer naturalist.
So I hunted this time for abstract principles in one of the more poorly mapped domains of evolutionary biology. As a result of my experience in zoogeography, I was drawn by the idea of the fountainheads of evolution, places in which dominant groups rise and spread to the rest of the world. Serious discussion of the phenomenon had begun in 1915 with William Diller Matthew, curator in invertebrate paleontology at the American Museum of Natural History and later professor of paleontology at the University of California at Berkeley. In his monograph Climate and Evolution he constructed a picture of dominance in mammals and other vertebrate animals. Matthew instructed the reader to look at a north polar projection of the globe. Europe, Asia, and North America are so close together as to form a single supercontinent. Citing evidence from his own and previous studies of fossil mammals, he audaciously posited that the dominant groups arise on this supercontinent and spread outward, displacing formerly dominant groups southward to the peripheral regions of tropical Asia, Africa, and South America. In our own time, he noted, the winners include deer, camels, pigs, and the most familiar of the rats and mice, members of the family Muridae. Among the losers retreating to the edges of the world are horses, tapirs, and rhinoceroses. Like a true Aryan biologist, Matthew suggested that competitive superiority is the result of the adaptation of the dominant groups to the harsh, constantly changing environments of the northern latitudes.
Then along came Philip Darlington with a new twist, presented first in a 1948 article in the Quarterly Review of Biology and later in his 1957 full-dress treatise Zoogeography: The Geographical Distribution of Animals. Matthew, Darlington wrote, was only half right. The fossil remains he had studied were biased toward the Northern Hemisphere, where most collecting had been concentrated during the early years of paleontology. Thirty years after Climate and Evolution, Darlington argued, we have more data on fossils to examine, and these come from all over the world, including Matthew’s “peripheral” areas. In addition we must examine more carefully the evidence from the distribution of living groups, especially fishes, frogs, and other cold-blooded vertebrates, animals for which most of the new evidence is available. When we put all the pieces together, we see that the evolutionary crucible is not the north temperate land mass, but the Old World tropics. For the past 50 million years or so, groups of vertebrates have arisen in the vast greenhouse region that comprises southern Asia, sub-Saharan Africa, and, until recent geological times, much of the Middle East. The most dominant of the animal groups pressed on northward to Europe and Siberia, across the Bering Sea, a barrier periodically breached by the rise of isthmuses, and into the New World. People living in North America or Europe today need only look around to see the current hegemonic groups: cervid deer, canid dogs, felid cats, murid rats and mice, ranid frogs, bufonid toads, and other pioneer groups familiar to every child. Their species are extending their ranges into, not out of, the harsher climates.
I was enchanted by the idea of dominant animals and the succession of dynasties. There was a main center of evolution on the land, and Darlington seemed closer than Matthew to pinning it down. Either way, there remained a second question of major importance: What is the biological nature of dominance? More precisely, What hereditary traits cause an assemblage of species to spread into new lands and overwhelm the old endemics? The surrender of any group is all the more puzzling because endemics have had thousands or millions of years to become adapted to the habitats they occupy before the invaders appear.
This problem of the biological cause of dominance was not clearly in my mind as I began my own biogeographic study, a monograph on the ants of New Guinea and surrounding regions. But Matthew and Darlington, who never asked the question directly themselves, had primed me to formulate it. All I needed, I realize in retrospect today, was a small set of data to fall into place in order for the question to form somewhere in my subconscious. Then, driven by the power of the mythic conqueror archetype evoked by Matthew and Darlington, I would put together a tentative scenario, a story, and a phrase to capture it all, in the manner of Shakespeare’s muse,
And as imagination bodies forth
The forms of things unknown, the poet’s pen
Turns them into shapes, and gives to airy nothing
A local habitation and a name.
I asked the question and got a persuasive and verifiable answer because in the course of my taxonomic drudge-work I had sketched out on paper the geographic ranges of the ant species one at a time and possessed a large body of quantitative information. I knew what I was talking about. Matthew and Darlington had developed their images in a coarser resolution, at the level of entire genera and families of animal species. I understood the ants of the western Pacific, not all larger groups of land vertebrates of the world, but in more detail than they had available. As part of my omnibus approach, I had collected a great many data on the places the ant species lived, their nest sites, their colony size, what they ate, and anything else that might find use—somehow, I hoped, somewhere, someday. Because I considered all the information valuable in its own right, I had swept up specimens like a vacuum cleaner in the field and continued close work on them in the museum. My ultimate aim was to find interesting patterns of evolution, but I would have continued on to the end of my descriptive work even if I had expected nothing of superordinate value.
A pattern did emerge, however. Evolutionary biology always yields patterns if you look hard enough, because there are a hundred parameters and a thousand patterns awaiting examination. It came clear this time as I mapped ranges of one species after another. I saw that some of the ants were in the early stages of invading New Guinea and the eastern Melanesian archipelagoes. Other species, apparently survivors from older invasions, were splitting off as forms limited to one island or another. Some had fragmented into many such endemic residents. And still other ensembles of species were clearly in retreat, their populations now scattered here and there in pockets of island terrain. Finally, a small percentage had begun to expand again, this time from New Guinea. It dawned on me that the whole cycle of evolution, from expansion and invasion to evolution into endemic status and finally into either retreat or renewed expansion, was a microcosm of the worldwide cycle envisioned by Matthew and Darlington.
To find the same biogeographic pattern in miniature was a surprise then, although in retrospect it seems almost self-evident. But for some reason I just didn’t anticip
ate that particular sequence at the time. It came within a few minutes one January morning in 1959 as I sat in my first-floor office next to the entrance of the Biological Laboratories, sorting my newly sketched maps into different possible sequences—early evolution to late evolution. Which came first, which came late? I occasionally glanced up at the giant metallic rhinoceros outside the window and the intermittent stream of students and faculty walking into and out of the building. My mind drifted round and about, home, museum, field trips, lectures. I looked back down to the maps, and up again, and at some point the pattern became obvious, the only one possible.
Discovery of the cycle of advance and retreat was followed immediately by recognition of another ecological cycle. As I reflected on the expanding and retreating species, I drew on my memories of the long walks in New Guinea. I saw that the expanding, hence dominant, species are adapted for ecologically marginal habitats, in which relatively small numbers of ant species occur. Such places include the savannas, the monsoon forests, the sunny margins of lowland rain forest, and the salt-lashed beaches. They are marginal not just in having smaller numbers of ant species than the inland rain forests, but also in a purely geographic sense. Located near river banks and sea coast, they are staging areas from which it is easiest to disperse by wind and by floating vegetation from one island to another. The marginal species, I also realized, are most flexible in terms of the places in which they live. Because they face only a small number of competitors, they have been ecologically “released,” able to live in more habitats and in denser populations than would otherwise be possible. It seemed likely that these ants not only could move more easily but also would tend to press older native species back into the inner rain forests, reducing their dispersal power and shattering their populations into fragments prone to evolve into endemic species.
I knew I had a candidate for a new principle of biogeography. Though far from definitive, and attributable to only one animal group—ants—the concept is at least rested on solid data. I gathered my maps, stepped next door to the office of my old companion in Cuba, Grady Webster, spread the papers out, and recited my scenario. What did he think? Reasonable, he said, looks good. Congratulations! (What did he really think? It didn’t matter. I was too pleased with myself to worry.) Over the next few months I presented a full-dress review to the evolutionary biologists I considered most knowledgeable in zoogeography: Bill Brown, Phil Darlington, the geneticist Theodosius Dobzhansky, Ernst Mayr, and the senior entomologists Alfred Emerson, Carl Lindroth, and Elwood Zimmerman. This is the way it is done on the path to publication, especially by young scientists. And these luminaries all wrote back: Okay, they said, no obvious flaws that stand out.
I named the phenomenon the taxon cycle. Let me explain here that a taxon is any subspecies, species, or group of species, such as a genus, recognized in taxonomic classifications as being similar by virtue of common descent and labeled as such in taxonomic classifications. The grizzly bear, Ursus horribilis, being a species, is a taxon; so is the genus Ursus, containing the grizzly and all the other species of bear, including the black bear and brown bear, close enough to each other to be reasonably considered to share a recent common ancestor. I conjectured that if the principle held for species, it would hold for other taxa as well. In two articles, I refined my analysis.* The expanding species, I reported, have certain characteristics associated with life in the marginal habitats. The colonies are more populous and tend to nest in the soil rather than in decaying logs and tree limbs on the ground. The workers possess more spines on the body, an armament used against enemies in the open spaces of the marginal habitats. They orient more frequently by odor trails laid by scouts over the ground.
These traits are not the source of the dominance, however. They are only adaptations to life in the marginal habitats. I had no basis to infer the existence of special “dominance genes,” a powerful ichor flowing in the blood of warrior ants. All that mattered in the history of the fauna was a happenstance: the dominant species had become adapted to the marginal habitats, which served as potent dispersal centers. Like the people of some island civilizations, a few ant species achieve dominance simply by their ability to cross the sea.
The taxon cycle led me to reconsider a very old concept, that of the balance of Nature: when one species is established, eventually another species has to go. But the replacement is rarely so precise; in fact, nothing in evolution ever is. The principle is more correctly defined as a statistical generalization. If a hundred species invade a certain ecological guild, say night-flying fruit eaters or orchid-pollinating bees, roughly a hundred comparable species will disappear, with many exceptions accruing to special places and times. The rule was reinforced in my mind by the discovery of a simple relation between the area of each of the Melanesian islands and the number of ant species found in it. The greater the area, the larger the number of species. When I plotted the logarithms, the points formed an approximately straight line. I expressed the area-species curve simply as follows: S = CAZ, where S is the number of species found on the island, A is the area of the island, and C and z are fitted constants. In 1957 Darlington had expressed the same relation in the reptiles and amphibians of the West Indies not as an equation but as the following general rule: with each tenfold increase in island area, the number of species on the island doubles. There are, for example, approximately forty species of reptiles and amphibians on Jamaica, and eighty-five on the nearby island of Cuba, which has about ten times the area of Jamaica. His expression is more readily understood in the many cases in which it applies, but the logarithmic equation is the more precise and flexible expression and therefore more generally true.
I did not grasp its significance just then, but the area-species relation that Darlington and I had defined would soon lead to a deeper understanding of the balance of species diversity. In order to explain clearly and congenially how that next step was taken, however, I need first to describe certain developments that were unfolding in biology as a whole and in Harvard biology in particular during the 1950s and 1960s.
*E. O. Wilson and W. L. Brown, “The Subspecies Concept and Its Taxonomic Application,” Systematic Zoology 2(3) (1953): 97–111.
*W. L. Brown and E. O. Wilson, “Character Displacement,” Systematic Zoology 5(2) (1956): 49–64·
*“Adaptive Shift and Dispersal in a Tropical Ant Fauna,” Evolution 13(1) (1959): 122–144; “The Nature of the Taxon Cycle in the Melanesian Ant Fauna,” American Naturalist 95 (1961): 169–193.
chapter twelve
THE MOLECULAR WARS
WITHOUT A TRACE OF IRONY I CAN SAY I HAVE BEEN BLESSED with brilliant enemies. They made me suffer (after all, they were enemies), but I owe them a great debt, because they redoubled my energies and drove me in new directions. We need such people in our creative lives. As John Stuart Mill once put it, both teachers and learners fall asleep at their posts when there is no enemy in the field.
James Dewey Watson, the codiscoverer of the structure of DNA, served as one such adverse hero for me. When he was a young man, in the 1950s and 1960s, I found him the most unpleasant human being I had ever met. He came to Harvard as an assistant professor in 1956, also my first year at the same rank. At twenty-eight, he was only a year older. He arrived with a conviction that biology must be transformed into a science directed at molecules and cells and rewritten in the language of physics and chemistry. What had gone before, “traditional” biology—my biology—was infested by stamp collectors who lacked the wit to transform their subject into a modern science. He treated most of the other twenty-four members of the Department of Biology with a revolutionary’s fervent disrespect.
At department meetings Watson radiated contempt in all directions. He shunned ordinary courtesy and polite conversation, evidently in the belief that they would only encourage the traditionalists to stay around. His bad manners were tolerated because of the greatness of the discovery he had made, and because of its gathering aftermath. In the 1950s and
1960s the molecular revolution had begun to run through biology like a flash flood. Watson, having risen to historic fame at an early age, became the Caligula of biology. He was given license to say anything that came to his mind and expect to be taken seriously. And unfortunately, he did so, with a casual and brutal offhandedness. In his own mind apparently he was Honest Jim, as he later called himself in the manuscript title of his memoir of the discovery—before changing it to The Double Helix. Few dared call him openly to account.
Watson’s attitude was particularly painful for me. One day at a department meeting I naively chose to argue that the department needed more young evolutionary biologists, for balance. At least we should double the number from one (me) to two. I informed the listening professors that Frederick Smith, an innovative and promising population ecologist, had recently been recruited from the University of Michigan by Harvard’s Graduate School of Design. I outlined Smith’s merits and stressed the importance of teaching environmental biology. I proposed, following standard departmental procedure, that Smith be offered joint membership in the Department of Biology.
Watson said softly, “Are they out of their minds?”
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