Letters to a Young Scientist

by Edward O. Wilson

  In general, where close ecological equivalents met during the interchange, the North American elements prevailed. In this part of the world at least, Matthew’s theory was vindicated. The North American mammals also attained a higher degree of diversification, as measured by the number of genera. A genus is a group of related species and a group of genera is a family. The genus Canis, for example, comprises domestic dogs, wolves, and coyotes; other genera in the dog family Canidae include Vulpes (foxes), Lycaon (African wild dogs), and Speothos (South American bush dogs). During the interchange, the number of genera rose sharply in both North and South America and remained high thereafter. In South America it began at about seventy and has reached 170 at the present time. The swelling of numbers has come principally from speciation and radiation of the World Continent mammals after they arrived in South America. The old, pre-invasion South American elements were not able to diversify significantly in either North or South America. So the mammals of the Western Hemisphere as a whole now have a strong northern cast. Nearly half of the families and genera of South America belong to stocks that have immigrated from North America during the past 2.5 million years.

  Why did the northern mammals prevail? No one knows for sure. The answer has been largely concealed by complex events imperfectly preserved in the fossil record—the paleontologist’s equivalent of the fog of war. The question remains before us, part of the larger unsolved problem toward which our understanding of dynastic succession is directed. Evolutionary biologists keep coming back to it compulsively, as I did one night while camping at Fazenda Dimona, in the Brazilian Amazon, surrounded by mammals of World Continent origin. What comprises success and dominance?

  Success in biology is an evolutionary idea. It is best defined as the longevity of a species with all its descendants. The longevity of the Hawaiian honeycreepers will eventually be measured from the time the ancestral finchlike species split off from other species, through its dispersal to Hawaii, and finally to that time when the last honeycreeper species ceases to exist.

  Dominance, in contrast, is both an ecological and evolutionary concept. It is best measured by the relative abundance of the species group in comparison with other, related groups, and by the relative impact it has on the life around it. In general, dominant groups are likely to enjoy greater longevity. Their populations, simply by being larger, are less prone to sink all the way to extinction in any given locality. With greater numbers, they are also better able to colonize more localities, increasing the number of populations and making it less likely that every population will suffer extinction at the same time. Dominant groups often are able to preempt the colonization of potential competitors, reducing still further the risk of extinction.

  Because dominant groups spread farther across the land and sea, their populations tend to divide into multiple species that adopt different ways of life: dominant groups are prone to experience adaptive radiations. Conversely, dominant groups that have diversified to this degree, such as the Hawaiian honeycreepers and placental mammals, are on average better off than those composed of only a single species: as a purely incidental effect, highly diversified groups have better balanced investments and will probably persist longer into the future. If one species comes to an end, another occupying a different niche is likely to carry on.

  The mammals of North American origin proved dominant as a whole over the South American mammals, and in the end they remained the more diverse. Over two million years into the interchange, their dynasty prevails. To explain this imbalance, paleontologists have forged a widely held theory, an evolutionary-biologist kind of theory, in other words a rough consensus consistent with the largest number of facts. The fauna of North America, they note, was not insular and sharply different like that of South America. It was and remains part of the World Continent Fauna, which extends beyond the New World to Asia, Europe, and even Africa. The World Continent is by far the larger of the two landmasses. It has tested more evolutionary lines, built tougher competitors, and perfected more defenses against predators and disease. This advantage has allowed its species to win by confrontation. They have also won by insinuation, like raccoons and pack-forming wild dogs; many were able to penetrate sparsely occupied niches more decisively, radiating and filling them quickly. With both confrontation and insinuation, the World Continent mammals gained the edge.

  The testing of this theory, first conceived on a rough grand scale by William Diller Matthew and Philip Darlington, has just begun. Right or wrong, whether decisive in empirical support or not, its pursuit alone promises to link paleontology in interesting new ways to ecology and genetics. That synthesis will continue as the study of biological diversity expands in widening circles of inquiry to other disciplines, to other levels of biological organization, and across farther reaches of time. You have a place in it if animals and plants interest you in their own right, and especially if you like epics and the clash of worlds.

  The author identifying insects at an osprey nest, Florida Keys, March 19, 1968. Photograph by Daniel Simberloff.

  Nineteen

  THEORY IN THE REAL WORLD

  IT MAY SEEM to you that science, having grown so large and complex in fact and theory, would be a difficult profession to enter. Perhaps you worry that most of the opportunities in research and application are closed, that competition for the rest is tight and daunting, and most of the epics and big pictures have been filled in. You would be wrong. The researchers of my generation and others before you accomplished a lot. But they did not close all pathways and enter all unknown regions. Instead, they opened new ones. In science every answer raises more questions. I will ramp up that important truth to an exponential degree: in science every answer creates many more questions. Thus has it ever been, even before Newton held up a prism to a sunbeam and Darwin puzzled over variation among the Galápagos mockingbirds.

  It was also Newton who famously said, for all scientists into the future, “If I see further than others, it is by standing on the shoulders of giants.” I will now tell you a story of shoulders and giants.

  It could begin at any one of several times, but I will start on December 26, 1959, at the annual meeting of the American Association for the Advancement of Science, in Washington, D.C., when a mutual friend introduced me to Robert H. MacArthur. Robert (he resisted being called Bob) and I were relatively young. He was twenty-nine and I was thirty. We were both very ambitious, each searching self-consciously for the opportunity to make a major advance in science. MacArthur was brilliant. He was widely thought the new avatar of theoretical ecology, having already made several seminal advances. He was an avid naturalist and expert on birds, and in addition (very important in our case) an able mathematician. Thin, sharp in face and disposition, he had an intense and withdrawn manner that warned off fools. He was not the kind who placed hand on shoulder and slapped backs, nor did he often laugh out loud. Although we spent a great deal of time together, MacArthur and I never became close friends. Looking back today, I realize we never finished taking the measure of each other.

  His mentor at Yale, the first giant in this story, had been G. Evelyn Hutchinson, who was bringing ecology into the Modern Synthesis of evolutionary biology. He was famous for the earnest brilliance of his students. Under his tutelage, MacArthur had already made his mark by showing how complex ecological processes such as competition in community organization and the evolution of reproductive rates could be simplified into a form amenable to useful mathematical analysis. We were both, ten years later, to be elected to the U.S. National Academy of Sciences, also at an exceptionally young age. In 1972, at the peak of his creativity, MacArthur died of kidney cancer. Science was thus stripped of his future greatness, a huge loss.

  Coming together for meetings during the early 1960s, we both saw ecology and evolutionary biology as potentially one continuous discipline filled with opportunity for innovation in theory and field research. This was a new concept heralded by G. Evelyn Hutchinson. But we had another,
equally pressing motivation. By the 1960s, the revolution of molecular and cellular biology was already well under way. The second half of the twentieth century was clearly going to be their golden years, and one of the most transformative periods of all time in the history of science. Molecular biology and cellular biology were propelled not only by the extraordinary opportunities they provided for innovation, but also by the massive funding they received due to their obvious relevance to medicine.

  MacArthur and I understood clearly what was happening. We also saw that one negative result in science was the proportionate downgrading of our own disciplines, ecology and evolutionary biology. We had no equivalent of the double helix, no direct link to physics and chemistry, as did molecular and cellular biology. Rachel Carson’s seminal Silent Spring had been published in 1962, launching the modern environmental movement, which might have provided a nourishing source of funding equivalent to medicine, but that beneficence was still in its infancy. The new disciplines of conservation biology and biodiversity studies did not emerge until the 1980s.

  Furthermore, aside from population genetics and some very abstract principles of ecology, we had few ideas that could be solidly linked together in the expected manner of mature natural sciences. Molecular biologists and cellular biologists were filling faculty openings in research universities, unconcerned about biology at the levels of the organism and of the population. In their judgment, if they bothered to form one at all, our disciplines were old-fashioned and hopelessly unproductive. The frontiers of biology, it appeared, had shifted decisively leftward, in the direction of physics and chemistry. It was not so much that this new generation of biologists considered the old guard unimportant. It was more that they expected to do a better job of the research when, someday, they got around to it themselves. The pathways were there for MacArthur and me and other young ecologists, but they proved difficult to follow.

  My difficulties at Harvard were intensified by the fact that I was the only young tenured Harvard professor in what was later to be called organismic and evolutionary biology. The elder and more distinguished faculty members in the same disciplines were either wholly absorbed in tending their personal academic gardens or else in denial—aloof and disinclined to deal with the threat.

  The ultimate in noblesse non oblige was the venerable George Gaylord Simpson, the second giant in the story. He was a world authority in vertebrate paleontology and one of the authors of the Modern Synthesis. He had devised a brilliant picture of the evolution and movement of faunas around the world. But his withdrawal from engagement with others was legendary. Aging and ill by the time he came to Harvard, crippled by a falling tree during a recent visit to the Amazon, he preferred to work alone in his office deep in the bowels of the Museum of Comparative Zoology. When on one occasion Robert MacArthur visited the Department of Biology, I made an appointment for him to see Simpson. A meeting of first-rate minds, I thought, across the generations. I escorted him to the great man’s office, then left the two alone so as not to intrude on their conversation. (I expected to hear all about it later anyway.) I returned to my office and began to catch up on some paperwork. Scarcely fifteen minutes later MacArthur reappeared at my door. “He hardly said a word,” Robert reported. “He just refused to talk.”

  Simpson’s taciturnity, and from my viewpoint his indifference toward addressing the intellectual imbalance of biology at Harvard, had already played a role in the introduction of the term “evolutionary biology.” In 1960, the faculty members of the Department of Biology working on ecology and evolution, being outgunned and outfunded and soon to be outnumbered, decided to form a committee to organize and unify our efforts. I arrived early for the first meeting, and soon was followed by Simpson, who sat across from me (silently) to await our colleagues.

  “What shall we call our subject?” I ventured.

  “I have no idea,” he responded.

  “What about ‘real biology’?” I continued, trying for humor. Silence.

  “Whole-organism biology?”

  No response. Well, those were bad ideas anyway.

  There was a pause, then I added, “What do you think of ‘evolutionary biology’?”

  “Sounds all right to me,” Simpson said, perhaps just to keep me quiet.

  Other committee members began to file in, and when all were settled, I seized the opportunity to assert, “George Simpson and I agree that the right term for the overall subject we represent is ‘evolutionary biology,’” the name I had made up on the spot.

  Simpson said nothing, whereupon our group became the Committee on Evolutionary Biology. In time it grew to be the official Department of Organismic and Evolutionary Biology. Thus was born the name of a scientific discipline. If there was an earlier independent birth elsewhere, and I’ve heard of none, at least the most influential use of the name was made at a time when it was most needed.

  Envy and insecurity are among the drivers of scientific innovation. It won’t hurt if you have a dose of them also. For MacArthur and myself, the desire to create a new theory was reinforced by the recognition that what we were now calling evolutionary biology, and its more quantitative subdivision of population biology, required a rigor comparable to that of molecular and cellular biology. We needed quantitative theory and definitive tests of the ideas spun from the theory and vivid connections to real-life phenomena. Such hallmarks of excellence were relatively sparse in the subjects our efforts addressed. It was time to search for them in a focused manner.

  I spoke to MacArthur about islands I had visited around the world, and their use in studying the links between the formation and geography of species. I could see that he was not thrilled by the complexity of the subject. He became much more interested in the species-area curves that I had also been plotting. These displayed in a simple form the geographic areas (as in square miles or square kilometers) of islands in different archipelagoes of the world, principally the West Indies and western Pacific, and the number of bird, plant, reptile, amphibian, or ant species found on each island. We could see plainly that with an increase of area from one island to the next, the number of species increased roughly to the fourth root. This means, for example, that if one island in an archipelago is ten times the size of another in the same archipelago, it would contain approximately twice the number of species. We also observed that islands more distant from the mainland had fewer species than those close by.

  When I talked about equilibrium I spoke of the islands near and far as being “saturated.” MacArthur said, “Let me think about this for a while.” I trusted him to come up with something. I’d already seen evidence of MacArthur’s ingenuity in breaking down complex systems into simpler ones.

  MacArthur soon wrote a letter to me in which he postulated the following:

  Start with an empty island. As it fills up with species there are fewer species available from other islands to arrive as immigrants, and so the rate of immigration falls. Also, as the island fills up with species, it becomes more crowded and the average population size of each species decreases. As a result the rate of species extinction rises. Therefore, as the island fills up, the immigration rate falls, and the extinction of the species already present rises. Where the two curves cross, the extinction rate equals the immigration rate, and the number of species is at equilibrium.

  To continue, on small islands the crowding of the species is more severe, and the extinction rate curve is steeper. On distant islands, immigration is less, and the immigration curve less steep. In both cases the result is a smaller number of species at equilibrium.

  In 1967, MacArthur and I applied this simple model with every scrap of data on related subjects in ecology, population genetics, and even wildlife management we could find, and fitted it together, as best we could, in The Theory of Island Biogeography. The book enjoyed and continues to enjoy considerable influence in the disciplines from which it was constructed. It also played a role in the creation of the new discipline of conservation biology durin
g the decades to follow. It was a good example of the principle I’ve urged you to remember: in research define a problem as precisely as possible, and choose if need be the one or two partners needed to solve it.

  Even so, I wasn’t completely satisfied with our product. I asked myself even while it was unfolding, how can we put such theory to a test? The equilibrium we envisioned might require centuries to achieve. So, how does one conduct an experiment with Cuba, Puerto Rico, and the other islands of the West Indies? One doesn’t. Instead one looks for another, more tractable system. You may recall another principle of scientific research I offered you in an earlier letter. It is that for every problem there exists a system ideally suited for its solution. In biology the system is usually an organism of a particular species, such as the bacterium Escherichia coli for problems in molecular genetics. I was looking for something located higher in the scale of biological organization. I needed an ideal ecosystem.

  I was driven by two intense desires. I wanted to go on working on islands, whatever the excuse. And I wanted to do something radically new in biogeography. I reasoned that I might accomplish both if I chose an ecosystem small enough to be manipulated.

  A solution then presented itself. Insects—my specialty—are almost microscopic in size compared to the mammals, birds, and other vertebrates that had been featured in earlier biogeographic studies. They weigh a few milligrams or less, where vertebrates are measured in grams or more. There are large numbers of tiny islands on which insects can live and breed for generations. Instead of just one or several islands the size of Cuba, Barbados, or Dominica, where birds and mammals can be studied, there are hundreds of thousands of islands around the world with an area of a hectare or less. Somehow, I thought, the insect, spider, and other invertebrate faunas of a few could be altered so that the rates of immigration and extinction onto them could be measured. From these data multiple tests could be devised to test hypotheses, to evaluate theory itself, and to discover new phenomena.