Letters to a Young Scientist (16 page)
In an early stage of pheromone research a biochemist, my friend John Law, and I set out to identify the trail substance used by the imported fire ant, which by that time had become one of the more noxious insect pests of the American South. We thought that in order to have plenty of the pheromone we should collect tens of thousands or even hundreds of thousands of the ants for extraction of the critical substance. That seemed quite practicable, because each fire ant colony contains upward of two hundred thousand workers. And I happened to know a way to gather that many fire ants quickly and efficiently. The imported fire ant, as a native to the floodplains of South America, has a unique way to avoid rising water. When the ants sense the approach of a flood from around and below them, they move to the surface of the nest, carrying with them all the young of the colony—the eggs, the grublike larvae, and the pupae—while nudging the mother queen upward as well. When the water reaches the nest chambers, the workers form a raft of their bodies. The whole colonial mass then floats safely downstream. When the ants contact dry land, they dissolve their living ark and dig a new nest.
It occurred to me that if we simply excavated fire ant nests and dumped them and the soil into nearby pools of water, the colony would rise to the surface and gather as an ant-pure raft while the dirt settled to the bottom. We tried this crude method on roadsides outside Jacksonville, Florida, and it worked. We came back with the requisite one hundred thousand worker ants (roughly estimated, not counted!) and my hands covered with itching welts from the stings of many angry ants.
Back at Harvard in Law’s laboratory, the search for the fire ant trail pheromone at first went well. The crucial substance appeared to be a relatively simple molecule—a terpenoid—and its complete molecular structure seemed within reach. Then came frustration, and a mystery. As the chemists attempted to purify the substance in order to characterize it definitively, and we proceeded to assay the reactions they produced by laying artificial trails in the laboratory, the response to the fraction supposedly containing the pheromone grew progressively weaker. Was the pheromone an unstable compound? Thinking that to be a good possibility, and concluding that the substance probably couldn’t be identified with the equipment and material available, we quit. To help others making the attempt we published a note in the science journal
Nature
—one of the few articles the editors have ever accepted that reported a failed experiment.
Years later, Robert K. Vander Meer, a natural product chemist working on fire ant pheromones in Florida, discovered the reason for our failure. The trail substance, it turned out, is not a single pheromone, but a medley of pheromones, all released from the sting onto the ground. One attracts nestmates of the trail layer, another excites them into activity, and still another guides them through the active space created by the evaporating chemical streaks. All of the components need to be present to evoke the full response in a fire ant worker seen in the field and laboratory. By not realizing this complexity, and thereby taking aim only at one of the components, we had failed to identify any of them.
In the 1960s and 1970s research on pheromones deepened and expanded, becoming an important part of the new discipline of chemical ecology. Researchers worked out with increasing accuracy what proved to be the complex pheromone codes of ant and honeybee colonies. Our theory of engineering by natural selection proved out well. However, recognizing that we had dealt with biology, and the independent events of natural selection, the correlations we proposed were only roughly met. A few strange, idiosyncratic exceptions were found, some of which to this day await further theory and experimental testing.
Ecosystems, with their rich complexes of interacting plants, animals, fungi, and microorganisms, came to be seen in a new way and the theories that guide ecology were altered accordingly. There was a different sensory world to be understood, one wholly invisible to human sight and hearing. The signals are in the air, spread over the ground, and beneath in the soil, and in pools of water. They form a crisscrossing of odors and scents, a riot of voices unheard by us that variously broadcast, threaten, or summon:
Check me as I approach you, I am a member of your colony. I have discovered an enemy scout, now hurry, follow me. I am a plant whose flowers have opened up this night and I wait here for you, come to me for a meal of pollen and nectar. I am a female cecropia moth calling, so if you are a male cecropia moth, follow my scent upwind, come to me. I am a male jaguar, alone on my territory, if you have detected this scent, you are trespassing, so get out, get out now
.
By science and technology we have entered this world, but we have only begun to explore it. Only when it becomes better known will we gain a part of the knowledge needed to understand how ecosystems are put together and, from that, how to save them.
Now I hope you see how theories are made, and how they work. The process can be messy, but the product can be true and beautiful. As factual information grows about any subject—in this case chemical communication—we dream about what it all means. We make propositions about how the phenomena we discovered work and how they came into existence. We find a way to test these various hypotheses. We look for a pattern that emerges when we put the parts together, like a jigsaw puzzle. If we find such a pattern, it becomes the working theory—we use it to think up new kinds of investigation, in order to move the whole subject forward. If this extension doesn’t work very well and now facts appear that contradict the theory, we adjust it. When things get bad enough, we junk the theory and create a new one. With each such step, science moves closer to the truth—sometimes rapidly, sometimes slowly. But always closer.
Woolly mammoths, a now-extinct species of the World Continent Fauna. Modified from the original painting. © Natural History Museum Picture Library, London.
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IOLOGICAL
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HEORY ON A
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Y SECOND EXAMPLE
of the growth of theory is from biogeography, the science that explains the distribution of plants and animals. In its global reach of space and time, biogeography is the ultimate discipline of biology—in the same sense that astronomy is the ultimate discipline of the physical sciences. When the mapping of species around the world is added to the study of how they got there, biogeography acquires a noble grandeur. At least, that was how I felt when as a college student in my late teens I looked up from my studies of descriptive natural history to study the processes of evolution. I learned to ask: What kind of process creates biodiversity? What other kind scatters species into their current geographic ranges? Neither kind occurs at random, I read. Both are the products of understandable causes and effects. I was already totally devoted to making a career in natural history, as an expert on insects. A government entomologist, perhaps, or a park ranger, or a teacher. Now I rejoiced. I could also be a real scientist!
The first revelation for me came from the Modern Synthesis of evolutionary theory. Put together mostly in the 1930s and 1940s, it united the original Darwinian theory of evolution by natural selection with advances being made in the modern disciplines of genetics, taxonomy, cytology, paleontology, and ecology. I was especially impressed by Ernst Mayr’s 1942 synthesis,
Systematics and the Origin of Species
, which I could immediately apply to my knowledge of taxonomy, the systematic classification of organisms. Suppose you yourself were working on a particular subject, say the colors of gems or the taste of wines, and you came upon a theoretical work that seemed to make sense of everything you already knew. You would have the same kind of transformative experience.
Later, as a graduate student at Harvard, I discovered a remarkable work on theory of biogeography only occasionally noticed by previous scientists: William Diller Matthew’s “Climate and Evolution,” published in a 1915 issue of
Annals of the New York Academy of Sciences
. In it the eminent vertebrate paleontologist, who worked as a curator of mammals in the American Museum of Natural History located in New York City, proposed a grand scheme for the origin and spread of mammals around the world. The kinds of mammals destined to be dominant in this way have originated, he wrote, in the great Eurasian landmass of the north temperate zone, roughly present-day England all the way to present-day Japan. Being competitively superior, they eliminated older, formerly dominant groups that had occupied the same niches. The early rulers were not extinguished entirely, however. They still flourished in areas not yet colonized by the newcomers. Think of the present great northern landmass formed by Europe, northern Asia, and North America as the hub of a wheel. To the south, Matthew said, tropical Asia to Africa, Australia, and Central and South America are the spokes of the wheel. Dominants originate in the hub and spread through the spokes. At the time of his account, Matthew’s theory seemed to fit the facts.
The dominant groups of the North, Matthew went on, are superior because they evolved in rugged, severely seasonal climates, which required a general toughness and ability to adapt to change. These most recent winners include animals familiar to all Eurasians and North Americans: mice and rats (taxonomic family Muridae), deer (Cervidae), cattle (Bovidae), weasels (Mustelidae), and, of course, us (Hominidae). Former dominants, now confined to the southern spokes, are the rhinoceroses (Rhinoceratidae), elephants (Elephantidae), and primates exclusive of man.
Right or wrong, and by evidence available in Matthew’s time it seemed right (although much less so now), I saw the theory as prehistory on a global scale. It was biology lifted to the maximum in space and time. And it was scientific natural history, the subject I had chosen!
In 1948 Philip J. Darlington, whom years later I was to succeed as curator of insects at Harvard’s Museum of Comparative Zoology, presented a different story for the reptiles, amphibians, and freshwater fishes, no less grand than that of Matthew’s for mammals. These cold-blooded vertebrates, he said, arose not in the north temperate zone as supposed by Matthew for the warm-blooded mammals, but in the vast tropical forests and grasslands that once covered most of Europe, northern Africa, and Asia. They then spread south into the peripheral continents, much reduced in diversity of species, and northward into the north temperate zone. It also turned out from the new wave of fossil research that humanity originated not in Eurasia but in the tropical savannas of Africa.
I was raised, so to speak, more on Darlington than on Matthew, but Matthew I found to be right in one important respect. There was indeed a global pattern of dominant groups arising in large, ecologically varied portions of the world’s landmass.
Then came the equally grand theory of the World Continent Fauna, the existence of which supported the overall theme developed by both Matthew and Darlington. For tens of millions of years South America was isolated from North America by a broad seaway that submerged the present-day Isthmus of Panama, thereby connecting the Pacific Ocean to the Caribbean Sea and isolating the continents on either side. Mammals, except bats, as a rule could not cross the broad stretch of ocean water. As a result those in South America evolved independently from those in North America. But the two faunas converged in outward appearance and in the niches they filled. In the north there were horses, in the south horselike litopterns. The rhinos and hippos of the north were duplicated, roughly, by South American toxodonts, and tapirs and northern elephants respectively by southern astrapotheres and pyrotheres. Shrews, weasels, cats, and dogs were matched in varying degree by the diverse members of the South American family Borhyaenidae. The fearsome saber-toothed tiger of North America was approached in overall appearance by an equivalent in South America, even though they remained very different in another way: the northern saber-tooth was a placental (fetus carried throughout in the uterus) and the South American one was a marsupial (fetus carried part of the way in an outside pouch).
This evolutionary convergence was the greatest on the land that the world has ever seen. Imagine that we could travel back in time to South America as it was ten million years ago and make a safari across its savanna, much as tourists do today in East Africa:
Say we are there back then on the edge of a lake, early one sunny morning, turning our gaze slowly through a full circle. The vegetation looks much like modern savanna. Out in the water a crash of rhinoceros-like animals browse belly-deep through a bed of aquatic plants. On the shore something resembling a large weasel drags an odd-looking mouse into a clump of shrubs and disappears into a hole. A creature vaguely like a tapir watches immobile from the shadows of a nearby copse. Out of the high grass a big, catlike animal suddenly charges a herd of—what?—animals that are not quite horses. Its mouth is thrown open nearly 180 degrees, knife-shaped canines projecting forward. The horse look-alikes panic and scatter in all directions. One stumbles, and . . .
This independent kingdom of wildlife disappeared over a million years ago, long before the arrival of human beings, while its North American equivalent persisted mostly intact until only about ten thousand years ago, after skilled human hunters arrived and began to spread over the continent. Each appeared to have reached a balance within its own domain. Why, then, did the southern kingdom decline while the northern kingdom lived on?
This obvious disparity in survival brought biogeographers to the interesting question implied by the balance of nature: What happens when two full-blown, closely similar dynasties meet head-on? If it were possible to play God with geological spans of time to wait and watch, the ideal experiment would be this: Allow two isolated parts of the world to fill up with independent adaptive radiations of plants and animals, so that the majority of species in each theater have close ecological equivalents in the other theater; then connect the two regions with a bridge and see what happens. When the organisms intermingle, would those from one theater replace the other, so that a single fauna and flora comes to occupy the entire range?
The grand experiment has in fact been performed once in relatively recent geological time, and we can deduce a great deal of what happened by comparing fossil and living species. Two and a half million years ago the Isthmus of Panama rose above the sea, bridging the ancient Pacific-to-Caribbean seaway and allowing the mammals of South America to mingle with the mammals of North and Central America. Species from each continent spread into the other.
The change in biodiversity that occurred can best be measured at the taxonomic level of the family. Examples of mammalian families are the Felidae, or cats; Canidae, dogs and their relatives; Muridae, the common mice and rats; and of course Hominidae, human beings. The number of mammalian families in South America before the interchange was thirty-two. It rose to thirty-nine soon after the Isthmus of Panama connection, and then subsided gradually to the present-day level of thirty-five. The history of the North American fauna was closely comparable: about thirty families before the interchange, rising to thirty-five, and subsiding to thirty-three. The number of families crossing over was about the same from both sides.
When all this information was put together, the stage was set for another kind of theory. When biologists see a number go up following a disturbance and then fall back to the original level, whether body temperature, density of bacteria in a flask, or biological diversity on a continent, they suspect that an equilibrium exists in the system. The restoration of the numbers of mammalian families in both North and South America points to such a balance of nature. In other words, there appears to be a limit to diversity, in the sense that two very similar major groups cannot coexist in their fully radiated condition. A closer examination of the ecological equivalents on both continents, dwellers in the same broad niche, reinforces this conclusion. In South America marsupial big cats and smaller marsupial predators were replaced by their placental equivalents. Toxodonts gave way to tapirs and deer. Still some unusual specialists—the wild cards—were able to persist. Anteaters, tree sloths, and monkeys continue today to flourish in South America, while armadillos are not only abundant throughout tropical America but are represented by one species that has expanded its range throughout the southern United States.
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?