Everything in the preceding chapters — the caste of an ant, the dance of a bee, the altruism of a sterile worker, the vast synthesis of sociobiology — rests upon a single foundation: the theory of evolution by natural selection. Edward O. Wilson was, before all his particular enthusiasms, an evolutionist, a scientist for whom the Darwinian account of how life changes was not one idea among many but the organizing principle of biology entire. He did not devote a separate book to evolutionary theory as such; rather, it ran through everything he wrote, the engine beneath every phenomenon he described. This chapter draws that engine into the open, gathering Wilson's understanding of how evolution actually works — how natural selection shapes adaptation, how new species arise, how the diversity of life is generated, and how the small changes of the moment accumulate into the grand transformations of deep time.
At the base of Wilson's evolutionary thought lay the mechanism Darwin had identified: natural selection acting on heritable variation. Wilson understood this mechanism in the modern, quantitative form the twentieth century had given it, grounded in the mathematics of population genetics. He recognized that evolution, at its most fundamental level, is a change in the frequencies of genes within populations over time, driven by four basic forces: selection, which favors the genes that build more successful organisms; mutation, the ultimate source of new genetic variation; migration, the flow of genes between populations; and genetic drift, the random fluctuation of gene frequencies, especially powerful in small populations. These forces, working together across the generations, are the machinery of evolutionary change, and Wilson grounded his whole science of behavior in this population-genetic bedrock, insisting that any serious account of life must be built upon it.
Mutation supplies the raw material, and Wilson was careful to give it its due as the ultimate source of all evolutionary novelty. Genetic mutation — the alteration of the hereditary material — generates the variation on which selection then acts, and without this ceaseless welling-up of new variants, evolution would grind to a halt for want of anything to select among. But mutation alone is directionless, a source of random variety; it is natural selection that gives evolution its direction, preserving the rare useful variant and discarding the many useless or harmful ones. Wilson stressed this division of labor between the forces: mutation proposes, selection disposes, and the interplay of the two, filtered through the dynamics of populations, produces the exquisite fit between organism and environment that so struck Darwin and every naturalist since.
Wilson liked to ground these abstractions in the concrete history of the group he knew best, and the origin of the ants themselves served him as a worked example of evolution in action. He and Hölldobler traced the derivation of ants from within the vespoid wasps, using the methods of cladistic analysis to reconstruct the branching history by which a lineage of wasps gave rise, tens of millions of years ago, to the first ants. They identified the key innovations that underlay the ants' subsequent success — anatomical novelties such as the metapleural gland, which secretes antibiotic substances that help protect the crowded colony from disease, and the distinctive waist and other features that mark the ant body. Such key innovations, arising by ordinary evolutionary means, can open the way to great subsequent diversification, equipping a lineage to exploit possibilities closed to its ancestors. In the ants Wilson could point to the whole evolutionary story made concrete: an origin from a particular ancestral stock, a set of enabling innovations, and a subsequent radiation into the thousands of species that rule the insect world today.
That fit — adaptation — was for Wilson the central marvel that evolutionary theory exists to explain. The features of an organism, and of an insect society, are shaped by natural selection to serve survival and reproduction, and Wilson examined adaptation everywhere he looked, from the antibiotic secretions of an ant's metapleural gland to the elaborate caste systems of the leafcutters. He treated adaptation not as a vague notion of usefulness but as a precise consequence of selection: a trait spreads because, in the environment where it arose, its bearers left more descendants than their rivals. Yet Wilson was no naive adaptationist. He knew that not every feature is an adaptation, that history and chance and constraint leave their marks, and that the adaptive landscape — the metaphor population geneticists use to picture the terrain of fitness across which populations move — is rugged, full of peaks and valleys that a lineage may or may not be able to cross. Adaptation, for Wilson, was real and pervasive but never automatic, always the outcome of a contest between selection and the many forces that oppose or deflect it.
Not all selection favors mere utility, and Wilson was fascinated by the ways that evolution produces beauty as well as function. The extravagant plumage of a bird of paradise, useless or worse for simple survival, arises through sexual selection — the form of natural selection driven by competition for mates, in which traits spread not because they help their bearers survive but because they help them reproduce, whether by attracting mates or by besting rivals. Wilson examined how such processes can drive the evolution of ornament and display to extraordinary extremes, producing forms of a splendor that seems, at first, to defy the austere logic of survival. The beauty of the living world, on this understanding, is not an accident nor a gift from outside but a product of evolutionary processes as lawful as any other, sexual selection sculpting extravagance wherever the reproductive advantage of display outweighs its cost. That evolution should generate not only the functional but the beautiful was, for Wilson, one more reason for wonder at the reach of the Darwinian mechanism.
If adaptation is how lineages change, speciation is how they multiply, and here Wilson made contributions of his own to evolutionary theory. The formation of new species, he explained, centers on the evolution of intrinsic isolating mechanisms — the biological barriers that prevent populations from interbreeding, so that once-continuous lineages split into separate, reproductively isolated species. The most common route begins with geographic isolation: when populations are physically separated, by a barrier or simply by distance, they diverge under the independent action of selection and drift, accumulating differences until, should they meet again, they can no longer interbreed. Wilson had seen this process written across the islands of the Pacific, where the ant faunas of separate islands had diverged into distinct forms, and it informed his lifelong sense of speciation as an ongoing, observable process rather than a remote historical event. Life, he insisted, is proliferating, mutating, and changing constantly all around us; speciation is not finished but perpetually underway.
Wilson attended closely to the role of chance in speciation, and especially to the genetic consequences of small founding populations. When a new population is established by a few colonists — a handful of insects blown to a distant island, say — those founders carry only a limited and possibly unrepresentative sample of the genetic variation present in their source population. This founder effect means that the new population begins its independent evolutionary career already genetically distinct from its parent, its starting point shifted by the accident of which few individuals happened to found it. Combined with the small population's heightened susceptibility to genetic drift, the random fluctuation of gene frequencies, the founder effect can set a young population on an evolutionary trajectory quite different from that of its ancestors, hastening the divergence that leads to new species. Wilson saw in such effects a reminder that evolution is not solely the deterministic grinding of selection but is shot through with contingency, with the lasting consequences of chance events that might easily have fallen out otherwise.
He was struck, too, by the great variation in the pace of speciation across different groups and settings. Some lineages speciate with remarkable rapidity — Wilson pointed to orchids exploiting the innumerable microsites of tropical mountains, throwing off new species at a prodigious rate — while others remain little changed for immense spans of time. The rate at which new species form, he recognized, depends on the interplay of many factors: the availability of empty ecological space, the presence of barriers that isolate populations, the generation time and population structure of the organisms, the ruggedness of the adaptive landscape they must traverse. This variation in tempo is itself an important feature of the evolutionary process, helping to explain why some groups are enormously diverse while others, no less ancient, contain but a handful of species. The generation of diversity, Wilson saw, is uneven, pulsing faster here and slower there according to circumstance.
Among the mechanisms of divergence, one was Wilson's own discovery, and it deserves particular notice: character displacement. Wilson, with William Brown, described how closely related species, where they come into contact and compete, may be driven by selection to diverge in their traits — in body size, in feeding apparatus, in behavior — precisely so as to reduce the competition between them. Two species that overlap in what they need are pushed apart by natural selection until they need different things, and can therefore coexist. This process, which Wilson named and analyzed, is a beautiful demonstration of selection actively shaping the differences among species, sculpting the diversity of a community from the pressure of competition. It reveals evolution not merely as change within a lineage but as an interactive process, in which species evolve in response to one another, the presence of a competitor becoming itself a force of natural selection.
Another concept that bore Wilson's personal stamp was the taxon cycle, which he formulated from his study of ant distributions across the island archipelagoes of the Pacific. Wilson observed that species seem to pass through a characteristic cycle in their occupation of an archipelago: a species expands across the islands, spreading widely; then, over time, it specializes, its populations adapting to particular local conditions and narrowing their range; and eventually it contracts, its scattered populations dwindling until the species may go locally extinct, leaving room for the next expanding form. This taxon cycle, a repeating pattern of expansion, specialization, contraction, and local extinction, gave dynamic shape to what might otherwise seem a static distribution of species, revealing the ceaseless turnover beneath the apparent stability of an island fauna. It was a characteristically Wilsonian idea, drawing a general evolutionary and biogeographic pattern out of the patient accumulation of particular observations about ants on islands, and it fed directly into the theory of island biogeography he would develop with Robert MacArthur (see Chapter 9, The Theory of Island Biogeography).
Wilson recognized, too, that competition and coexistence are woven together in evolution, for character displacement is one of the ways that similar species partition the resources of a habitat and so manage to live side by side. He returned to the theme repeatedly, showing how the divergence driven by competition allows many species to coexist where otherwise a few dominant ones might exclude the rest. This linking of the evolutionary process to the ecological question of how so many species can share a world was characteristic of Wilson's integrative cast of mind. He urged, from his earliest theoretical work, the integration of short-term ecological dynamics with longer-term evolutionary processes — the recognition that ecology and evolution are not separate subjects but two aspects of a single unfolding, the ecological interactions of the moment being the very stuff from which evolutionary change is wrought over time.
One especially rich expression of this interweaving of ecology and evolution is coevolution, the reciprocal evolutionary change of interacting species, and Wilson found examples of it throughout the world of the social insects. He framed the relationships between ant colonies and the many organisms that exploit or depend on them — the parasites that breach a colony's defenses, the symbionts that live within its walls — as coevolving systems, in which the defenses of the colony and the counter-adaptations of its exploiters drive one another forward in an escalating arms race. Each improvement in a colony's chemical recognition system selects for parasites better able to forge the colony's chemical passwords; each advance in forgery selects, in turn, for a more secure system. Over evolutionary time such reciprocal pressures can produce astonishingly intricate adaptations on both sides, the intricacy a record of the long contest between them. Coevolution, for Wilson, was evolution at its most dynamic and interactive, a demonstration that the environment to which a species adapts is composed largely of other species, themselves adapting in response, so that the living world is a vast web of mutual evolutionary influence, every lineage shaping and shaped by the others around it.
When Wilson turned from the origin of single species to the generation of whole arrays of them, he reached for the concept of adaptive radiation, one of the grand patterns of evolutionary history. A lineage entering empty ecological space — a colonist arriving on a new island, or a group that has evolved some key innovation — may diversify rapidly into a whole array of specialized descendant forms, each adapted to a different way of life. The classic examples became touchstones of Wilson's writing: the finches of the Galápagos that so impressed Darwin, diversified from a single ancestral stock into a spectrum of species with beaks shaped for different foods; the honeycreepers of Hawaii, radiating across the archipelago into a dazzling variety of forms. Adaptive radiation, Wilson argued, is one of the central means by which life fills the world with diverse forms, exploiting the many ways of making a living that any environment offers. It is evolution in its most creative and exuberant mode, a single lineage flowering into many.
Wilson delighted in the sheer range of forms that a single lineage could produce, and he gathered examples from across the living world to convey it — the astonishing diversity of sharks, spanning forms from the great filter-feeders to the swift predators, all elaborated from a single ancient body plan; the thousands of orchid species contriving their endless variations on the theme of the flower. Each such radiation testified to the creative reach of evolution, its capacity to work a single inherited design into a whole spectrum of specialized forms. And beneath all this diversity Wilson stressed a deep unity: the unbroken continuity of descent that links all living things, through their common ancestors, across billions of years. However different two organisms may appear, they are cousins, joined by an unbroken chain of ancestry reaching back to the origin of life; the diversity of the living world is variation upon a single, continuous inheritance. This double vision — of boundless diversity springing from fundamental unity — was, for Wilson, the essential Darwinian insight, and the source of much of biology's intellectual beauty.
Radiation has a counterpart that Wilson found equally illuminating: convergence, the independent evolution of similar solutions in unrelated lineages. Where radiation shows one lineage diversifying into many forms, convergence shows many lineages arriving at the same form, driven by the same selective pressures toward the same adaptive answer. Wilson had seen convergence at its most striking in the social insects, where ants and termites, separated by hundreds of millions of years of evolution, had independently evolved elaborate castes, nest-building, and ecological dominance. Convergence testifies, as radiation does, to the power and the lawfulness of natural selection: presented with similar problems, evolution tends to find similar solutions, so that the living world is patterned, its diversity channeled along recurring lines rather than scattered at random. Radiation and convergence together, the one multiplying forms and the other constraining them, shape the overall architecture of life's variety.
Wilson was insistent that evolution operates across a whole hierarchy of levels, and that its full understanding requires attention to processes ranging from the genetic to the environmental. He distinguished microevolution, change at the level of genes within populations, from macroevolution, change at the larger level of species and environments over long spans of time — the two joined in a continuous process but requiring, he thought, somewhat different tools of study. At the grandest level he entertained the concept of species selection, the possibility that selection can act not only on individuals within a species but on whole species within a lineage, influencing which kinds of species tend to persist and proliferate and which tend to dwindle and vanish. This was a bolder and more contested idea, extending the logic of selection upward to a higher unit, and Wilson considered it as part of his effort to integrate the various forces of evolution into a coherent account of how the diversity of life is generated and shaped across all its scales.
What Wilson sought, in the end, was integration — a synthesis of the many forces of evolution into a single coherent account. Mutation and selection, migration and drift, geographic isolation and character displacement, adaptive radiation and convergence, speciation and extinction, the whole array of processes operating from the level of the gene up to the level of the whole biosphere: these were not competing explanations but complementary aspects of one grand process, and Wilson labored to see them whole. He emphasized that the generation of biological diversity is the joint outcome of all these forces working together, no single one sufficient by itself, their interplay producing the living world in all its variety. This integrative ambition was the hallmark of his evolutionary thought, as it was of his thought in general. He was never content to isolate a single mechanism and rest there; he wanted always to see how the pieces fit together into the larger picture, how the forces combined across the scales of space and time to yield the fecund, ever-changing world that so moved him.
This attention to levels reflected a conviction that ran through the whole of Wilson's biology: that life is organized as a hierarchy, from the gene to the cell to the organism to the population to the ecosystem, and that evolutionary processes operate, in different ways, at each level. The superorganism of the ant colony had taught him to see the colony as a genuine level of organization above the individual, and he carried this hierarchical vision into his general evolutionary thought, situating each phenomenon within the nested levels of biological order. The forces of evolution — selection above all — could in principle act at any of these levels, and much of the deepest theoretical dispute in Wilson's field, including the controversy over group selection treated elsewhere in this book, turns precisely on the question of which levels selection acts upon and how strongly (see Chapter 5, The Evolution of Social Behavior). The hierarchy of life was, for Wilson, not a static classification but a dynamic arena in which the evolutionary forces play out at every scale.
Late in his career, in Genesis, Wilson set his evolutionary thinking within the largest possible frame, adopting the framework of the major transitions of evolution developed by John Maynard Smith and Eörs Szathmáry. This framework identifies a series of great thresholds in the history of life — the origin of the first replicating molecules, the assembly of genes into genomes, the origin of the complex cell, the emergence of multicellular organisms from single cells, and the rise of animal societies — at each of which formerly independent units combined into a new and higher level of organization, a new kind of individual. Wilson seized on this framework because it unified, under a single principle, the whole sweep of evolutionary history from the molecule to the human society, and because it placed his own lifelong subject, the origin of societies, among the grand transitions of life. The evolution of eusociality, on this view, was not a mere curiosity of the insects but one of the major creative events in the history of the planet, of a kind with the origin of the cell and the organism.
Genesis represented Wilson's attempt to push the evolutionary account of society further back in time than he had ever ventured before, to trace the deep origin of social life to its ultimate roots in the history of the planet. He aimed to show that the great social systems of the living world — the colonies of the insects, the societies of the higher animals, and ultimately the civilization of humankind — are not late and separate developments but expressions of tendencies written deep into the evolutionary process, emerging again and again wherever conditions allow. By placing the origin of societies within the framework of the major transitions, Wilson gave his lifelong subject its widest possible setting, as one of the recurring creative achievements of evolution across the whole of deep time. The evolution of social behavior, which had occupied him since his first studies of the ants, thus found its place in a grand narrative reaching from the origin of life itself to the complex societies of the present — a narrative unified throughout by the single, tireless process of evolution by natural selection, building order upon order across the ages. It was the culminating expression of the integrative vision that had animated his science from the beginning.
For all the grandeur of this vision, Wilson never lost sight of a sobering implication of the Darwinian account, one he stated with unflinching clarity in his later reflections: that evolution has no goal, no foresight, no design. The human mind itself, he wrote, is the product of accidental evolutionary steps rather than of any purpose working toward it — a contingent outcome of a blind process, not the intended endpoint of a directed one. This is the hard core of Darwinism, and Wilson did not flinch from it: natural selection is a mechanical process, favoring whatever happens to reproduce, with no eye to the future and no aim beyond the moment's differential survival. That so blind a process should have produced the ordered splendor of the living world, up to and including the mind capable of understanding it, was for Wilson not a diminishment but a deepening of the wonder. The absence of design did not empty life of meaning; it located the source of life's order in the patient, unguided work of selection over immensities of time.
If the mind was not designed, it was nonetheless shaped, and Wilson devoted much thought to how natural selection had molded the human brain and its capacities. The brain, he argued, is an organ like any other, built by selection to promote the survival and reproduction of its bearers, and the general architecture of the mind is best understood as a suite of adaptations — its powers of perception, memory, reasoning, and emotion each honed by selection to solve the recurrent problems our ancestors faced. He developed, in this connection, the concept of prepared learning: the observation that humans acquire some behaviors and fears far more readily than others, because evolution has biased the mind to learn certain associations easily, the ones that mattered for survival in our ancestral past. The ease with which we come to fear snakes, and the difficulty of unlearning that fear, exemplifies such prepared learning, a readiness built into the brain by selection. Even our cognitive and emotional biases, Wilson suggested, including some that mislead us in the modern world, can be understood as adaptations that once guided ancestral decision-making well enough. The mind, in short, bears everywhere the marks of the evolutionary process that produced it, and cannot be understood apart from that process — a theme that runs from his evolutionary theory into his account of human nature (see Chapter 18, The Psychology of Human Nature).
From this understanding Wilson drew a warning that grew more urgent in his final years. Having grasped that human nature is the product of natural selection acting over millions of years, he viewed with alarm the prospect that humanity might soon abandon natural selection altogether, redesigning its own biology and nature at will through the new powers of genetic technology. He counseled caution, worried that in remaking ourselves we might destroy, carelessly and irreversibly, the evolved human nature that is the ground of everything we value. This was not a rejection of science but a scientist's sober respect for the depth and subtlety of what evolution had wrought, and a fear that our technical power might outrun our wisdom. The same evolutionary understanding that revealed the contingency of human nature revealed also its preciousness, as a thing produced by a process we are only beginning to comprehend and would be reckless to discard.
Wilson's evolutionary theory, then, was no dry mechanism but a vision of the living world in its full temporal depth — a world of ceaseless change, in which species form and diverge and radiate and sometimes vanish, in which adaptation is wrought by selection from the raw material of mutation, in which competition drives divergence and similar pressures drive convergence, and in which the whole vast process, operating across every level of the biological hierarchy and across billions of years, has filled the world with forms. It was the framework within which everything else he studied made sense. The ants he loved were a twig on the great tree of life, shaped by the same forces that shaped all the rest; the societies he analyzed were among the major transitions of evolutionary history; the human nature he sought to understand was one contingent product of the blind and patient process of descent with modification.
To read Wilson on evolution is to be returned, again and again, to the sense of wonder that first drew him to biology as a boy: wonder at the sheer fecundity of life, its endless proliferation of forms, its constant mutation and change, the way it fills every available space with variety. Natural selection, for him, was the key that unlocked this wonder without dispelling it, the mechanism that explained the diversity of life while leaving intact, even heightening, its power to astonish. He spent his career tracing the workings of that mechanism through his beloved ants and outward to the whole of the living world, and he never ceased to marvel that so simple a process — heritable variation, differential reproduction, time — could have produced so much. The chapters that follow turn to the fruits of this evolutionary process: to the theory of island biogeography, where evolution and ecology meet (see Chapter 9), and to the diversity of life it has generated, and to the urgent task of preserving that diversity before it is lost (see Chapter 11, Biodiversity, and Chapter 12, Nature Conservation).