The Evolutionary Way of Knowing
Squirrel Island is one of those iconic places on the coast of Maine where an idyllic landscape of meadows, mossy slopes, and fragrant forests of spruce and pine meets the sea. A short boat ride from Booth-bay Harbor, it was the perfect spot to spend time with my girlfriend Edith as we picked berries, listened to warblers singing high in the canopy, and examined all that grew in this tame fragment of northern nature.
But it was the seashore that brought us here. A luxuriant cover of slippery rockweeds, their air-filled bladders crackling under the pressure of our boots, gave way to mats of blue mussels and then to stands of waving kelp fronds as we descended to the low water mark. All the expected inhabitants of the rocky shore were here: spiny green sea urchins nestled in pools and among the kelp holdfasts, large hairy horse mussels crammed into sand pockets around boulders, rock crabs and green crabs hiding beneath sponge-lined ledges, sluggish dog whelks massing in crevices or perched atop prey barnacles, limpets clinging to and scraping crusts beneath rocks, and three kinds of periwinkle feasting on seaweeds. Gulls, always on the lookout for crabs, patrolled overhead. To judge from their large size and the many closely spaced growth interruptions on their weather-beaten shells, the blue mussels had been living their sedentary lives on this shore for decades. To us as naturalists, the shore and its surroundings exemplified the serenity and beauty that give nature and life meaning. To us as scientists, they posed questions and invited inquiry into the ways of the living world.
Grinding winter ice, breaking waves, intense competition for light and space, and a variety of predators—crabs, lobsters, sea stars, whelks, fish, and gulls—are part of life for every creature on this shore. Yet despite these dangers and despite the conflicting interests of the seaweeds and animals in this community, the species are well adapted to each other and to their rigorous physical surroundings. The secure attachment by flexible threads secreted from the foot protects mussels from being dislodged by shifting ice or powerful predators from rocks. Even when the threads fail and the mussels become detached, the bivalves can produce new threads and re-fasten themselves if they land in the right place. A mussel’s two shell valves can shut tightly around the soft tissues, so that would-be attackers homing in on the chemical plume that a mussel releases while actively feeding and respiring loses the scent and finds another victim. When snails like dog whelks and periwinkles become dislodged, they quickly withdraw the delicate soft parts—head, foot, and other organs—far back in the shell, whose opening is sealed with a flexible yet tightly fitting doorlike device. Predatory sea stars, when directly contacting potential prey snails, often elicit a vigorous escape response. Sponges beneath overhangs secrete chemicals that enemy sea slugs find repellent. I could fill a book documenting the adaptations of every species on this relatively simple shore, let alone every living and fossil species on the planet.
The fact of adaptation—the good fit between organism and environment—was already well established by the time Charles Darwin and his collaborator and competitor Alfred Russel Wallace ushered in the evolutionary age with the presentation of their paper before the Linnean Society in London in 1858. All living things, including humans, effectively perform the essential functions of life—growth, metabolism, food intake, defense, maintenance of the body, and reproduction—in the places these organisms inhabit. Darwin and Wallace proposed one mechanism for adaptation, based on an idea borrowed from the English political economist Thomas Malthus—the number of offspring produced in a population exceeds the number surviving to reproduce as adults. Survivors possess heritable characteristics—those passed from generation to generation—that enable living individuals to meet the challenges and capitalize on opportunities in their environment more effectively than the traits of those individuals that die before reaching maturity. Adaptation thus involves a selective process, which was called natural selection by Darwin, resulting from the culling of inadequately performing individuals in the struggle for life.
Bipedal locomotion—running and walking on two legs—is a good example of human adaptation. The bipedal condition evolved from the ancestral four-footed condition many times, as in kangaroos and several lines of dinosaurs, but in primates it is restricted to our own species (Homo sapiens) and other species of Homo, dating back to perhaps as early as two-and-a-half million years ago. Among running animals in general and bipeds in particular, humans are unusual in that they can run for long distances, up to six miles (ten kilometers) per day for someone in good physical health. Only a few other mammals—wolves, hyenas, and African dogs that hunt in packs, as well as migrating horses and African wildebeest—engage in comparable endurance running, but these animals run on all fours. Our two-legged, long-distance running is made possible by modifications in the leg bones, joints, tendons, muscles, and even our skin. Unlike other running mammals, we achieve high speed with a long stride of up to three-and-a-half meters (more than ten feet) in highly trained athletes. Traits contributing to this long stride include unusually long hind limbs, a relatively short and lightweight foot, the unique presence of the Achilles tendon connecting the heel with muscles in the foot, and short bundles of muscles that produce the forces necessary for sustained running. All the joints in our legs, from the foot to the pelvis, are conspicuously enlarged compared to those of apes and compared to the joints in our arms. This enlargement enables the body to absorb the strong forces produced when the feet hit the ground as we run. The copious heat generated by our muscles during running is lost through the skin, which in humans has unusually abundant sweat glands through which evaporation takes place. Human skin is also peculiar in lacking insulating hair over most of its surface. These and other departures from the primate norm represent adaptations that enabled our immediate ancestors to compete effectively with other predators for scarce, protein-rich meat in hot open country. Other distinctive human adaptations—a large brain, delayed sexual maturity, the use of long-distance weapons, and the domestication of animals and plants—came much later. In the early history of our genus Homo, however, running-related modifications gave us a decisive competitive edge in a place where the struggle for life was particularly intense.
Not every aspect of an organism represents an adaptation. Many features are simply expressions of how an individual grows, or how it is put together, much as the seam of a plastic cup indicates how the cup was manufactured. Most shells, for example, grow in the shape of a spiral, reflecting a pattern of growth in which new shell material is added only at the expanding end of what is effectively a conical enclosure. The spiral can be variously modified in adaptive ways, ranging from the loose, rapidly expanding spiral of a clam shell or a cap-shaped limpet to the tightly coiled spiral of many snails; but the spiral form itself is not a direct expression of adaptedness. Likewise, the growth marks on the exterior of a mussel shell or in the bones of reptiles and some dinosaurs indicate periodic interruptions in growth rather than some subtle adaptation, but here again many animals have fashioned growth marks into larger features that impart greater shell strength and other protective functions.
Individual traits may even be harmful in some situations. An actively feeding mussel, which pumps water through its gills in order to extract food particles as well as oxygen, necessarily releases metabolic products with the expelled water. These are beacons for potential predators like sea stars and snails. The disadvantages associated with this inadvertent advertisement must be compensated for by an ability to recognize the nearby presence of hungry predators and to respond quickly by shutting the shell temporarily so that the scent disappears.
The catalog of potentially harmful traits in humans is also long. In the human eye, for example, there is no vision in the hole through which the optic nerve passes from the light-gathering surface of the retina to the brain. There is thus a blind spot located some thirty degrees to the right of the point of focus of the right eye, and another thirty degrees to the left of the focal point of the left eye.
It may even be that most evolutionary changes are neutral or nearly neutral, and therefore impervious to natural selection. In all organisms, genetic material (DNA and RNA) consists of helically wound chains of nucleotides of four kinds: adenine, cytosine, guanine, and thymine (or uracil). Some of this genetic material provides instructions for building proteins, which consist of long chains of twenty kinds of amino acids. In the sequence of three nucleotides that specifies a given amino acid, the third nucleotide of the sequence is typically silent—that is, a switch from one nucleotide to another in this position leaves the amino-acid specification unchanged. These alterations are likely to be common, but they have no effect on an organism’s adaptation; they are thus effectively neutral. Even mutations with mildly deleterious effects may become established under some conditions. In very small populations of five or fewer individuals, a mutation with a tiny disadvantage can persist because a random fluctuation in population size by one or two individuals has a much larger effect on the fate of that mutation than selection does on the trait that the mutation specifies. If species often originate as tiny populations of this size, as many biologists believe, this so-called random fixation through genetic drift may be an important non-adaptive cause of genetic change. In still other cases, a new mutation or a new trait may be protected from selection because it is expressed in a very safe environment, as in the embryo inside the mother, where it is sheltered from the usual agencies comprising the struggle for life. In such situations, traits are relatively free to vary, and they therefore provide the raw material on which the agencies responsible for natural selection can act.
The features of an organism may be nothing more than byproducts of construction, or they may be functionless legacies of the past, but living things are adapted units whose parts and characteristics enable them to thrive and propagate. Generations of such successful organisms form a lineage, whose members have accumulated enough adaptive information about their surroundings to pass on their traits and the instructions for those traits to the next generation. A lineage can persist for millions of years as long as its members keep working adequately. Often, a lineage divides, giving rise to two or more lineages in an evolutionary branching event. All the lineages of life—ancient as well as modern—can thus be thought of as forming an evolutionary tree, whose innumerable twigs can be traced back through time to a single common ancestor. At every point on the tree, organisms were successful enough to leave offspring. The tree of life is therefore a tree of unbroken success.
A more provocative way of thinking about adaptation is to say that living things are well fitted to their surroundings because they have evolutionarily formulated and tested an adequate hypothesis about their situation. Such an adaptive hypothesis consists of the totality of form, physiology, and behavior of an organism. It incorporates information about potential causes of failure and death—competition, predation, disease, and weather—as well as potential opportunities such as food or mates into a material body, a metabolizing “survival machine.”
The concept of an adaptation as being equivalent to a scientific hypothesis, first proposed by the
Austrian marine biologist Wolfgang Sterrer, introduces three core ideas of this book. First, a living thing is an adapted whole, all of whose characteristics affect its fate in the struggle for life. Natural selection due to enemies and allies operates on wholes, not on genes or on isolated traits encoded by those genes. Selection may change the frequency of gene variants controlling such traits as eye color, blood type, or susceptibility to diseases like hemophilia, but it is the organism’s phenotype—its form, physiology, and behavior—that is exposed to selection and that constitutes the adaptive hypothesis. Our fellow meat-eaters on the Pliocene plains of East Africa competed with whole people, not with our adapted parts or our genes. Their fates and ours hinged on how well the bodies of competing individuals worked.
This idea is most obvious when applied to individual organisms, but it applies also to cells within the living body. In the nervous system, for example, axons—long extensions of nerve cells—grow more or less at random, but they persist only when they reach their target as indicated by an electrical signal, that is, when a connection is established. Other axons wither away, leaving an orderly, flexible, and well-functioning network of nerve connections that allows an animal to sense and respond to its environment. This trial-by-success pattern of axon growth and death yields an adapted whole in the same way that natural selection does at the higher level of biological organization: that of individual organisms.
The notion of an organism as hypothesis also points to the role that living things play in modifying their environment. Organisms do not simply respond physiologically or behaviorally to their surroundings, or evolve through natural selection in response to a new situation; they also help create their environment. The good fit between organism and environment is a combination of acting and being acted upon, a feedback rather than a one-way relationship. Organisms are active participants in their adaptation, not passive players unable to influence their own fates.
Third, the trial-by-success procedure that lies at the heart of adaptation in nature is essentially identical to the human scientific way of knowing. In nature, feedbacks between body and environment impose selection and yield adaptive wholes by distinguishing between those that survive and those that do not. In the scientific search for objective reality, a scientist consciously uses observations and prior evidence to formulate a hypothesis, which is then tested and modified with additional evidence gathered through further observation and experiment. Evolutionary adaptation is achieved without a guiding sentient mind, whereas science for the most part proceeds deliberately and purposefully; but the two adaptation-producing methods have in common a feedback between the environment and a meaningful, predictive representation of that environment.
To see this parallel between adaptation and science in more concrete terms, let’s return to the plants and animals on the coast of Maine. The organisms Edith and I met on Squirrel Island all belong to species that are native to the North Atlantic. It therefore seemed reasonable to conclude that their distinguishing features, including their adaptations, evolved in that ocean, and to assume that the dangers and opportunities important to individual members of species on these shores are representative of conditions faced by each species as a whole both now and in the past. These ideas were my preliminary hypotheses about the origins of the adaptive characteristics that these common species displayed so clearly.
But when I began to examine these intuitions, I quickly discovered that my hypotheses would require substantial revision. For example, it became clear that the environments in which we observed our seashore species were far different from those that existed four hundred years ago and earlier, when many of these same species were flourishing. In those few centuries, humans had greatly altered the complement of predators, competitors, and food sources in the waters of New England and southeastern Canada. Gone were the vast numbers of lobsters and cod—major predators of clams and sea urchins—that French fishermen and early English settlers encountered in the seventeenth century. Three species—the great auk, Labrador duck, and sea mink—that had once been significant consumers of shore animals and fish had disappeared before 1920. The Atlantic gray whale, a voracious bottom-feeding species, had been hunted to extinction by 1675. Meanwhile, species from elsewhere arrived on these shores, including a rockweed, the common periwinkle, and the green crab, all accidentally introduced with ships’ ballast rocks from Europe before 1840. The adaptations displayed by the native shore species in Maine clearly had been honed under conditions quite different from those that prevail in the present day. More refined hypotheses about the nature and origins of these adaptations were clearly needed.
To pinpoint the origins of New England’s marine life and its adaptations, I needed to trace lineages back through millions of years of evolutionary time. The categories—families and genera—in which our New England species were classified are found in other geographic areas as well, which means that an adequate hypothesis of evolutionary origin and change would have to consider many species besides those found in Maine. Our species might have arisen in places far away and in environments that no longer existed. Their adaptive hypotheses might work in the human-altered conditions prevailing today, but they were evolutionarily formulated and honed elsewhere.
These possibilities began to engage my attention fifteen years later on the other side of the continent. Edith (by then my wife of fourteen years), our four-year-old daughter Hermine, and I had settled in for a spring sabbatical of research and teaching at the Friday Harbor Laboratories on San Juan Island in the state of Washington. Friday Harbor is an idyllic place, combining a rich and easily accessible marine fauna, a forest preserve perfect for long morning walks, and a stimulating intellectual atmosphere. Contemplating the specimens I collected during our stay, I realized that many of Friday Harbor’s seaweeds and marine animals have close counterparts in the biologically much more impoverished floras and faunas of the North Atlantic. Even though the Pacific and Atlantic species were now separated by the impenetrable ice barrier in the Arctic Ocean, they must have had a common origin. But where was this place of origin, and what were conditions like when the adaptations of these species were evolving?
I settled on two hypotheses. Either the common ancestors of Atlantic-Pacific species pairs originated in the North Pacific and spread northward through the Arctic Ocean to the Atlantic, or they originated somewhere in the North Atlantic and moved through the Arctic into the Pacific. Either way, the spread of a temperate-zone ancestor from one northern ocean to another required a much warmer Arctic Ocean than the ice-covered ocean of the last eight hundred thousand years. Furthermore, the land bridge that linked Alaska and Siberia for tens of millions of years would have to have been breached by the sea to form the Bering Strait, which connects the North Pacific and Arctic Oceans. Testing and refining these hypotheses would involve gathering evidence from many sources. And so began a research project that has engaged me off and on for more than twenty years.
One obvious source of evidence, especially to a paleontologist like me, is the fossil record. Happily for me, seashells are abundant as fossils, and most species of living clams and snails have a fossil record that can be traced back for millions of years. If the lineage leading up to a living species extends further back in time in the Pacific than in the Arctic or Atlantic Oceans, it likely originated in the Pacific. Using genetic characteristics that can be observed in fossils, such as details of shell form, paleontologists can link ancestors with descendants. With the advent of molecular techniques, in which DNA sequences of related species are compared, it has become much easier to identify species lineages and to determine the geographic locations and times of lineage splitting. Geological evidence can help to specify when the Bering Strait was open, and when the Arctic Ocean was warm or when it was covered by sea ice.
Excerpted from The Evolutionary World by Geerat J. Vermeij.
Copyright © 2010 by Geerat J. Vermeij.
Published in 2010 by Thomas Dunne Books.
All rights reserved. This work is protected under copyright laws and reproduction is strictly prohibited. Permission to reproduce the material in any manner or medium must be secured from the Publisher.