The Second Meaning of Evolutionary Theory How Does Life Evolve

Darwin's On the Origin of Species (1859) immediately established evolution as a serious scientific issue in the minds of scholars throughout the Western world. Although others before him (going back, some would say, to the ancient Greeks) had entertained notions of the interconnectedness of all life, nonetheless it was Darwin's book that finally forced the thinking world to take the idea of evolution seriously. And although Darwin presented biological patterns, especially of the nested groupings of organisms, it is clear that he succeeded where others had failed in establishing the probable truth of evolution because he was able to suggest a mechanism—a causal theory of just how it is that the features of organisms can be modified over time. (Indeed, Darwin used the term "descent with modification" throughout his text instead of the term evolution.)

That mechanism was natural selection, an idea developed simultaneously by the naturalist Alfred Russell Wallace. Natural selection remains the prevailing explanation of why biological nature appears to be so well designed— that is, how giraffes came to have their long necks for browsing leaves from high shrubs and trees; how bats attained the ability to echolocate (that is, navigate and find insect prey by emitting ultrahigh-frequency sounds that bounce back to the ear in radarlike fashion): in short, all the adaptations of the biological world. Natural selection has been observed occurring in the wild, has been the subject of many laboratory experiments, and has been analyzed mathematically in great detail.

Natural selection results from the simple fact that resources are limited in the natural world, so that those organisms best able to obtain energy and nutrient supplies (as well as survive disease and predation) will tend to survive longer and produce more progeny than those less able to cope. Because organisms tend to resemble their parents, what we now realize to be the underlying genetic information for successful living is passed on in relatively greater amounts to the next generation. But Darwin reasoned that if the environment should change, the natural variation in a population might very well include organisms with other features that then might have an advantage in the new conditions—and thus natural selection would favor the other variants that were not selected for in the past. In that way, the features of an entire species of organisms would be changed to meet the new conditions.

Darwin was frustrated, however, in not knowing how, exactly, offspring tend to resemble their parents, or how new variant features arise from time to time. The science of genetics had yet to be born. But during Darwin's lifetime, the Austrian monk Gregor Mendel performed simple experiments breeding peas in his garden, and in so doing discovered that features seem to be inherited through particles that could be separated and recombined, important rules that lay the foundation of the modern science of genetics.

Mendel's work was largely ignored in his lifetime, but several individuals and teams of biologists all rediscovered Mendel's work just at the turn of the nineteenth century—thus jump-starting the serious scientific study of heredity. Most of the early progress came from the laboratories of Thomas Hunt Morgan at Columbia University in New York. There, experiments on fruit flies soon revealed that the particulate nature of inheritance discovered by Mendel was caused by the existence of such particles, soon dubbed "genes," arranged in a linear fashion along strands in the cell nuclei, called "chromosomes." Also, sudden changes in heritable information—called "mutations"—were soon discovered. Here, at last, was sound knowledge of the mechanisms of both heredity itself and the origin of novel genetic information.

At first, however, the findings of the new science of genetics seemed to be at odds with the Darwinian notion of evolution through natural selection. One botanist, Hugo Devries, for example thought that the sudden appearance of mutations in the flowers of the evening primrose was in itself sufficient to explain how organisms change through time in evolution. Geneticists came to assume that the sort of natural history practiced by Darwin and Wallace was old fashioned, and that natural selection itself was no longer necessary as an explanation for how evolution occurs.

Some of the conflicts between the idea of natural selection and the early results of genetics included the observation that most mutations are deleterious—that is, harmful to organisms—including some that are downright lethal. Also, Darwin had talked of selection gradually modifying traits—such as hair length in mammalian coats, for example—whereas genetics stressed the particulate either/or nature of inheritance, such as the yellow/green or smooth/wrinkled dichotomies in Mendel's original data of pea genetics. It took thirty years before geneticists observed mutations that were small scale and either neutral or even beneficial in their effects. Eventually, too, geneticists learned that many genes can combine to determine a trait, allowing them to reconcile their new theories of inheritance with the sort of continuous variation in size and shape on which Darwin had focused.

Thus the way was finally cleared, by the 1920s, to reconciling genetics with the concept of natural selection. This work was achieved by three mathematically inclined geneticists: Sewall Wright in the United States, and Ronald Fisher and J. B. S. Haldane in England. Their approach essentially founded the mathematical study of evolution—a discipline still known as "population genetics."

The reconciliation of natural selection with the newer science of genetics inspired still more work in evolutionary theory. The Russian-born geneticist Theodosius Dobzhansky migrated to Morgan's lab at Columbia University and launched into a brilliant series of studies of evolutionary processes in natural (wild) populations of fruit flies. Dobzhansky, along with the ornithologist Ernst Mayr (also in New York, at the American Museum of Natural History), became intrigued at a pattern they thought had been overlooked by Darwin and all his successors up to the 1930s: discontinuity in the natural world. In particular, they saw that species—especially those living in the same regions—are almost invariably different from each other, and, in particular, do not interbreed with one another.

Dobzhansky, in particular, observed that, at the genetic level, genes are particulate and discrete. But at the level of the population, characteristics tend to be continuous: a spectrum of variation in the size and shape of the antlers of deer, for example, or the tails of humpbacked whales. But at the level of species, discontinuity once more seemed to be the rule. And, he supposed, this must be the result of some additional evolutionary factors—as natural selection would be expected to produce a continuous array of variation within a species.

Thus was born the notion of speciation— that is, the circumstances leading to a new species evolving from an ancestral species, from which it becomes reproductively isolated. Developed at first by Mayr and Dobzhan-sky and still a subject of ongoing research in evolutionary biology, speciation is generally thought to occur when a portion of an ancestral species becomes physically isolated from the main part of the species. If natural selection modifies that population sufficiently so that mating no longer is possible, then we say that a new species has evolved. This is very different from Darwin's original view, as Darwin thought that new species were just the consequence of long periods of gradual change within a species, such that, given enough time, we would recognize that an ancestral species had slowly evolved into what we would call a new species. We now know that new species can evolve very quickly—on the order of hundreds or a few thousands of years.

The 1940s, despite the outbreak of war, saw a remarkable coming together of evolutionary disciplines in what came to be called the Synthetic Theory of Evolution. Paleontologists (led by American Museum paleontologist George Gaylord Simpson), botanists, ecologists, systematists, cytologists (biologists who study cells), and developmental biologists all more or less agreed that the new integration of genetics with Darwinian selection was sufficient to explain all the major features of the evolution of life. By 1959—the centennial year of Darwin's publication of On the Origin of Species—many biologists firmly believed that a complete theory of the mechanisms of evolution was at hand. They were mistaken.

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