The Modern Synthesis

Fisher, Wright, and the other scientists who were the first to show how genetics fuel evolution were not field biologists. They were mainly lab‑bound experimentalists and mathematically inclined theoreticians. But by the 1930s, other researchers began to apply their ideas to the real world: to the patterns of diversity found among living species and to the evidence in the fossil record. Just as Fisher and Wright had melded genetics and evolutionary theory, this next generation of scientists added new ingredients from ecology, zoology, and paleontology. By the 1940s, non‑Darwinian explanations for evolution ideas about inner forces directing some sort of Lamarckian transformation, or giant mutations creating new species in a single generation–had become hopelessly old‑fashioned.

One major step toward the modern synthesis came in 1937 with the publication of a book called Genetics and the Origin of Species, by a Soviet scientist named Theodosius Dobzhansky. Dobzhansky had come to the United States nine years earlier to work in the laboratory of Thomas Hunt Morgan at Columbia University, where biologists studying the fruit fly Drosophila melanogaster were discovering the true nature of mutations. Dobzhansky was an oddball in the lab; to the other members of the Fly Room, fruit flies lived only in milk bottles in cluttered laboratories. But Dobzhansky had studied insects in the wild since he was a child in Kiev. When he was a teenager, his goal in life was to collect every species of ladybird beetle in the region. “Seeing a lady beetle still produces in me a flow of a love hormone,” Dobzhansky would say many years later. “The first love is not easily forgotten.”

Dobzhansky developed a sharp eye for the natural variations among different populations of ladybird beetles, and when he read about Morgan’s work on mutations, he wondered if it might reveal their underpinnings. But the genetics of the ladybird beetle were too complicated for Dobzhansky to work out, so he switched his research to Drosophila melanogaster , Morgan’s well‑studied fly.

Dobzhansky quickly earned a reputation as a brilliant student of genetics, and at age 27 he was invited to come to New York to learn the newest methods of the Fly Room. When Dobzhansky and his wife arrived at Columbia, they found the Morgan lab a dreary, roach‑infested place. But in 1932 things improved: Morgan packed up shop and moved out to the California Institute of Technology. Dobzhansky followed him and settled happily among the orange groves.

In California, Dobzhansky finally began to answer the questions he had asked as a teenager: What were the genetics that determined the differences between populations of a species? Most biologists at the time assumed that in any given species, the animals all had practically identical genes. After all, it had taken Morgan years to find a naturally occurring mutation arising in his fruit flies. But these were assumptions bred in the lab.

Dobzhansky began studying the genes of wild fruit flies, traveling from Canada to Mexico to catch members of the species Drosophila pseudoobscura. Today biologists can sequence every letter in a species’ entire genetic code, but in Dobzhansky’s day, the technology was far cruder. He could only judge differences between chromosomes by looking at them through a microscope. Even with such simple methods, he found that different populations of D. pseudoobscura did not have identical sets of genes. Each population of fruit flies he studied bore distinctive markers in their chromosomes that set them off from other populations.

Decades later, when geneticists had invented more precise ways to compare DNA, they would find that the variability Dobzhansky had found among his fruit flies was the rule, not an exception. Among humans, for example, many biologists once thought that races carried dramatically distinct sets of genes. Some even went so far as to claim that they were separate species. But research on human genetics now shows that these old ideas were wrong. “The biological notion that we used to have of races is not compatible with the reality of the genetics that we’re finding today,” says Marcus Feldman, a geneticist at Stanford University.

Out of 25,000 or so genes in the human genome, an estimated 6,000 genes exist as different versions (known as alleles). The distinctions that we conventionally use to divide the species into races–skin color, hair, and the shape of faces–are controlled by only a few genes. The vast majority of variable genes do not respect so‑called racial boundaries. There is far more variability within any given population of humans than between populations. If all the humans on Earth were wiped out except a single tribe in a remote New Guinea valley, the survivors would still preserve 85 percent of the genetic variability of our entire species.

Dobzhansky’s discovery of so much genetic variability in a single species raised a deep question: If there was no standard set of genes that distinguished a species, what kept species distinct from one another? The answer, Dobzhansky correctly realized, was sex. A species is simply a group of animals or plants whose members reproduce primarily among themselves. Two animals belonging to different species are unlikely to mate, and even if they do, they will rarely produce viable hybrids. Biologists already knew that hybrids often died before they hatched or they grew into sterile adults. Dobzhansky ran experiments on fruit flies that demonstrated that this incompatibility is caused by specific genes carried by one species that clash with the genes from another species.

In Genetics and the Origin of Species Dobzhansky sketched out an explanation for how species actually came into existence. Mutations crop up naturally all the time. Some mutations are harmful in certain circumstances, but a surprising number have no effect one way or the other. These neutral changes pop up in different populations and linger, creating a variability that is far greater than anyone had previously imagined. And all this variability can be a good thing, evolutionarily speaking, because if conditions change, a mutation’s neutral effects can become useful and be favored by natural selection.

Variability is also the raw material for making new species. If a population of flies starts breeding only among themselves, their genetic profile will grow distinct from the rest of its species. New mutations would crop up in the isolated population, and natural selection might help them to spread until all the flies carried them. But because these isolated flies were breeding only within their own population, the mutations could not spread to the rest of the species. The isolated population of flies would become more and more genetically distinct. Some of their new genes would turn out to be incompatible with the genes of flies from outside their own population.

If this isolation lasted long enough, Dobzhansky argued, the flies might lose the ability to interbreed completely. They might simply lose the ability–or interest–to mate with the other flies. Even if they did produce offspring, the hybrids might become sterile. If the flies were now to come out of their isolation, they could live alongside the other insects but still continue mating only among themselves. A new species would be born.

Dobzhansky’s 1937 book captivated biologists far beyond the confines of genetics. In the mountains of New Guinea, an ornithologist named Ernst Mayr found Genetics and the Origin of Species to be an enormous inspiration. Mayr specialized in discovering new species of birds and mapping out their ranges. The work was hard, and not just because of the malaria and the headhunters. Like other ornithologists, Mayr had a difficult time determining exactly when a group of birds deserved the title of species. A bird of paradise species might be recognizable by the color of its feathers, but from place to place it might have a huge amount of variation in other traits–on one mountain it might have an extravagantly long tail and on another its tail would be cut square.

Biologists typically tried to bring order to this chaos by recognizing subspecies–local populations of a species that were distinct enough to warrant a special label of their own. But Mayr saw that the subspecies label was far from a perfect solution. In some cases, subspecies weren’t actually distinct, but graded into each other like colors in a rainbow. In other cases, what looked like a subspecies might turn out to be a separate species of its own.

When Mayr read Genetics and the Origin of Species, he realized that these puzzles of species and subspecies shouldn’t be considered a headache: they were actually a living testimony to the evolutionary process Dobzhansky wrote about. Variations emerge in different parts of a species’ range, creating differences between populations. In one part of a bird’s range they may create a long tail, in others a tail that is cut square. But because the birds mate with their neighbors, they do not become isolated into a species of their own.

One of the starkest examples of what this flow of genes can do is a phenomenon known as the “ring species.” In the North Sea, for instance, there is a species of bird known as the herring gull. It has a gray mantle and pink legs. If you move west through its range, you come across more herring gulls in Canada, which look essentially the same as the ones in the North Sea, except for a few minor differences in their coloring. But by the time you reach Canada, the differences become stark, and in Siberia, the gulls have a dark gray mantle and legs that are less pink than yellow. Yet despite these differences, they are still scientifically classified as the herring gull (although their common name is the vega gull). Keep moving through Asia and into Europe, and the gulls continue to get darker and more yellow‑legged. You will find dark, yellow‑legged gulls extending even farther west, all the way to the North Sea where your trip began. Here these gulls, known as the lesser black‑backed gulls, live alongside the gray‑mantled, pink‑legged herring gulls.

Because the two groups of birds look so different and do not mate, they are treated as two separate species. Yet lesser black‑backed gulls and herring gulls live at two ends of a continuous ring, inside of which all the birds can mate with their immediate neighbors. A ring species is exactly what you’d expect given the way mutations arise and spread.

A population of birds can evolve into its own species if it gets cut off from its neighbors. Mayr argued that the easiest way to cut them off is by geographical isolation. A glacier may thrust across a valley, isolating the birds on the mountains on either side. A rising ocean may turn a peninsula into a chain of islands, stranding birds on each of them. This sort of isolation doesn’t have to last forever; it need only form a barrier long enough to let the isolated population become genetically incompatible with the rest of its species. Once the glacier melts, or the ocean drops and turns the islands back into a peninsula, the birds will be unable to interbreed. They will live side by side but follow separate evolutionary fates.

Biologists such as Mayr and Dobzhansky helped assemble the modern synthesis by studying living animals. But if they were right, then the same processes should have been at work for billions of years, and the fossils should record their effects. Yet even in the 1930s, many paleontologists were not yet convinced that natural selection could account for what they saw among their bones. They saw long‑term trends in the evolution of animals that seemed to follow a built‑in direction. Horses seemed to evolve steadily from dog‑sized creatures into bigger and bigger forms; at the same time, their toes steadily shrank away until their feet turned into hooves. The ancestors of elephants were originally the size of pigs, and their descendants evolved into colossal sizes over tens of millions of years; at the same time, their teeth seemed to steadily expand and become more complex. There was no sign, paleontologists claimed, of the open‑ended, irregular experimentation that natural selection might produce.

Henry Fairfield Osborn, the president of the American Museum of Natural History, declared that these trends were proof that much of evolution was not governed by natural selection. Each of the mammal lineages began with the potential already within them to become horses or elephants–“something which in time may appear,” as he put it. Only through a struggle against the elements and other animals could a species discover that potential. “Disprove Lamarck’s principle and we must assume that there is some third factor in Evolution of which we are now ignorant,” he declared in 1934.

But one of Osborn’s students, the paleontologist George Gaylord Simpson, never thought much of this sort of reheated Lamarckism. Simpson was more impressed by Dobzhansky’s ability to link genetics and natural selection. After Simpson read Genetics and the Origin of Species, he decided to see whether the fossil record could be accounted for in the same way.

Simpson took a closer look at the trends in the fossil record that Osborn claimed were evidence for directed evolution. Under his scrutiny, the linear trends broke up into bushy trees of lineages branching off in many directions. Horses, for example, had evolved into many different sizes and hoof anatomies over the past 50 million years; many of these branches are long extinct and had nothing to do with the origin of living horses.

If the natural selection that scientists were studying in labs was in fact responsible for the transformations of the fossil record, it would have to work at a rate that was fast enough to produce the changes paleontologists could see. The Fly Room researchers had made careful measurements of how quickly mutations emerged in fruit flies, and how quickly they could spread with the help of natural selection. Simpson invented his own ways to measure rates of evolutionary change in fossils. He looked over the enormous collections of bones that paleontologists had gathered in the previous century, measuring their dimensions, and plotted how they changed over time. Simpson found that lineages could evolve at fast or slow rates, and even within a single lineage evolution could speed up and slow down over time. And Simpson discovered that the fastest rates of change he found in fossils were outstripped by the speed of evolution documented in fruit flies. The modern synthesis, and not some mysterious Lamarckian process, was all Simpson needed to make sense of his bones.

By the 1940s, the architects of the modern synthesis had shown that genetics, zoology, and paleontology were all telling much the same story. Mutations are the foundation of evolutionary change; combined with Mendelian heredity, the flow of genes, natural selection, and geographical isolation, they could create new species and new forms of life; and over millions of years they could create the transformations recorded in fossils. The success of the modern synthesis has turned it into a driving force behind the evolutionary research of the past 50 years.