Modern geology has helped solve another puzzle that troubled Darwin—the existence of oddly similar terrestrial species on separate continents. How, for example, to explain the emus of Australia, ostriches of Africa and rheas of South America— large, flightless, long-necked birds with the same distinctive sternums? Early evolutionists, following Darwin, invoked scenarios such as long-gone land bridges stretching thousands of miles to explain how apparently related species could wind up so far apart. The outrageous truth wasn't revealed until the 1960s, when scientists discovered plate tectonics and confirmed that the continents, far from being permanent fixtures of land surrounded by water, were giant rafts floating on molten rock. This discovery justified the nagging suspicion of middle school students everywhere that the continents should fit together into a giant jigsaw puzzle, as indeed they once had. In Darwin's time, the idea that once-contiguous continents shifted apart, separating sister species one from another, would have been nearly as audacious as evolution itself.
Evolution explains the vast diversity of life on earth, with single species becoming many as they adapt to different environments. "Remarkably," says the evolutionary biologist Edward O. Wilson, "although his masterwork was entitled On the Origin of Species, Darwin really didn't pay much attention to how one species splits and multiplies into many." Darwin did acknowledge the importance of this process, called speciation, at the very end of Origin: "Life, with its several powers, having been originally breathed into a few forms or into one...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." But, says Wilson, Darwin focused on "how one species was transformed by some force or other into another species through time, not how species could multiply."
Darwin's famous Galápagos finches—more than a dozen species all descended from the same South American ancestor—would become the iconic example of speciation. But understanding the process would have to wait for the work of Wallace in the mid-1860s. "Wallace clearly expressed [speciation] in a major study made of butterflies of the Malay Archipelago," Wilson says. Wallace, working in an area with tens of thousands of islands, showed that a single butterfly species could slowly become many as it adapted to the specific conditions encountered on each island. "From then on biologists put more time into thinking about multiplication of species," Wilson says, "and by the turn of the century they had a pretty clear idea of how species originate. But that was something that Darwin held back a little."
Darwin knew that plant and animal species could be sorted into groups by similarity, such that birds clustered into songbirds and raptors, say, with each group subdivided again and again down to dozens or hundreds of distinct species. He also saw that the individuals within any given species, despite many similarities, also differed from one another—and some of those differences were passed from parents to their offspring. And Darwin observed that nature had a brutally efficient method of rewarding any variation that helped an individual live longer, breed faster or leave more progeny. The reward for being a slightly faster or more alert antelope? The lions would eat your slower neighbors first, granting you one more day in which to reproduce. After many generations and a great deal of time, the whole population would run faster, and with many such changes over time eventually become a new species. Evolution, Darwin's "descent with modification through natural selection," would have occurred.
But what was the source of variation and what was the mechanism for passing change from generation to generation? Darwin "didn't know anything about why organisms resemble their parents, or the basis of heritable variations in populations," says Niles Eldredge, a paleontologist at the American Museum of Natural History in New York City.
In Darwin's era, the man who did make progress on the real mechanism of inheritance was the Austrian monk Gregor Mendel. In his abbey garden in the late 1850s and early 1860s, Mendel bred pea plants and found that the transmission of traits such as flower color and seed texture followed observable rules. For instance, when plants with certain distinct traits were bred with each other, the hybrid offspring did not have a trait that was a blend of the two; the flowers might be purple or white, but never an intermediate violet. This surprising result helped point the way toward the concept of "units" of inheritance—discrete elements of hereditary information. An offspring inherits a set of these genetic units from each parent. Since the early 1900s, those units of inheritance have been known as genes.
Mendel knew Darwin's work—his German copy of Origin was sprinkled with handwritten notes—but there's no evidence that Mendel realized that his units of inheritance carried the variation upon which Darwinian selection acted. "The interesting thing is that Mendel had both pieces of the puzzle in his hands, but he never put it together," says Michael Ruse, a historian and philosopher of science at Florida State University. "He never once said, 'Ah hah, I've got the answer to Darwin's problem.'" Mendel's discoveries remained obscure until after he died in 1884, and Darwin never knew of them. But what if he had? "If Darwin had read Mendel's papers, he might have picked up on it," Ruse says, "but I'm not sure it would have made much difference."
Today, comparative genomics—the analysis of whole sets of genetic information from different species—is confirming the core of Darwin's theory at the deepest level. Scientists can now track, DNA molecule by DNA molecule, exactly what mutations occurred, and how one species changed into another. (In one particularly fitting example, researchers are now working out the molecular changes that allowed Darwin's Galápagos finches to evolve different beaks in response to their different feeding strategies.) Darwin himself made a stab at drawing a "tree of life," a diagram that traces the evolutionary relationships among species based on their similarities and differences. But scientists are now constructing the most detailed tree of life ever, as part of the Encyclopedia of Life project (sponsored in part by the Smithsonian Institution), using DNA sequence data as well as traditional anatomical and behavioral characteristics to trace the precise evolutionary relationships among thousands and thousands of species.