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thing, long-tongued anteaters may come to dominate the population and become the norm. Over successive generations the species will change. In contrast to earlier theories, Darwin observed that it was not characteristics acquired during a being’s lifetime that were passed on to offspring. Years of obsessive measurements led him to establish the principle that for each and every trait—an anteater’s tongue, hair color, and so on—it was the variation of that characteristic across the population that conferred advantage for an individual. Those traits would become more common in a population because the advantage they conferred would play out as greater reproductive success.
    There are other significant selective forces, such as the complexities of sex, where males puff themselves up to win a female, or females exercise seemingly whimsical choice over males. But natural selection is the overarching force that has shaped the living world in which we live. It’s a system of trial, error, and revision. Evolution is blind and has no direction. Species are not more or less evolved, nor are they higher or lower, as they were once and sometimes still are described. Through iteration, they are merely better adapted to survive in their environments. In an experiment that really should not take place, an orangutan wouldn’t last two minutes in the boiling submarine waters of a hydrothermal vent, despite being sophisticated enough to use tools in the jungles of Borneo. Down in the hot sea of a vent, though, hundreds of species, including the six and a half foot long giant tubeworm and dozens of species of bacteria, are quite content to eke out their existence. Change is the norm, and adaptation is success.
    In the 150 years since the
Origin of Species
was published, millions of scientists have poked and pulled at the theory of evolution; unpicked, tweaked, and yanked at it in every conceivable way. They have observed countless species from aardvarks (or anteaters) to zebras to study their behavior. They have created simulations of myriad populations first with mathematical models, later with computers, and have pushed and pressurized their artificial environments to see how they adapt over successive generations. They have bred and crossbred inestimable species to observe how inheritance works, to see where advantage lies in the next generation. They have let tanks of bacteria reproduce for decades and witnessed descent with modification in action. In the modern era, we have deciphered a host of genetic codes and seen exactly the differences in DNA that reflect one species becoming two, each finding a niche into which they are better suited. We’ve seen populations of bacteria adapt to the hostile action of antibiotics and, distressingly, emerge resistant. While the initial model that Darwin laid out has been modified and fleshed out, the “one big argument,” as he described it, has survived intact throughout the necessary battering that an idea of this magnitude demands. This is why it is called the theory of evolution by natural selection. The colloquial meaning of the term
theory
as a hunch, or guess, or plain old stab in the dark, is woefully puny next to its scientific meaning. When scientists talk about a theory they are aiming at the top of the pile of ideas: a set of testable concepts that all point to and predict a description of reality that is so robust it is indistinguishable from fact.
    Darwin drew up his masterpiece as the study of cells was beginning to emerge from the stagnant pond of spontaneous generation. But his description of evolution is not about the beginning of new life; it is, as is implicit in the title, about the origin of new species. These arise when an organism has acquired so many mutated traits that they are no longer capable of reproducing with what were once their kin. When we are taught natural selection, typically we refer to prominent visible traits—antlers,