Why Evolution Is True Read Online Free PDF

of evolutionary theory is what Darwin clearly saw as his greatest intellectual achievement: the idea of natural selection. This idea was not in fact unique to Darwin—his contemporary, the naturalist Alfred Russel Wallace, came up with it at about the same time, leading to one of the most famous simultaneous discoveries in the history of science. Darwin, however, gets the lion’s share of credit because in The Origin he worked out the idea of selection in great detail, gave evidence for it, and explored its many consequences.
    But natural selection was also the part of evolutionary theory considered most revolutionary in Darwin’s time, and it is still unsettling to many. Selection is both revolutionary and disturbing for the same reason: it explains apparent design in nature by a purely materialistic process that doesn’t require creation or guidance by supernatural forces.
    The idea of natural selection is not hard to grasp. If individuals within a species differ genetically from one another, and some of those differences affect an individual’s ability to survive and reproduce in its environment, then in the next generation the “good” genes that lead to higher survival and reproduction will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.
    Here’s a simple example. The wooly mammoth inhabited the northern parts of Eurasia and North America, and was adapted to the cold by bearing a thick coat of hair (entire frozen specimens have been found buried in the tundra). 3 It probably descended from mammoth ancestors that had little hair—like modern elephants. Mutations in the ancestral species led to some individual mammoths-like some modern humans—being hairier than others. When the climate became cold, or the species spread into more northerly regions, the hirsute individuals were better able to tolerate their frigid surroundings, and left more offspring than their balder counterparts. This enriched the population in genes for hairiness. In the next generation, the average mammoth would be a bit hairier than before. Let this process continue over some thousands of generations, and your smooth mammoth gets replaced by a shaggy one. And let many different features affect your resistance to cold (for example, body size, amount of fat, and so on), and those features will change concurrently.
    The process is remarkably simple. It requires only that individuals of a species vary genetically in their ability to survive and reproduce in their environment. Given this, natural selection—and evolution—are inevitable. As we shall see, this requirement is met in every species that has ever been examined. And since many traits can affect an individual’s adaptation to its environment (its “fitness”), natural selection can, over eons, sculpt an animal or plant into something that looks designed.
    It’s important to realize, though, that there’s a real difference in what you’d expect to see if organisms were consciously designed rather than if they evolved by natural selection. Natural selection is not a master engineer, but a tinkerer. It doesn’t produce the absolute perfection achievable by a designer starting from scratch, but merely the best it can do with what it has to work with. Mutations for a perfect design may not arise because they are simply too rare. The African rhinoceros, with its two tandemly placed horns, may be better adapted at defending itself and sparring with its brethren than is the Indian rhino, graced with but a single horn (actually, these are not true horns, but compacted hairs). But a mutation producing two horns may simply not have arisen among Indian rhinos. Still, one horn is better than