Why Evolution Is True Read Online Free PDF Page B

no horns. The Indian rhino is better off than its hornless ancestor, but accidents of genetic history may have led to a less than perfect “design.” And, of course, every instance of a plant or animal that is parasitized or diseased represents a failure to adapt. Likewise for all cases of extinction, which represent well over 99 percent of species that ever lived. (This, by the way, poses an enormous problem for theories of intelligent design (ID). It doesn’t seem so intelligent to design millions of species that are destined to go extinct, and then replace them with other, similar species, most of which will also vanish. ID supporters have never addressed this difficulty.)
    Natural selection must also work with the design of an organism as a whole, which is a compromise among different adaptations. Female sea turtles dig their nests on the beach with their flippers—a painful, slow, and clumsy process that exposes their eggs to predators. Having more shovel-like flippers would help them do a better and faster job, but then they couldn’t swim as well. A conscientious designer might have given the turtles an extra pair of limbs, with retractable shovel-like appendages, but turtles, like all reptiles, are stuck with a developmental plan that limits their limbs to four.
    Organisms aren’t just at the mercy of the luck of the mutational draw, but are also constrained by their development and evolutionary history. Mutations are changes in traits that already exist; they almost never create brand-new features. This means that evolution must build a new species starting with the design of its ancestors. Evolution is like an architect who cannot design a building from scratch, but must build every new structure by adapting a preexisting building, keeping the structure habitable all the while. This leads to some compromises. We men, for example, would be better off if our testes formed directly outside the body, where the cooler temperature is better for sperm. 4 The testes, however, begin development in the abdomen. When the fetus is six or seven months old, they migrate down into the scrotum through two channels called the inguinal canals, removing them from the damaging heat of the rest of the body. Those canals leave weak spots in the body wall that make men prone to inguinal hernias. These hernias are bad: they can obstruct the intestine, and sometimes caused death in the years before surgery. No intelligent designer would have given us this tortuous testicular journey. We’re stuck with it because we inherited our developmental program for making testes from fishlike ancestors, whose gonads developed, and remained, completely within the abdomen. We begin development with fishlike internal testes, and our testicular descent evolved later, as a clumsy add-on.
    So natural selection does not yield perfection—only improvements over what came before. It produces the fitter, not the fittest. And although selection gives the appearance of design, that design may often be imperfect. Ironically, it is in those imperfections, as we’ll see in chapter 3, that we find important evidence for evolution.
    This brings us to the last of evolutionary theory’s six points: processes other than natural selection can cause evolutionary change. The most important is simple random changes in the proportion of genes caused by the fact that different families have different numbers of offspring. This leads to evolutionary change that, being random, has nothing to do with adaptation. The influence of this process on important evolutionary change, though, is probably minor, because it does not have the molding power of natural selection. Natural selection remains the only process that can produce adaptation. Nevertheless, we’ll see in chapter 5 that genetic drift may play some evolutionary role in small populations and probably accounts for some nonadaptive features of DNA.
    These, then, are the six parts of evolutionary theory. 5 Some parts