Why Evolution Works (and Creationism Fails)

Chapter 10: How Evolution Works

86 Chapter 10 How Evolution Works Nothing in biology makes sense except in the light of evolution. —Theodosius Dobzhansky, pioneering geneticist and evolutionary biologist Creationists like to stress chance, as if chance automatically ruled out evolutionary change. Chance is indeed a factor in evolution, but it is not the whole story. Indeed, chance can often lead to very predictable behavior.Thus, before we go on to discuss evolution, let us examine a physical phenomenon that, like evolution, is governed by chance but leads to very lawlike behavior: diffusion. Suppose that we prepare a shallow dish and fill it with water. In the dead center of the dish we inject a small droplet of dye (figure 7, left).With time, the droplet will spread out, and the color will become less intense (figure 7, center, right). Eventually, the dye will uniformly fill the entire dish. From that time on, the appearance of the solution will not change, assuming that we do not let the water evaporate. How does the dye come to fill the dish? By a process known as diffusion. Diffusion is the result of random molecular motions—chance. Each molecule of the dye is constantly buffeted from all directions by the water molecules Now Later Still later 7. A droplet of dye injected into the center of the dish gradually diffuses outward until the dye fills the whole dish uniformly. CH010.qxd 3/26/09 5:29 AM Page 86 How Evolution Works 87 (and other dye molecules, but there are many fewer of them). Its path is therefore completely random—determined by chance. Figure 8 shows the paths of three hypothetical dye molecules. Each follows a random path, but after a great many steps, the odds that any one will remain near the center of the original droplet are very small. Suppose that we repeat the experiment with the same three molecules (a physical impossibility, but this is a thought experiment). On the second, third, fourth, fifth, . . . experiments, they will follow very different paths from those shown in figure 8. It is impossible to predict where they will end up after a few minutes, but few will remain stationary, and all will be found a measurable distance from the center of the dish. There are not three but billions of dye molecules. Instead of repeating the experiment with three molecules over and over, let us do it once with three billion molecules. Each follows a random path, but, on average, each drifts away from the center of the dish just as did the three molecules in figure 8. The result is what we have already seen in figure 7 and is highly repeatable.That is, if we know the temperature, the composition of the dye, and the liquid in which it is dissolved,we can measure the rate at which the droplet spreads out, or diffuses. If we repeat the experiment another time at the same temperature, we will find the same rate with very good accuracy.Thus, the rate of diffusion is highly predictable, even though it is the result of myriad random collisions. Diffusion is a well-known and indeed important physical process that is a result of chance—random collisions—but results in very lawlike behavior. So is evolution. For example, in a small breeding population a rare, yet important allele (form of a gene) may disappear from the population if the individual or 8. The paths of three hypothetical dye molecules. The chance that any one will remain near the center of the original droplet is small. CH010.qxd 3/26/09 5:29 AM Page 87 individuals carrying it fail to breed.This failure could be the result of a chance accident or disease. While this instance of genetic drift is random and due to chance, the probability of its happening in this small population is highly predictable. Diversity When a teacher gazes out over her classroom of fifteen-year-old students, she is aware of the diversity in that small population. She first notices the obvious diversities, like sex and ethnic diversity. Upon further analysis, she begins to notice diversity in hair color and degree of curliness, diversity in height and weight, and diversity in face shape and facial features. Especially because her students are teenagers, she may even begin to notice diversity in the timing of physical maturation. We can easily notice diversity among humans because we are so...