Evolution  (Page 3/9)

Natural selection (aka adaptive evolution) is, as Darwin pointed out 150+ years ago, analogous to the process by which animal breeders produce animals with novel traits (aka artificial selection). For example, a pigeon breeder might notice that one of his pigeons has an unusually large ruff of feathers around its neck. He breeds this pigeon with another pigeon, and selects the pigeons with the biggest ruffs from among the offspring to be the parents of the next generation. After a few cycles of this, some of the pigeon offspring will have very unusual and pronounced neck ruffs, and will look nothing at all like the original pigeon ancestor in that regard (figure 1, below). This common practice gets its name from the fact that the breeder selects , or chooses, specific animals to be the parents of the next generation. And it works; there are many examples where substantial changes in animal appearance, or behavior, can be brought about in just a few generations by applying this artificial method of selection.

Darwin’s genius, and the source of Huxley’s self-disparaging statement at the top of this chapter, was to recognize that this process could also occur in the absence of an individual who did the selecting. Natural selection , the idea for which Darwin is so famous, simply recognizes three well-known observations and puts them into a context that generates evolutionary change. Let’s look briefly at each of these three observations.

The first thing that Darwin postulated is that the variations seen in living things are due, to a greater or lesser degree, to heritable factors. In other words, there are heritable variations among the individuals in a population of organisms. Let’s break down that term a bit, and look at each of the words, using examples mostly from human populations.

Firstly, we know that there are variations among individuals in a population. Look around your classroom, or at your family picture album. You probably don’t look exactly like your brother or sister, and your mom and dad don’t look exactly like your uncles or aunts. So even in situations where the parents are the same, variation occurs among the offspring. Variation is even greater in a population of individuals who don’t share the same parents. Variation is normal, and easily observed.

What about that other word, heritable ? Again we now know that many of those variable traits are heritable, i.e. they are passed from one generation to the next. In humans, eye color, hair color, height, etc. are all characteristics that might be the same in you and your parents. If you have a dog or cat, and that dog or cat has offspring, you can often see aspects of the offspring (e.g. coat color, size) that are identical to those in the parental animal. One likely explanation for that observation is that you and your parents have some shared molecule or molecules that determine each of those traits. We now know (but Darwin didn’t) that the molecule is DNA, about which you will learn more later. On the other hand, some conditions are not heritable. For example, if you have a cat that lost its tail in a horrible and noisy accident involving a rocking chair and your 300-lb great-aunt, and if the cat has kittens, those kittens will have normal tails. The rocking chair might damage the cat’s tail, but not its DNA. At the time of Darwin, the mechanisms of heritability were not known (he knew nothing about genes), but everyone understood that some traits were heritable, and others were not. So again, the heritable variation that is necessary for evolution to occur is easily observed in the natural world.