The Way of Science

Now it's time for the facts of Evolution, and a working definition of what Evolution is.

I. Evolution Is Fact.

Based on extensive observations (150 years' worth) of fossils, comparative anatomy of plants and animals, DNA and chromosomes, etc., we can say with great confidence that populations of organisms change, genetically, as they produce new generations of offspring. (Darwin's basic definition fits well here: Evolution is descent with modification.) Note that these changes occur in populations of organisms, not in individuals. The genetic changes that have taken place, and continue to do so, are of two related kinds: changes within a species (called vertical evolution or adaptation), and changes that give rise to new species from existing ones (called horizontal evolution or speciation). "Speciation" is synonymous with an increase in diversity. There is one more variant on genetic change that leads to a new species. If a population shows gradual genetic change over long generational time, the descendants may be quite unlike their ancestors. Even though we still have one gene pool, the descendants may be so different that they are considered a new species. Paleontologists have superb examples of this kind of transition in hard-shelled marine fossils.

Later, when we examine some arguments that the anti-Evolutionists have proposed, we will look at some of the evidence in more detail.

II. THEORY OF EVOLUTION

After class discussion of the previous material, you should be comfortable with what we mean by the facts of Evolution, and be able to distinguish and describe adaptation and speciation. Now consider the Theory of Evolution: how can we explain "descent with modification? Here we enter the realm of "Darwin's Dangerous Idea," the title of an excellent book by Daniel Dennett, a philosopher. Note that Darwin has had far more effect on the average person's world-view than any of the remarkable scientific ideas of recent times, such as relativity, quantum physics, etc. This "dangerous idea," among other disturbing effects, removes humans from their central and privileged position in biology. It is also purely materialistic, with no need for anything supernatural.

Darwin's famous 1859 publication accomplished two major things. First, using fossils, the Galapagos finches, etc., he established that species were not constant, but evolved from previously existing species. He also speculated, correctly, that all species were/are descended from a single common ancestor. Second, Darwin proposed a mechanism ("theory") to explain how modification occurs within species. This mechanism is a two-step process, of which the second step - natural selection - is the simple but devastating insight. There are other kinds of selection, but this one is the most important for beginners to understand.

Here it is, then, this simple but powerful explanation of how Evolution happens. Never forget that the explanation has two steps, each of which is necessary for most kinds of evolutionary change.

1. Genetic variation

Each generation of offspring commonly consists of large numbers of offspring. These individuals are genetically different from each other and from their parents. This genetic variability is randomly generated, and there is no such thing as an "ideal" individual.

2. Natural selection

Those variants which are best equipped to cope with local conditions will produce more offspring (they will be "selected for," in biological terms) than those individuals more poorly endowed (these are "selected against"). The next generation will contain more genes from the first group, and thus each generation will show more resemblance to the more successful form. Simple, no? Your instructor may wish to demonstrate how quickly a computer model of this basic process can change a gibberish of letters into Shakespeare. Be sure that you understand that the evolutionary "two-step" is both random (step one, production of genetic variation) AND directed (step two, selection). Note also that the directionality is supplied purely materialistically, without need for the supernatural. There is no purpose, no goal, no "straight-line" pursuit of some ideal form.