Chapter Summary

The evolution of new species is an obvious result of the interacting processes of genetic variation, natural selection, and environmental change. New species arise when a population within an existing species becomes isolated. Isolation is usually geographic but may also be the result of a macromutation. In isolation, this new population experiences the processes of genetic variation and phenotypic adaptation separate from the parent species. Among the differentiating traits that result may be characters that act as reproductive isolating mechanisms, meaning that even if the populations once again have the opportunity to interbreed, they will probably not be able to do so. They will have become separate species.

The process of speciation, occurring countless times over the billions of years of life’s history, has produced the incredible array of living forms we know today and see in the fossil record. The species we know about only hint at the variety that exists and has existed in the past. When species have the opportunity, they are able to adaptively radiate into new niches, and the increased diversity of living forms is the result. It must be remembered, however, that all these forms are variations on the single theme of life that originated on this planet. All life, through speciation and adaptive radiation, is descended from a single origin.

We can depict evolution as a luxuriant bush, dense with innumerable twigs, each representing a new species. We use the bush as a metaphor for evolution for two reasons. First, we realize that the conditions for speciation are continuously being produced, so that speciation has probably taken place more often than we can imagine. Second, we understand that Darwin’s model of the origin of species, driven by the steady, gradual “fine-tuning” of natural selection, is not accurate. Natural selection is a conservative force, acting largely to maintain a species’ adaptation. What produces new species is geographic or genetic isolation, and these processes act relatively quickly.

The evidence for evolution is overwhelming. We are still examining the specific details of evolutionary theory, but the idea of evolution and our knowledge of the essential processes involved are hypotheses that have passes numerous scientific tests and failed none. They are theories in the scientific sense of the word. The evidence for evolution is as strong as, say, the evidence for the theory that the earth revolves around the sun. Nonetheless, some people, for reasons that have nothing to do with science, claim that evolution did not occur. These claims have been examined scientifically, and they have been shown to be scientifically invalid. Their proponents, however, continue to insist on their accuracy. These claims are thus pseudoscience, and they confuse the important relationship between science and belief systems; for, in fact, there is no necessary conflict between the two.

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