to the needs of a population or organism. In this sense, as Francois Jacob has written, evolution is a "tinkerer, not an engineer."2 Evolution does not design new organisms; rather, new organisms emerge from the inherent genetic variation that occurs in organisms.
Genetic variation is random, but natural selection is not. Natural selection tests the combinations of genes represented in the members of a species and allows to proliferate those that confer the greatest ability to survive and reproduce. In this sense, evolution is not the simple product of random chance.
The booklet Science and Creationism: A View from the National Academy of Sciences3 summarizes several compelling lines of evidence that demonstrate beyond any reasonable doubt that evolution occurred as a historical process and continues today. In brief:
Fossils found in rocks of increasing age attest to the interrelated lineage of living things, from the single-celled organisms that lived billions of years ago to Homo sapiens. The most recent fossils closely resemble the organisms alive today, whereas increasingly older fossils are progressively different, providing compelling evidence of change through time.
Even a casual look at different kinds of organisms reveals striking similarities among species, and anatomists have discovered that these similarities are more than skin deep. All vertebrates, for example, from fish to humans, have a common body plan characterized by a segmented body and a hollow main nerve cord along the back. The best available scientific explanation for these common structures is that all vertebrates are descended from a common ancestor species and that they have diverged through evolution.
In the past, evolutionary relationships could be studied only by examining the consequences of genetic information, such as the anatomy, physiology, and embryology of living organisms. But the advent of molecular biology has made it possible to read the history of evolution that is written in every organism's DNA. This information has allowed organisms to be placed into a common evolutionary family tree in a much more detailed way than possible from previous evidence. For example, as described in Chapter 3, comparisons of the differences in DNA sequences among organisms provides evidence for many evolutionary events that cannot be found in the fossil record.
Evolution is the only plausible scientific explanation that accounts for the extensive array of observations summarized above. The concept of evolution through random genetic variation and natural selection makes sense of what would otherwise be a huge body of unconnected observations. It is no longer possible to sustain scientifically the view that the living things we see today did not evolve from earlier forms or that the human species was not produced by the same evolutionary mechanisms that apply to the rest of the living world.
The following two sections of this chapter examine two important themes in evolutionary theory. The first concerns the occurrence of evolution in "real time"—how changes come about and result in new kinds of species. The second is the ecological framework that underlies evolution, which is needed to understand the expansion of biological diversity.
Evolution by natural selection is not only a historical process—it still operates today. For example, the continual evolution