Eugenie C. Scott has been the Executive Director of the National Center for Science Education (NCSE) since 1987. She is a physical anthropologist who taught at several universities prior to becoming director of NCSE.
Scot has published dozens of articles on evolution and creationism in the popular press and she is a frequent guest on television and radio broadcasts. She is one of the world's leading experts on explaining evolution to the general public (and to other scientists).
Although she meets all the qualifications, she was not included in Richard Dawkins' book: The Oxford Book of Modern Science Writing.
The following excepts are taken from her latest book Evolution vs. Creationism: An Introduction.
If a population at the end of a geographic range of a species is cut off from the rest of the species, through time it may become different from other populations. Perhaps natural selection is operating differently in its environment than it is in the rest of the species range, or perhaps the population has a somewhat different set of genes than other populations of the species. Just by the rules of probability, a small, peripheral population is not likely to have all the variants of genes that are present in the whole species, which might result in its future evolution taking a different turn.
No longer exchanging genes with other populations of the species, and diverging genetically through time from them, members of a peripheral, isolated population might reach the stage where, were they to have the opportunity to mate with a member of the "parent" species, they would not be able to produce offspring. Isolating mechanisms, most of which are genetic but some of which are behavioral, can arise to prevent reproduction between organisms from different populations. Some isolating mechanisms prevent two individuals from mating at all: in some insects, for example, the sexual parts of males and females of related species are so different in shape and size that copulation can't take place. Other isolating mechanisms come into effect when sperm and egg cannot fuse for biochemical or structural reasons. An isolating mechanism could take the form of the prevention of implantation of the egg or of disruption of the growth of the embryo after a few divisions. Or the isolating mechanism could kick in later: mules, which result from crossing horses and donkeys are healthy but sterile. Donkey genes thus are inhibited from entering into the horse species and vice versa. When member of two groups aren't able to share genes because of isolating mechanisms, we can say that speciation between them has occurred. (Outside of the laboratory, it may be difficult to determine whether two species that no longer live in the same environment are reproductively isolated.)
The new species would of course be very similar to the old one—in fact, it might not be possible to tell them apart. Over time, though, if the new species manages successfully to adapt to its environment, it might also expand and bud off new species, which would be yet more different from the parent—now "grandparent"—species. This branching and splitting has, through time, given us the variety of species that we see today.
... common to all ID [intelligent design] proponents is the rejection of "Darwinism." In ID literature, "Darwinism" becomes an epithet, though it is not always clear in any given passage exactly what is meant by "Darwinism." In evolutionary biology, "Darwinism" usually refers to the ideas held by Darwin in the nineteenth century. Usually the term is not used for modern evolutionary theory, which, because it goes well beyond Darwin to include subsequent discoveries and understandings, is more frequently referred to as "neo-Darwinism," or just "evolutionary theory." Evolutionary biologists hardly ever use "Darwinism" as a synonym for evolution, though it occasionally is used this way by historians and philosophers of science. In ID literature, however, "Darwinism" can mean Darwin's ideas, natural selection, neo-Darwinism, post-neo-Darwinian evolutionary theory, evolution itself, or materialist ideology inspired by "Godless evolution."
The public, on the other hand, is unlikely to make these distinctions, instead equating "Darwinism" with evolution (common descent). For decades, Creation Science proponents have cited the controversies among scientist over how evolution occured—including the specific role of natural selection—in their attempts to persuade the public that evolution itself—the thesis of common ancestry—was not accepted by scientists, or at least was in dispute. Within the scientific community, of course, there are lively controversies, including over how much of evolution is explained by natural selection and how much by additional mechanisms such as those being discovered in evolutionary developmental biology ("evo-devo"). No one says natural selection is unimportant; no one says that additional mechanisms are categorically ruled out. But these technical arguments go well beyond the understanding of laypeople and are easily used to promote confusion over whether evolution occurred. Intelligent Design proponents similarly exploit public confusion about "Darwinism" to promote doubt about evolution.