Charles Darwin's (1859) case for the fact of evolution was as crucial to his theory as was the mechanism he proposed. The idea of natural selection becomes interesting and compelling only in the context of all the great facts of natural history that it explains. Life on earth has changed. New forms have appeared, replacing forms that have ceased to exist. Origination and extinction are two of the great facts of natural history. They underlie the patterning of biological diversity. Case by case, these facts are explained more consistently, more simply, and more compellingly by the mechanisms of evolution than by any set of ad hoc explanations.

The realization that the continents are mobile and not fixed in position, and the discovery of the processes driving that mobility, is one of the great scientific achievements of the 20th Century. From the outset, fossil evidence has been important in reconstructing past continental positions, usually by providing data on ancient similarities and differences that appear at odds with present-day geographies. However, the fossil record does much more than provide evidence on ancient continental positions: it shows the diverse evolutionary effects that the dynamics of the Earth's crust have had on the passengers inhabiting those mobile continents.

In modern biology, natural selection is widely treated as the central mechanism of evolution. Darwin built his greatest book on it; largely ignored for the latter part of the 19th Century, its centrality was restored during the Modern Synthesis of the 1930s. Since then, its importance has sometimes been exaggerated into claims that selection drives adaptation toward optimality. Its effects have been studied in captive and wild populations of all sorts of organisms, and in past as well as present biotas. But it has also been downplayed as only one of many factors of evolutionary change, an editor with no creative force, incapable of guiding any real change in lineages. Natural selection is as intrinsic to ecological theory as to evolutionary theory, and links the two fields as few other concepts do. But like nearly all population processes, its effects on evolution in the long run remain an open question, difficult to test directly.

Change has been the rule in the history of life. Mammals today dominate the terrestrial habitats where dinosaurs once held sway. In modern oceans, ecologists can study many species of arthropods, but trilobites are long gone. Using data from the fossil record, David Raup estimated that only about one in a thousand species that ever lived on this planet is still alive today (Raup, 1991). On the other hand, the number of species and higher taxa has increased steadily over geologic time. Thus the history of life is essentially a history of turnover of species, lineages and higher taxa over time.

Any two biologic groups — populations, species, or groups of species — that are separated in time but connected by evolution provide us with the opportunity to measure a rate of evolution.

Rates are interesting because they vary greatly. Some species have survived for millions of years without appreciable change. On the other hand, some species have arisen from other, quite different species within just a few thousands or tens of thousands of years: very brief intervals of time. Similarly, some groups of organisms have diversified rapidly, producing many new species during brief intervals of geologic time, whereas others have added new species very slowly.

The study of biodiversity can be divided into two major aspects. One aspect is concerned with the numbers of species, genera, families or other taxonomic units that are present within a given group of organisms, or a given region, or during a given period of time. This measure of diversity is termed richness. Richness may be represented at any geographic scale: local, such as the number of species in your back yard; regional, such as the number of species found in California; or global, such as the number of species in the entire world at present. Preservation of species richness in the present biosphere is clearly a matter of great social and scientific concern. Each species has a unique genetic makeup, and a distinctive place within an ecosystem. If a species is lost, the unique genes are lost also, and the effects on the ecosystem can be destabilizing, affecting the well-being of still other species.

For many of us, the word model may trigger remembrances of long-ago afternoons spent painstakingly gluing a small plastic car or airplane model together (for others, it may conjure up images of a somewhat emaciated young woman staring out of the cover of Vogue or Elle). The model car is not the same as a real car; it is made of different materials; it has many fewer parts, and it does not move. Nevertheless, it resembles a real automobile sufficiently that we recognize it as a realistic representation. Similarly, scientists use the term model to refer to a reconstruction of nature for the purpose of study (Levins, 1966). In other words, in order to understand nature, one may not always want to study it directly. Instead, understanding may come from studying a facsimile of nature that captures what is perceived to be its essential properties. In the same way, a child may learn how a car is built by building a plastic model of it, even if this model contains no moving parts.

Evolution is one of the most controversial concepts for teachers to teach. The source of opposition to evolution primarily comes from a perception that acceptance of evolution is somehow incompatible with religion. In this paper, I shall discuss the sometimes bumpy relationships between science, religion and evolution, and what this history means for teachers. First, however, I should define the critical terms evolution, science, and religion.

The Creationist/ Scientist conflict, centering on the teaching of evolution in public, has become an ideological battleground in some parts of the U.S. The constitutional requirement for separation of Church and State is being countered by sophisticated maneuvering that attempts to present Creationism as an alternate, non-religious, legitimate scientific paradigm (model). On the other hand, the counter-claim that a religion-free approach is itself an alternative belief system, and therefore should also be prohibited by the Constitution, is not satisfactorily resolved. Left in the middle are children, students and lay people for whom not the constitutional issue, but the actual out-of-court resolution of the conflict between science and religion, is of considerable importance.