Among the liveliest disputes in evolutionary biology today are disputes concerning the role of chance in evolution—more specifically, disputes concerning the relative evolutionary importance of natural selection vs. so-called “random drift”. The following discussion is an attempt to sort out some of the broad issues involved in those disputes. In the first half of this paper, I try to explain the differences between evolution by natural selection and evolution by random drift. On some common construals of “natural selection”, those two modes of evolution are completely indistinguishable. Even on a proper construal of “natural selection”, it is difficult to distinguish between the “improbable results of natural selection” and evolution by random drift.

In the second half of this paper, I discuss the variety of positions taken by evolutionists with respect to the evolutionary importance of random drift vs. natural selection. I will then consider the variety of issues in question in terms of a conceptual distinction often used to describe the rise of probabilistic thinking in the sciences. I will argue, in particular, that what is going on here is not, as might appear at first sight, just another dispute about the desirability of “stochastic” vs. “deterministic” theories. Modern evolutionists do not argue so much about whether evolution is stochastic, but about how stochastic it is.

Russell (1948), Reichenbach (1956), and Salmon (1975, 1979) have argued that a fundamental principle of science and common sense is that “matching” events should not be chalked up to coincidence, but should be explained by postulating a common cause. Reichenbach and Salmon provided this intuitive idea with a probabilistic formulation, which Salmon used to argue for a version of scientific realism. Van Fraassen (1980, 1982) showed that the principle, so construed, runs afoul of certain results in quantum mechanics. In this paper a new formulation of the principle is offered that emerges from its use in evolutionary theory. This characterization identifies fairly general conditions in which postulating common causes will be more explanatory than postulating separate causes.

A precise formulation of the structure of modern evolutionary theory has proved elusive. In this paper, I introduce and develop a formal approach to the structure of population genetics, evolutionary theory's most developed sub-theory. Under the semantic approach, used as a framework in this paper, presenting a theory consists in presenting a related family of models. I offer general guidelines and examples for the classification of population genetics models; the defining features of the models are taken to be their state spaces, parameters, and laws. The suggestions regarding the various aspects of the characterization of population genetics models provide an outline for further detailed research.

The discovery that some B vitamins are constituents of respiratory coenzymes led to the development of an interfield theory of the kind discussed by Darden and Maull (1977). In this paper it is shown that the development of a useful interfield connection was made possible by two reconceptualizations: (1) a re-conceptualization that united two then-distinct fields giving rise to the concept of vitamins as dietary substances; and (2) another reconceptualization that united two approaches to respiratory metabolism producing the idea that coenzymes are transport vehicles.

Although it is possible to regard a species as a set with a special internal structure, it is preferable to regard a species as an individual precisely to emphasize this internal structure. It is necessary to recognize, moreover, that two organisms that are part of a single entity with respect to one process need not be part of a single entity with respect to another process. Furthermore, choosing to regard two entities (with respect to one process) as conspecific is not to deny that there are two entities within this species. Thus, the systematist need not propose formal names for every entity he discovers, but the names he proposes ought to be as faithful to the relationship among the entities as is possible.

I defend a view of the species category, pluralistic realism, which is designed to do justice to the insights of many different groups of systematists. After arguing that species are sets and not individuals, I proceed to outline briefly some defects of the biological species concept. I draw the general moral that similar shortcomings arise for other popular views of the nature of species. These shortcomings arise because the legitimate interests of biology are diverse, and these diverse interests are reflected in different legitimate approaches to the classification of organisms. In the final section, I show briefly how the pluralistic approach can help to illuminate some areas of biological and philosophical dispute.

Several years ago, I was asked to participate in a forum on evolution. Flattered, I accepted, only discovering later that I was to participate in a “debate”, involving me and a scientist squaring off against two Creationists, Henry M. Morris and Duane T. Gish. The topic for discussion was which doctrine has the greatest scientific merit: organic evolution through natural processes (like natural selection) set against the background of a very old earth (about 4½ billion years), or special instantaneous appearance of all organisms, about 6000 years ago, followed shortly thereafter by a world-wide flood? I very much doubt that many minds were changed on that particular evening—the bleachers were packed with ardent Baptists bussed in from all over Wisconsin and Illinois. The 10 or so in the audience who believed in evolution went on believing in evolution. The 5000 in the audience who took the early chapters of Genesis absolutely literally went on taking the early chapters of Genesis absolutely literally. But, people's time was far from wasted. Morris and Gish got the attention they crave. Their supporters were thrilled by the sight of heretics publicly proclaiming doctrines for which they would surely earn eternal damnation. And, I learnt what a hollow sham modern day Creationism really is: crude, dogmatic, Biblical literalism, masquerading as genuine science, in order to avoid the strictures of the first amendment to the U.S. Constitution, which bars the teaching of religion in schools.