ABSTRACT
The dynamical and statistical descriptions of evolutionary theory are often used interchangeably, or admixed indiscriminately. Perhaps the thought is that these are simply alternative, yet equivalent, ways of talking about the same theory. But, it seems to us that there are important differences between these conceptions. If evolutionary theory is a theory of forces it isn't a theory about the statistical structure of populations (and vice versa). Our task in this paper is to articulate and contrast the statistical and dynamical conceptions of evolutionary theory, and to make a case for the statistical conception. We restrict our attention to natural selection and drift, in the hope that the lessons learned there will generalize. Selection and drift are not forces acting on populations; they are statistical properties of an assemblage of "trial" events: births, deaths and reproduction. The only genuine forces going on in evolution are those taking place at the level of individuals (or lower) and none of these (and no aggregate of these) can be identified with either selection or drift.
