Philosophers of science have long been uneasy with biology, preferring instead to focus on physics. At the heart of this preference is a mistrust of uncertainty. Science is supposed to be the study of what is true everywhere and for all times, and the phenomena of science are supposed to be repeatable, arising from universal laws, rather than historically contingent. After all, if something pops up only on occasional Tuesdays or Thursdays, it is not classified as science but as history. Philosophers of science have thus been fascinated with the fact that elephants and mice would fall at the same rate if dropped from the Tower of Pisa, but not much interested in how elephants and mice got to be such different sizes in the first place.

Philosophers of science have not been alone in claiming that science must consist of universal laws. Some evolutionary biologists have also acceded to the general intellectual disdain for the merely particular and tried to emulate physicists, constructing their science as a set of universal laws. In formulating the notion of a universal "struggle for existence" that is the engine of biological history or in asserting that virtually all DNA evolves at a constant clocklike rate, they have attempted to find their own versions of the law of gravity. Recently, however, some biologists have questioned whether biological history is really the necessary unfolding of universal laws of life, and they have raised the possibility that historical contingency is an integral factor in biology.

To illustrate the difference between biologists favoring universal, deterministic laws of evolutionary development and those leaving room for historical contingency, consider two favorite statements of philosophers (both of which appear, at first sight, to be universal assertions): "All planets move in ellipses" and "All swans are white." The former is truly universal because it applies not only to those planets that actually do exist, but also to those that could exist—for the shape of planetary orbits is a necessary consequence of the laws governing the motion of objects in a gravitational field.

Biological determinists would say that "All swans are white" is universal in the same way, since, if all swans were white, it would be because the laws of natural selection make it impossible for swans to be otherwise: natural selection favors those characteristics that increase the average rate of offspring production, and so traits that maximize flexibility and the ability to manipulate nature will eventually appear. Nondeterminist biologists would deny this, saying that "swans" is merely the name of a finite collection of historical objects that may happen all to be white, but not of necessity. The history of evolutionary theory has been the history of the struggle between these two views of swans.