Zero force laws

This is an example of what philosophers call a zero force law. Zero force laws play a very important role in scientific theories, because we can't begin to understand what a force does until we understand what would happen in the absence of any forces. Consider Newton's famous dictum:

An object in motion tends to remain in motion in a straight line. An object at rest tends to remain at rest.

or (as you may remember from introductory physics)

$$F = ma \quad.$$

If we observe an object accelerating, we can immediately infer that a force is acting on it, and we can infer something about the magnitude of that force. However, if an object is not accelerating we cannot conclude that no forces are acting. It might be that opposing forces act on the object in such a way that the resultant is no net force. Acceleration is a sufficient condition to infer that force is operating on an object, but it is not necessary.

What we might call the ``First Law of Population Genetics'' is analogous to Newton's First Law of Motion:

If all genotypes at a particular locus have the same average fecundity and the same average chance of being included in the breeding population, allele frequencies in the population will remain constant.

For the rest of the semester we'll be learning about the forces that cause allele frequencies to change and learning how to infer the properties of those forces from the changes that they induce. But you must always remember that while we can infer that some evolutionary force is present if allele frequencies change from one generation to the next, we cannot infer the absence of a force from a lack of allele frequency change.