The concept of fitness as used in mathematical models of evolution

Problems have arisen in the field of philosophy of science with the term fitness. This is because of how we as biologists have created multiple definitions and uses for the term. To clear up this confusion I aim to create a concise definition for the term. Fitness is a random variable characterized by a distribution and can be measured by descendants at a specific future time interval. This does not clear up debate regarding whether an individual or an individual’s trait is fundamental in evolutionary theory. We state that an individual is what is fundamental in theory. This is because trait fitness only arises in special case highly derived population genetics models and is calculated from simplifying assumptions stemming from the individual. These models have led to a debate over their interpretation: statistical or causal. Here we argue that the statistical interpretation draws its conclusions from a misinterpretation of where mathematics enters our evolutionary theory. The covariances, variances, and regressions are themselves mathematical definitions and not statistical parameters being applied to a data set. Finally, there is more debate about whether or not individual fitness can be a cause of evolution. With understanding causal inference models and applying our new definition of fitness we can model how fitness can be a cause of evolution. This is by showing a change in expected distribution to the actual distribution.