The complex nexus of evolutionary fitness

The propensity nature of evolutionary fitness has long been appreciated and is nowadays amply discussed (Abrams, 2009, 2012; Ariew & Ernst, 2009; Ariew & Lewontin, 2004; Beatty & Finsen, 1989; Brandon, 1978; Drouet & Merlin, 2015; Mills & Beatty, 1979; Millstein, 2003, 2016; Pence & Ramsey, 2013; Sober, 1984, 2001, 2013, 2019; Walsh, 2010; Walsh et al., 2016; etc). The discussion has, however, on occasion followed long standing conflations in the philosophy of probability literature between propensities, probabilities, and frequencies. In this paper, I apply a more recent conception of propensities in modelling practice (the ‘complex nexus of chance’, CNC) to some of the key issues, regarding the mathematical representation of fitness and how it may be regarded as explanatory. The ensuing complex nexus of fitness (CNF) emphasises the distinction between biological propensities and the probability distributions over offspring numbers that they give rise to; and how critical it is to distinguish the possession conditions of the underlying dispositional (physical and biological) properties from those of their probabilistic manifestations.