Evolutionary contingency as non-trivial objective probability: Biological evitability and evolutionary trajectories

Contingency-theorists have put forth differing accounts of evolutionary contingency. The bulk of these accounts abstractly refer to certain causal structures in which an evolutionarily contingent outcome is supposedly embedded. For example, an outcome is evolutionarily contingent if it is at the end of a ‘path-dependent’ or ‘causally dependent’ causal chain. However, this paper argues that many of these proposals fail to include a desideratum – the notion of biological evitability or that evolutionary outcomes could have been otherwise – that for good theoretical reasons ought to be part of an account of evolutionary contingency. Although an inclusion of this desideratum might seem obvious enough, under some existing accounts, an outcome can be contingent yet inevitable all the same. In my diagnosis of this issue, I develop the idea of trajectory propensity to highlight the fact that there are plausible biological scenarios in which causal structures, alone, fail to exhaustively determine the biological evitability of evolutionary forms. In the second half of the paper, I present two additional desiderata of an account of evolutionary contingency and, subsequently, proffer a novel account of evolutionary contingency as non-trivial objective probability, which overcomes the shortcomings of some previous proposals. According to this outcome-based account, contingency claims are probabilistic statements about an evolutionary outcome's objective probability of evolution within a specifically defined modal range: an outcome, O, is evolutionarily contingent in modal range, R, to the degree of objective probability, P (where P is in between 1 and 0).