Much philosophical progress has been made in elucidating the idea of evolutionary contingency in a recent re-burgeoning of the debate. However, additional progress has been impaired on three fronts. The first relates to its characterisation: the underspecification of various contingency claims has made it difficult to conceptually pinpoint the scope to which 'contingency' allegedly extends, as well as which biological forms are in contention. That is-there appears to be no systematic means with which to fully specify contingency claims which has led to a tendency for authors to talk past each other. Secondly, on the matter of evidence, recent research has focused on the evidential import of (genuine) convergent evolution which is taken to disconfirm the evolutionary contingency thesis. However, there has been a neglect of convergent evolution's converse: 'evolutionary idiosyncrasies' or the singular evolution of certain forms, which I argue is evidentially supportive of evolutionary contingency. Thirdly, evolutionary contingency has often been claimed to vary in degrees and that the debate, itself, is a matter of 'relative significance' (sensu Beatty). However, there has been no formal method of evaluating the strength of contingency and its relative significance in a particular domain. In this paper, I address all three issues by (i) proposing a systematic means of fully specifying contingency theses with the concept of the modal range. Secondly, I (ii) propose an account of evolutionary idiosyncrasies, investigate the explanations for their occurrences, and, subsequently, spell out their significance with respect to the evolutionary contingency thesis. Finally, having been equipped with the evidential counterpart to convergent evolution, I shall (iii) sketch a likelihood framework for evaluating, precisely on the basis of a sequence of opposing data, the strength and relative significance of evolutionary contingency in a particular domain. With this in hand, the relative observations of idiosyncrasies and convergences can be informative of the strength and relative significance of contingency in any particular domain.

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 'pathdependent' 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).

Contingency-theorists have gestured to a series of phenomena such as random mutations or rare Armageddon-like events as that which accounts for evolutionary contingency. These phenomena constitute a class, which may be aptly called the 'sources of contingency'. In this paper, I offer a probabilistic conception of what it is to be a source of contingency and then examine two major candidates: chance variation and genetic drift, both of which have historically been taken to be 'chancy' in a number of different senses. However, contra the gesturing of contingency-theorists, chance variation and genetic drift are not always strong sources of contingency, as they can be non-chancy (and hence, directional) in at least one sense that opposes evolutionary contingency. The probabilistic conception offered herein allows for sources of contingency to appropriately vary in strength. To this end, I import Shannon's information entropy as a statistical measure for systematically assessing the strength of a source of contingency, which is part and parcel of identifying sources of contingency. In brief, the higher the entropy, the greater the strength. This is also empirically significant because molecular, mutational, and replicative studies often contain sufficient frequency or probability data to allow for entropies to be calculated. In this way, contingency-theorists can evaluate the strength of a source of contingency in real-world cases. Moreover, the probabilistic conception also makes conceptual room for the converse of sources of contingency: 'sources of directionality', which ought to be recognised, as they can interact with genuine sources of contingency in undermining evolutionary contingency.

The Gouldian argument for evolutionary contingency found in Wonderful Life (1989) can be dissected into three premises: a palaeontological, a macroevolutionary, and a developmental. Discussions of evolutionary contingency have revolved primarily around the developmental. However, a shift in methodological practice and new palaeontological evidence subsequent to the book's publication appears to threaten the palaeontological premise that asserts high Cambrian disparity or, roughly, that morphological differences between the Cambrian species were high. This presents a prima facie problem: did the Cambrian consist of enough anatomical variety to fuel a different outcome upon a replay of the tape of life? I investigate the role that disparity plays in the Gouldian argument for evolutionary contingency and conclude that the reduced disparity view does not undermine the argument, per se. As for the macroevolutionary premise, Gould never offered any explicit justification for the idea that species sorting was indiscriminate or 'lottery-like' (though he certainly gestured at it). Putting aside the exegetical, I consider certain empirical avenues for underwriting the macroevolutionary premise and argue that the most plausible route is to appeal to the special status of mass extinctions, a move that is not entirely unfamiliar to macro-evolutionists. That is -mass extinctions can be good sources of contingency due to their being 'chancy' in, at least, three respects. In sum, I update the palaeontological and macroevolutionary premises as to reconstrue the Gouldian argument for evolutionary contingency in its strongest form. 1 Introduction 2 The Gouldian Argument for Evolutionary Contingency 2.1 The Palaeontological Premise 2.2 The Role of Disparity 2.3 The Macroevolutionary Premise 2.4 The Developmental Premise 3 Macroevolutionary Sampling: Biased or Unbiased? 4 The Special Status of Mass Extinctions 4.1 Low Probability of Occurrence 4.2 Random Survivorship Rules: Field of Bullets or Wanton? 4.3 Aftermath Effects: 'Random' Origination

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