Zachary J. Mayne

This paper analyzes an episode of scientific work that was prompted by observations of a novel defect in early embryonic development, which was unexpectedly induced in an experimental context and has tentatively been dubbed “xenogastrulation.” The researchers worked to individuate this as a novel phenomenon—both by distinguishing it from what it is not (e.g., exogastrulation) and by forming a positive conception of what it is—in order to facilitate further inquiry. Our analysis provides new insights into the role of explanatory reasoning in nascent experimental research programs. We argue that the researchers’ efforts to individuate the novel phenomenon were entangled with their efforts to explain it. By this, we mean that tentative answers to the individuative question, “what is it?” also served as tentative answers to the explanatory question, “by what means does it occur?” This case study therefore demonstrates that explanation need not wait until an explanandum has been clearly individuated but instead can be deeply entangled with the process of individuating the explanandum in the first place.

The propensity interpretation of fitness (PIF) holds that evolutionary fitness is an objectively probabilistic causal disposition (i.e., a propensity) toward reproductive success. I characterize this as the conceptual foundation of the PIF. Reproductive propensities are meant to explain trends in actual reproductive outcomes. In this paper, I analyze the minimal theoretical and ontological commitments that must accompany the explanatory power afforded by the PIF’s foundation. I discuss three senses in which these commitments are less burdensome than has typically been recognized: the PIF’s foundation is (i) compatible with a principled pluralism regarding the mathematical relationship between measures of individual and trait reproductive success; (ii) independent of the propensity interpretation of probability; and (iii) independent of microphysical indeterminism. The most substantive ontological commitment of the PIF’s foundation is to objective modal structures wherein macrophysical probabilities and causation can be found, but I hedge against metaphysically inflationary readings of this modality.