Density-Dependent Selection Revisited: Mechanisms Linking Explanantia and Explananda

Interest in the most general theory of the evolutionary ecology of life histories ever proposed—that of density-independent (r) versus density-dependent (K) selection—peaked in the 1980s and 1990s respectively but declined somewhat thereafter because of arguments over its theoretical coherence and some empirical failures; although as is shown here, not so much as is commonly supposed. In this article, some conceptual analyses and clarification of terms in the conditions for its application (density relative to resources as a variable with spatial, temporal, and niche boundaries fixed), and in the explanantia of the theory (costs and frequencies, stability and instability within and among growing and declining populations, multiple levels of selection, and ultimately homogeneous and patchy environments) are presented. These make intelligible plausible mechanisms (consumption and digestion, production, and reproduction) linking the explanantia and the explananda of the theory that are both theoretically coherent and clarify what predictions it does not and does make including about senescence.