The concept of “fitness” is a notion of central importance to evolutionary theory. Yet the interpretation of this concept and its role in explanations of evolutionary phenomena have remained obscure. We provide a propensity interpretation of fitness, which we argue captures the intended reference of this term as it is used by evolutionary theorists. Using the propensity interpretation of fitness, we provide a Hempelian reconstruction of explanations of evolutionary phenomena, and we show why charges of circularity which have been levelled against explanations in evolutionary theory are mistaken. Finally, we provide a definition of natural selection which follows from the propensity interpretation of fitness, and which handles all the types of selection discussed by biologists, thus improving on extant definitions.

Two experiments examined rates of shuttle box avoidance responding in 3 strains of rats as a function of classical and instrumental contingencies. In Experiment 1, during classical conditioned-stimulus-unconditioned-stimulus (CS-US) pairings in the absence of an avoidance contingency, there were large differences between the Fischer, Long-Evans, and Lewis strains in rates of anticipatory responding to the CS. The same pattern of differences was observed in Experiment 2, when the avoidance contingency was added. None of the instrumental contingencies of CS termination, US termination, or the avoidance contingency differentially affected the strains. Classically elicited anticipatory responses and their compatibility with the required avoidance response were viewed as central factors in both the acquisition and maintenance of skeletal avoidance responses.

1. Two experimental approaches were employed to assess the relation between food consumption rate and maintenance requirements in male weanling rats. The first approach involved restricting food intake in rats previously given free access to food from weaning to 59 d of age. The second approach involved restriction of food intake to various levels after weaning. Maintenance requirements (g foodid per g body-weight (W)) were estimated by dividing the rate of food consumption by the resulting equilibrium W (EBW) for each animal. In addition, food consumption was partitioned into growth-independent (maintenance) and growth-dependent (gain) components by alternately setting W and specific growth rate (W') to zero in an equation relating food intake rate to W and W . Coupling coefficients representing maintenance consumption (g food/d per g W) and gain consumption (g food/g gain) were estimated for each animal by least squares.2. Both techniques for estimating maintenance consumption provided similar estimates within and across experiments, and regardless of when food restriction was imposed or its severity, consumption for maintenance was about 5 % W/d.3. The EBW to which animals in each treatment group aspired was directly proportional to that group's food intake rate.4. Coventional measures of growth efficiency were also related to food intake; efficiency decreased with decreasing food intake. Partitioning food consumption into maintenance and gain components revealed that as the rate of food intake decreased, the proportion of total intake consumed for maintenance increased. The results suggest that growth efficiency declines during food intake restriction because proportionately more of total intake is used for maintenance, leaving less available for gain.

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Anticipatory skeletal responses can be directly elicited during classical aversive conditioning in the shuttlebox. Under the classical CS-UCS contingency Long-Evans rats shuttle at moderate rates, while Fischer344 rats display high rates of anticipatory responding, even though the instrumental avoidance contingency is absent. The classical contingency is also exceedingly important in determining the differences between these strains, as well as their initial and terminal rates of anticipatory responding, when the avoidance contingency is present.