The Evolution of Body Size: What Keeps Organisms Small?

Abstract

It is widely agreed that fecundity selection and sexual selection are the major evolutionary forces that select for larger body size in most organisms. The general, equilibrium view is that selection for large body size is eventually counterbalanced by opposing selective forces. While the evidence for selection favoring larger body size is overwhelming, counterbalancing selection favoring small body size is often masked by the good condition of the larger organism and is therefore less obvious. The suggested costs of large size are: (1) viability costs in juveniles due to long development and/or fast growth; (2) viability costs in adults and juveniles due to predation, parasitism, or starvation because of reduced agility, increased detectability, higher energy requirements, heat stress, and/or intrinsic costs of reproduction; (3) decreased mating success of large males due to reduced agility and/or high energy requirements; and (4) decreased reproductive success of large females and males due to late reproduction. A review of the literature indicates a substantial lack of empirical evidence for these various mechanisms and highlights the need for experimental studies that specifically address the fitness costs of being large at the ecological, physiological, and genetic levels. Specifically, theoretical investigations and comprehensive case studies of particular model species are needed to elucidate whether sporadic selection in time and space is sufficient to counterbalance perpetual and strong selection for large body size.

Effects of larval crowding on quantitative variation for development time and viability in Drosophila melanogaster

“…Hence, our result hints at the existence of a trade‐off between viability and body size that is exerted in the preadult stage in Drosophila under competitive conditions. A potential trade‐off between viability and body size has been proposed to explain the persistence of small‐bodied species (Blanckenhorn, 2000), and our findings suggest that such a trade‐off might be limited to individual life‐cycle stages.…”

Sexual selection constrains the body mass of male but not female mice

“…We are able to assess the possibility of male sexual selection constraining body size because our study design and species selection control for three of the four costs of large body size suggested by Blanckenhorn (2000). By releasing all mice as adults we control for juvenile viability selection and by excluding predators and most parasites, while providing ample access to food/water we greatly reduce the pressure of adult viability selection.…”

“…Blanckenhorn (2000) suggested four costs due to larger body size: (1) viability costs in juveniles due to longer development (or faster growth); (2) viability costs in adults due to predation, parasitism, or starvation; (3) decreased mating success of large males due to lack of agility or high energy costs; and (4) decreased fitness in both sexes due to late reproduction associated with longer development. These four hypotheses include pressures due to both natural (1 and 2) and sexual (3 and 4) selection; however, supporting evidence in vertebrates has been difficult to obtain for the two sexual selection hypotheses.…”

Can individual variation in phenotypic plasticity enhance population viability?

“…Moreover, an individual's body condition at the end of the winter period may influence fitness in the following season (Harrison et al, 2011). Body mass dynamics are, therefore, a key element that can affect life-history processes of a species, including survival and reproduction (Blanckenhorn, 2000). Thus, we might expect natural selection to favor response mechanisms that permit individuals to compensate for an environmentally induced period of slow growth (Metcalfe and Monaghan, 2003).…”

“…Compensatory growth allows individuals to compensate by accelerating growth rates to reduce the risk of having a sub-optimal size during a future stressful period (Ali et al, 2003;Metcalfe and Monaghan, 2001). Compensation may occur in structural components as well as body mass (Abrams et al, 1996;Metcalfe and Monaghan, 2001;Nicieza and Metcalfe, 1997) and, in both cases, influence an individual's fitness (Blanckenhorn, 2000;Stearns, 1992). The fact that growth rates vary among individuals within a population (Kvist and Lindström, 2001), suggests that there may be plasticity in growth rates among individuals due to differences in body mass since growth rates respond to the individual's current body condition or state (Hornick et al, 2000;Metcalfe and Monaghan, 2001).…”