The evolution of body size: What keeps organisms small?

It is widely agreed that fecundity selection and sexual selection are the m ajor evolutionary forces that select for larger body size in moss organisms . The general, equilibrium view is that selection for large body size is ev entually counterbalanced by opposing selective forces. While the evidence f or selection favoring larger body size is overwhelming, counterbalancing se lection favoring small body size is often masked by the good condition of t he larger organism and is therefore less obvious. The suggested costs of la rge size are: (1) viability costs in juveniles due to long development and/ or fast growth; (2) viability costs in adults and juveniles due to predatio n, parasitism, or starvation because of reduced agility, increased detectab ility, higher energy requirements, heat stress, and/or intrinsic costs of r eproduction; (3) decreased mating success of large males due to reduced agi lity and/or high energy requiremats;and (4) decreased reproductive success of large females and males due to late reproduction. A review of the litera ture indicates a substantial lack of empirical evidence for these various m echanisms and highlights the need for experimental studies that specificall y address the fitness costs of being large at the ecological, physiological , and genetic levels. Specifically, theoretical investigations and comprehe nsive case studies of particular model species are needed to elucidate whet her sporadic selection in time and space is sufficient to counter balance p erpetual and strong selection for large body size.