Accumulation of noncoding DNA and therefore genome size (Cvalue) may be under strong selection toward increase of body size accompanied by low metabolic costs. C-value directly affects cell size and specific metabolic rate indirectly. Body size can enlarge through increase of cell size and͞or cell number, with small cells having higher metabolic rates. We argue that scaling exponents of interspecific allometries of metabolic rates are by-products of evolutionary diversification of C-values within narrow taxonomic groups, which underlines the participation of cell size and cell number in body size optimization. This optimization leads to an inverse relation between slopes of interspecific allometries of metabolic rates and C-value. To test this prediction we extracted literature data on basal metabolic rate (BMR), body mass, and C-value of mammals and birds representing six and eight orders, respectively. Analysis of covariance revealed significant heterogeneity of the allometric slopes of BMR and C-value in both mammals and birds. As we predicted, the correlation between allometric exponents of BMR and C-value was negative and statistically significant among mammalian and avian orders.allometry ͉ genome size ͉ body size optimization ͉ cell number

Summary1. Species' body size distributions are right-skewed, symmetric or left-skewed, but right-skewness strongly prevails. 2. Skewness changes with taxonomic level, with a tendency to high right-skewness in classes and diverse skewness in orders within a class. Where the number of lower taxa allows for analysis, skewness coefficients have normal distributions, with the majority of taxa being right-skewed. 3. Skewness changes with geographical scale. For a broad range, distributions in a class are usually right-skewed. For a narrower scale, distributions remain right-skewed or become symmetric or even close to uniform. 4. The prevailing right-skewness of species' body size distributions is explained with macroevolutionary models, the fractal character of the environment, or body size optimization. 5. Macroevolutionary models assume either size-biased speciation and extinction, or the existence of a constraint on small size. Macroevolutionary mechanisms seem insufficient to explain the pattern of species' body size distributions, but they may operate together with other mechanisms. 6. Optimization models assume that directional and then stabilizing selection works after speciation events. There are two kinds of optimization approaches to study species' body size distributions. Under the first approach, it is assumed that a single energetic optimum exists for an entire taxon, and that species are distributed around this optimum. Under the second approach, each species has a separate optimum, and the species' body size distribution reflects the distribution of optimal values. 7. Because not only energetic properties but also mortality are important in determining optimal sizes, only the second approach, that is, seeking the distribution of optimal values, seems appropriate in the context of life-history evolution. This approach predicts diverse shapes of body size distributions, with right-skewness prevailing.