This week we welcome Njal Rollinson.

The evolution of body size in endotherms and ectotherms; or, why they might not be giants

Strong selection and abundant genetic variation appear to be widespread in nature. Adaptive phenotypic evolution should therefore be common. Yet, stasis tends to dominate the temporal dynamic of traits in natural populations. This is the paradox of stasis. Additionally, for body size, not only is selection strong, it is typically positive. There is therefore an additional dimension to the paradox of stasis for body size: not only should we expected widespread adaptive evolution, we should expect a trend towards the evolution of larger size. Contrary to this expectation, body size does not respond to directional selection in contemporary populations. Drawing from the principles of life-history evolution, I outline a potential resolution to the size paradox. I test this hypothesis by reanalyzing phenotypic selection data, and by performing phylogenetically explicit analyses in the Amphibia. A new understanding of body-size evolution emerges, one that depends critically on life-history trade-offs, maternal effects, and oxygen limitation. The sum of this work underlines the importance of life-history and maternal effects in understanding patterns of body-size evolution at micro- and macro-evolutionary timescales.