Monday, April 30, 2012
Mouse to elephant
Research published in the Proceedings of the National Academy of Sciences (The maximum rate of mammal evolution) describes increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.
The asymmetry between rates can potentially be explained by distinct but not necessarily mutually exclusive mechanisms. One possibility is that there are fewer physical, biological, and environmental constraints to decreasing as opposed to increasing size. Pedomorphic processes are good candidates as mechanisms of size reduction, because all animals must pass through a smaller size during their ontogeny. We hypothesize it is easier to halt the developmental program and reproduce early than to grow larger and delay maturity. Another possibility is that selection favors size decreases because smaller animals have higher rates of reproduction with life histories characterized by rapid maturity, high birth rates, and short lifespans. Finally, decreases in size may reflect adaptation to a more generalized ecological niche, whereas increases in size require novel adaptations to obtain more food and space to fuel higher whole-organism metabolic rates.