Studies of phylogenetic diversification often show evidence for slowdowns in diversification rates over the history of clades. Recent studies seeking biological explanations for this pattern have emphasized the role of niche differentiation, as in hypotheses of adaptive radiation and ecological limits to diversity. Yet many other biological explanations might underlie diversification slowdowns....

the hormone relied on the sympathetic nervous system, the branch that pumps out adrenaline during emergencies but at other times emits a hormonal trickle to maintain a low level of stimulation. This baseline activity was lower in mice lacking the leptin receptor, indicating a connection between leptin and the sympathetic nervous system. When that connection was broken by deleting the adrenaline...