The latitudinal gradient in rates of evolution for bird beaks, a species interaction trait

Where is evolution fastest? The biotic interactions hypothesis proposes
that greater species richness creates more ecological opportunity, driving
faster evolution at low latitudes, whereas the “empty niches” hypothesis
proposes that ecological opportunity is greater where diversity is low,
spurring faster evolution at high latitudes. We tested these contrasting
predictions by analyzing rates of beak evolution for a global dataset of
1141 avian sister species. Rates of beak size evolution are similar across
latitudes, with some evidence that beak shape evolves faster in the
temperate zone, consistent with the empty niches hypothesis. The empty
niches hypothesis is further supported by a meta-analysis showing that
rates of trait evolution and recent speciation are generally faster in the
temperate zone, whereas rates of molecular evolution are slightly faster
in the tropics. Our results suggest that drivers of evolutionary
diversification are either similar across latitudes or more potent in the
temperate zone, thus calling into question multiple hypotheses that invoke
faster tropical evolution to explain the latitudinal diversity gradient.