Linking ecological specialization to its macroevolutionary consequences: An example with passerine nest type

A long-standing hypothesis in evolutionary biology is that the evolution
of resource specialization can lead to an evolutionary dead end, where
specialists have low diversification rates and limited ability to evolve
into generalists. In recent years, advances in comparative methods
investigating trait-based differences associated with diversification have
enabled more robust tests of this idea and have found mixed support. We
test the evolutionary dead end hypothesis by estimating net
diversification rate differences associated with nest site specialization
among 3,224 species of passerine birds. In particular, we test whether the
adoption of hole-nesting, a nest site specialization that decreases
predation, results in reduced diversification rates relative to nesting
outside of holes. Further, we examine whether evolutionary transitions to
the specialist hole-nesting state have been more frequent than transitions
out of hole-nesting. Using diversification models that accounted for
background rate heterogeneity and different extinction rate scenarios, we
found that hole-nesting specialization was not associated with
diversification rate differences. Furthermore, contrary to the assumption
that specialists rarely evolve into generalists, we found that transitions
out of hole-nesting occur more frequently than transitions into
hole-nesting. These results suggest that interspecific competition may
limit adoption of hole-nesting, but that such competition does not result
in limited diversification of hole-nesters. In conjunction with other
recent studies using robust comparative methods, our results add to
growing evidence that evolutionary dead ends are not a typical outcome of
resource specialization.