Adaptive radiation without independent stages of trait evolution in a group of Caribbean anoles

Adaptive radiation involves diversification along multiple trait axes,
producing phenotypically diverse, species-rich lineages. Theory generally
predicts that multi-trait evolution occurs via a ‘stages’ model, with some
traits saturating early in a lineage’s history, and others diversifying
later. Despite its multidimensional nature, however, we know surprisingly
little about how different suites of traits evolve during adaptive
radiation. Here, we investigated the rate, pattern, and timing of
morphological and physiological evolution in the anole lizard adaptive
radiation from the Caribbean island of Hispaniola. Rates and patterns of
morphological and physiological diversity are largely unaligned, belying
independent selective pressures associated with structural and thermal
niches. Cold tolerance evolution reflects parapatric divergence across
elevation, rather than niche partitioning within communities. Heat
tolerance evolution and the preferred temperature evolve more slowly than
cold tolerance, reflecting behavioral buffering, particularly in
edge-habitat species (a pattern associated with the Bogert effect). In
contrast to the nearby island of Puerto Rico, closely related anoles on
Hispaniola do not sympatrically partition thermal niche space. Instead,
allopatric and parapatric separation across biogeographic and
environmental boundaries serves to keep morphologically similar close
relatives apart. The phenotypic diversity of this island’s adaptive
radiation accumulated largely as a by-product of time and historical
biogeography, with surprisingly few exceptional pulses of trait evolution.
A better understanding of the processes that guide multidimensional trait
evolution (and nuance therein) will prove key in determining whether the
stages model should be considered a common theme of adaptive radiation.