Diversity versus disparity and the role of ecological opportunity in a continental bird radiation

Schweizer, Manuel; Hertwig, Stefan; Seehausen, Ole; Ebach, Malte (2014). Diversity versus disparity and the role of ecological opportunity in a continental bird radiation. Journal of biogeography, 41(7), pp. 1301-1312. Wiley 10.1111/jbi.12293

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Aim
The Neotropical parrots (Arini) are an unusually diverse group which
colonized South America in the Oligocene. The newly invaded Neotropics may
have functioned as an underused adaptive zone and provided novel ecological
opportunities that facilitated diversification. Alternatively, diversification may
have been driven by ecological changes caused by Andean uplift and/or climate
change from the Miocene onwards. Our aim was to find out whether Arini
diversified in a classical adaptive radiation after their colonization of South
America, or whether their diversification occurred later and was influenced by
more recent environmental change.
Location
Neotropics.
Methods
We generated a time-calibrated phylogeny of more than 80% of all
Arini species in order to analyse lineage diversification. This chronogram was
also used as the basis for the reconstruction of morphological evolution within
Arini using a multivariate ratio analysis of three size measurements.
Results
We found a concentration of size evolution and partitioning of size
niches in the early history of Arini consistent with the process of adaptive radia-
tion, but there were no signs of an early burst of speciation or a decrease in speci-
ation rates through time. Although we detected no overall temporal shifts in
diversification rates, we discovered two young, unexpectedly species-rich clades.
Main conclusions
Arini show signs of an early adaptive radiation, but we
found no evidence of the slowdown in speciation rate generally considered a
feature of island or lake radiations. Historical processes and environmental
change from the Miocene onwards may have kept diversification rates roughly
constant ever since the colonization of the Neotropics. Thus, Arini may not yet
have reached equilibrium diversity. The lack of diversity-dependent speciation
might be a general feature of adaptive radiations on a continental scale, and
diversification processes on continents might therefore not be as ecologically
limited as in isolated lakes or on oceanic islands.

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