Island woodiness underpins accelerated disparification in plant radiations

Abstract

Summary The evolution of secondary (insular) woodiness and the rapid disparification of plant growth forms associated with island radiations show intriguing parallels between oceanic islands and tropical alpine sky islands. However, the evolutionary significance of these phenomena remains poorly understood and the focus of debate. We explore the evolutionary dynamics of species diversification and trait disparification across evolutionary radiations in contrasting island systems compared with their nonisland relatives. We estimate rates of species diversification, growth form evolution and phenotypic space saturation for the classical oceanic island plant radiations – the Hawaiian silverswords and Macaronesian Echium – and the well‐studied sky island radiations of Lupinus and Hypericum in the Andes. We show that secondary woodiness is associated with dispersal to islands and with accelerated rates of species diversification, accelerated disparification of plant growth forms and occupancy of greater phenotypic trait space for island clades than their nonisland relatives, on both oceanic and sky islands. We conclude that secondary woodiness is a prerequisite that could act as a key innovation, manifest as the potential to occupy greater trait space, for plant radiations on island systems in general, further emphasizing the importance of combinations of clade‐specific traits and ecological opportunities in driving adaptive radiations.

A new look at phenotypic disparity and diversification rates in island plant radiations

“…found higher diversification rates in the oceanic and sky‐island clades, as expected if ecological opportunities in island‐like environments act as an evolutionary stimulus (Stroud & Losos, ). Previous studies had found exceptionally high diversification rates in some island plant clades, including ones examined here, but Nürk et al . provide critical evidence that those rates represent acceleration of diversification in insular settings.…”

“…Using model‐based tests for evolutionary rate differences between insular and noninsular clades, Nürk et al . found higher diversification rates in the oceanic and sky‐island clades, as expected if ecological opportunities in island‐like environments act as an evolutionary stimulus (Stroud & Losos, ).…”

“…Lens et al ., ). Building on this past framework, Nürk et al . found that secondary woodiness (or perennial habit, in lupines) generally evolved at the base of oceanic or sky‐island clades and thus was tightly associated with elevated diversification and disparification.…”

“…In this issue of New Phytologist , Nürk et al . (, pp. 518–531) offers such a comparison, with a focus on four well‐studied clades that each include prominent evolutionary radiations from oceanic or sky islands: the Hawaiian silversword alliance and California Islands Deinandra (Madiinae, Asteraceae), Macronesian Echium (Boraginaceae), Andean Hypericum (Hypericaceae), and Andean Lupinus (Fabaceae).

‘Lacking in all of this activity have been quantitative comparisons of the magnitude of diversification … and disparification … between island plants and their noninsular relatives.’

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“…The question of whether insular clades conform to the central criterion of adaptive radiation – namely, the evolution of a diversity of ecological roles and associated traits (see Givnish, ) – and show greater evidence of such disparification than their noninsular relatives is more difficult to address than differences in diversification rates. With a focus on mean plant height as a functional trait predicted to vary in relation to ecological niche, Nürk et al . conducted evolutionary model‐based tests for differences between insular and noninsular clades in overall trait disparity and, in a separate trait‐saturation analysis, the proportion of overall trait space represented by each clade.…”