LETTER

Focus on an island rule may hide morphological disparity in insular plants LETTER Joshua I. Briana,1 and Nathanael Walker-Haleb,1

The island rule (1, 2) posits that large mainland species case of island gigantism in stature but dwarfism in evolve to be small on islands, while small mainland leaves. However, C. repens is more closely related species evolve to be large. Biddick et al. (3) assess to Coprosma petiolata (4), which appears to repre- the signal of an island rule in plant traits on islands sent a case of island dwarfism in both stature and off the coast of New Zealand, concluding that some leaves, indicating that this process can be extremely plant traits show evidence of an island rule while variable. others do not. We applaud Biddick et al. for their ap- We consider that this variation may represent a plication of the island rule to plants. However, we ob- process of island plants exploring more of trait space serve significant heterogeneity in their data, and while than mainland relatives (5, 6), even in situations of so- analysis on a trait-by-trait basis may reveal an island rule, called anagenesis (5, 7). Analyses of separate traits can it may obscure evidence of island lineages explor- reflect an island rule but hide divergent patterns of ing more of trait space than their mainland relatives. among-trait evolution. In general, we expect that pop- A single plant species may demonstrate consider- ulations colonizing islands will be more subject to drift able variation in growth form, as their data suggest. (8), which could drive nonadaptive morphological For example, they use Veronica elliptica for multiple change (9) and cause island lineages to explore more comparisons; leaf area shows a 5-fold difference be- of trait space for both traits and trait covariance. Al- tween 2 different sources (rows 38–40 vs. 114–116 in though in a separate study these authors did not de- their dataset S1). As both sources were literature de- tect allometric change for island vines (10), it remains rived for this within-species data, it is difficult to see to be tested for more plant groups. how such large variation is accounted for through This dataset represents a valuable resource to un- their modeling of “collection method” and “species derstand the evolution of plant morphology on islands. identity” as random effects; this may create a gradi- A more holistic approach, focusing on multivariate ent that reflects within-species variation rather than analysis of plant traits, making full use of the available an evolutionary trend. Additionally, this variation phylogenetic data for these taxa, is capable of revealing may disguise a different trend than size convergence. the dynamics of evolution on islands. Such dynamics For example, for observations that had both leaf and may be obscured when searching for size convergence stature measurements, 36 showed different patterns on a trait-by-trait basis. between leaf and stature, suggesting they are not uniformly becoming gigantic or dwarfed. Closely re- Acknowledgments lated species can also display significant heterogeneity. J.I.B. and N.W.-H. were supported by the Woolf Fisher Trust. Coprosma repens For example, the comparison of to We thank Caroline Parins-Fukuchi and Stephen Smith for their the closely related Coprosma chathamica reveals a helpful discussion.

1 J. B. Foster, Evolution of mammals on islands. Nature 202, 234–235 (1964). 2 L. Van Valen, A new evolutionary law. Evol. Theory 1,1–30 (1973). 3 M. Biddick, A. Hendriks, K. C. Burns, Plants obey (and disobey) the island rule. Proc. Natl. Acad. Sci. U.S.A. 116, 17632–17634 (2019). 4 J. T. Cantley, A. S. Markey, N. G. Swenson, S. C. Keeley, Biogeography and evolutionary diversification in one of the most widely distributed and species rich genera of the Pacific. AoB Plants 8, plw043 (2016). 5 K. C. Burns, Evolution in Isolation: The Search for an Island Syndrome in Plants (Cambridge University Press, Cambridge, UK, 2019). 6 N. M. Nürk, G. W. Atchison, C. E. Hughes, Island woodiness underpins accelerated disparification in plant radiations. New Phytol. 224, 518–531 (2019). 7 B. C. Emerson, J. Pati~no, Anagenesis, cladogenesis, and speciation on islands. Trends Ecol. Evol. (Amst.) 33, 488–491 (2018).

aDepartment of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom; and bDepartment of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom Author contributions: J.I.B. and N.W.-H. designed research, performed research, and wrote the paper. The authors declare no competing interest. Published under the PNAS license. 1To whom correspondence may be addressed. Email: [email protected] or [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1916554116 PNAS Latest Articles | 1of2 Downloaded by guest on September 26, 2021 8 E. Mayr, Change of Genetic Environment and Evolution, Evolution as a Process, J. Huxley, A. C. Hardy, E. B. Ford, Eds. (Allen and Unwin, London, 1954). 9 R. R. Ackermann, J. M. Cheverud, Detecting genetic drift versus selection in human evolution. Proc. Natl. Acad. Sci. U.S.A. 101, 17946–17951 (2004). 10 M. Biddick, I. Hutton, K. C. Burns, Independent evolution of allometric traits: A test of the allometric constraint hypothesis in island vines. Biol. J. Linn. Soc. Lond. 126, 203–211 (2018).

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