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Abstract
Insular woodiness (IW), referring to the evolutionary transition from herbaceousness toward woodiness on islands, has arisen more than 30 times on the Canary Islands (Atlantic Ocean). One of the IW hypotheses suggests that drought has been a major driver of wood formation, but we do not know in which palaeoclimatic conditions the insular woody lineages originated. Therefore, we provided an updated review on the presence of IW on the Canaries, reviewed the palaeoclimate, and estimated the timing of origin of woodiness of 24 insular woody lineages that represent a large majority of the insular woody species diversity on the Canaries. Our single, broad‐scale dating analysis shows that woodiness in 60%–65% of the insular woody lineages studied originated within the last 3.2 Myr, during which Mediterranean seasonality (yearly summer droughts) became established on the Canaries. Consequently, our results are consistent with palaeoclimatic aridification as a potential driver of woodiness in a considerable proportion of the insular woody Canary Island lineages. However, the observed pattern between insular woodiness and palaeodrought during the last couple of million years could potentially have emerged as a result of the typically young age of the native insular flora, characterized by a high turnover.
Highlights
Since the observations of Charles Darwin (1859) and Joseph Dalton Hooker (1867), marine islands have been regarded as ideal systems to unravel evolutionary processes that have shaped present-day life due to their isolation and defined boundaries (Helmus et al, 2014; Losos & Ricklefs, 2009; Patiño et al, 2017)
The most important events that have impacted the palaeoclimate on the Canary Islands are visually summarized in Figure 2 and Figure S13
The Messinian salinity crisis leading to a massive drying episode of the Mediterranean Sea (5.96–5.33 million years ago (Mya)) (Hsü et al, 1977; Jolivet et al, 2006), and the onset of desertification of Northern Africa (~7 Mya) (Schuster et al, 2006) have likely impacted the Canary Island vegetation, since most of the native Canary Island flora has its origin in the Mediterranean
Summary
Since the observations of Charles Darwin (1859) and Joseph Dalton Hooker (1867), marine islands have been regarded as ideal systems to unravel evolutionary processes that have shaped present-day life due to their isolation and defined boundaries (Helmus et al, 2014; Losos & Ricklefs, 2009; Patiño et al, 2017). Insular woodiness (IW) in a strict sense—defined as the evolution of the woody growth form on (sub)tropical islands after arrival of herbaceous colonizers (Figure 1)—is the best-known type of derived woodiness This evolutionary phenomenon explains why these islands harbor a higher proportion of woody plant species than nearby continents, which is without doubt the most striking feature of (sub)tropical insular floras (Carlquist, 1974; Lens, Davin et al, 2013; Whittaker & Fernández-Palacios, 2007). To extract the needed information to test these hypotheses, we searched for the most recently published molecular phylogenies with sufficient sampling of the woody Canary Island clade and their relatives and combined them with trait information on the life form (woodiness versus herbaceousness) via intensive screening of regional floras, field guides, taxonomic treatments, and the study of herbarium specimens and wood anatomical sections in some cases. We used a discrete MCMC model with BayesTraits version 3.0.1 (Meade & Pagel, 2017) on small subclades of the constructed megaphylogeny that include the Canary Island species and their closest continental sister lineages (Figures S1–S12)
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