Changing Ecological Opportunities Facilitated the Explosive Diversification of New Caledonian Oxera (Lamiaceae)

Abstract

Phylogenies recurrently demonstrate that oceanic island systems have been home to rapid clade diversification and adaptive radiations. The existence of adaptive radiations posits a central role of natural selection causing ecological divergence and speciation, and some plant radiations have been highlighted as paradigmatic examples of such radiations. However, neutral processes may also drive speciation during clade radiations, with ecological divergence occurring following speciation. Here, we document an exceptionally rapid and unique radiation of Lamiaceae within the New Caledonian biodiversity hotspot. Specifically, we investigated various biological, ecological, and geographical drivers of species diversification within the genus Oxera . We found that Oxera underwent an initial process of rapid cladogenesis likely triggered by a dramatic period of aridity during the early Pliocene. This early diversification of Oxera was associated with an important phase of ecological diversification triggered by significant shifts of pollination syndromes, dispersal modes, and life forms. Finally, recent diversification of Oxera appears to have been further driven by the interplay of allopatry and habitat shifts likely related to climatic oscillations. This suggests that Oxera could be regarded as an adaptive radiation at an early evolutionary stage that has been obscured by more recent joint habitat diversification and neutral geographical processes. Diversification within Oxera has perhaps been triggered by varied ecological and biological drivers acting in a leapfrog pattern, but geographic processes may have been an equally important driver. We suspect that strictly adaptive radiations may be rare in plants and that most events of rapid clade diversification may have involved a mixture of geographical and ecological divergence.

Phylogenomics, biogeography, and evolution of the blue‐ or white‐fruited dogwoods (Cornus)—Insights into morphological and ecological niche divergence following intercontinental geographic isolation

“…The evidence, combined with changes in abiotic habitat between the disjunct sisters observed in our study, supports the idea that accelerated species diversification is associated with the accelerated rate of morphological differentiation, which, at the very least, is indirectly driven by shifts in ecological niche. This finding aligns with many other studies that found correlations between species diversification and ecological factors (Hawkins et al, 2003;Lee et al, 2013;Trigas et al, 2013;Barrabé et al, 2019).…”

An updated tribal classification of Lamiaceae based on plastome phylogenomics

“…Ajugoideae are the third-largest subfamily within Lamiaceae and contain about 770 species in 23 genera [ 19 , 48 , 54 , 118 , 119 ] distributed worldwide but most common in tropical regions [ 1 ]. A possible synapomorphy of Ajugoideae may be pollen with branched to granular columellae [ 9 ].…”

Evolution of Plant Architecture, Functional Diversification and Divergent Evolution in the Genus Atractocarpus (Rubiaceae) for New Caledonia

“…The branched architectural class appears to be ancestral in Atractocarpus and our results indicated at least two independent origins of monocauly in New Caledonia. Other examples of derived monocauly have recently been revealed in flowering plants (Chomicki et al, 2017; Barrabé et al, 2018). The intermediate architectural class is evolutionary closer to the monocaulous habit since intermediate and monocaulous species are always sister taxa or very close relatives.…”