Analysis of intrinsic evolutionary factors leading to microendemic distributions in New Caledonian leaf beetles

Abstract

Microendemicity, or the condition of some species having local ranges, is a relatively common pattern in nature. However, the factors that lead to this pattern are still largely unknown. Most studies addressing this issue tend to focus on extrinsic factors associated with microendemic distributions, such as environmental conditions, hypothesising a posteriori about underlying potential speciation mechanisms, linked or not to these conditions. Here, we use a multi-faceted approach mostly focusing on intrinsic factors instead, namely diversification dynamics and speciation modes in two endemic sibling genera of leaf beetles with microendemic distributions, Taophila and Tricholapita, in a microendemicity hotspot, New Caledonia. Results suggest that the diversification rate in this lineage slowed down through most of the Neogene and consistently with a protracted speciation model possibly combined with several ecological and environmental factors potentially adding rate-slowing effects through time. In turn, species accumulated following successive allopatric speciation cycles, possibly powered by marked geological and climatic changes in the region in the last 25 million years, with daughter species ranges uncorrelated with the time of speciation. In this case, microendemicity seems to reflect a mature state for the system, rather than a temporary condition for recent species, as suggested for many microendemic organisms.

Figure 1

Uncorrelated-lognormal clock-constrained tree of New Caledonian Eumolpinae, with focal groups Taophila and Tricholapita highlighted. Moderately (posterior probability [PP] = 0.50–0.95) and highly (PP = 0.95–1.00) supported nodes are labelled and the HPD age intervals of the latter represented by bars for each of the nodes. Terminals colour-coded according to accompanying legend based on known species distributions, where “microendemic” refers to species known from a single or geographically close localities; “reduced range” to species found in a regional set of localities; and “widespread” to species found across Grande Terre.

Figure 2

(A) Diversification-through-time and lineage-through-time (LTT) trajectories inferred using BAMM for the lineage of sibling genera Taophila and Tricholapita. Net diversification (red) decomposed in speciation (green) and extinction (yellow) components. Visual information on the trajectory of confidence intervals through time available in Fig. S1. (B) Branch-specific speciation rates in the evolution of Taophila and Tricholapita, with changes in tendency indicated by white arrowheads as obtained with the ClaDS model. (Pictures obtained from iNaturalist and used with authors' permission: Taophila, Pierre-Louis Stenger; Tricholapita, Damien Brouste).

Figure 3

(a) Cartoon summary of the main spatial processes explained in the main text conducing to gradual occupation of habitat through successive speciation events and confinement to microendemic ranges in a scenario with reduced effects of extinction. (b) Histogram of the succession and variation in the number of phylogenetic splits along the evolution of Taophila and Tricholapita since the Oligocene, with gray horizontal bars showing confidence time intervals for these splits (shown in no particular order), together with contemporaneous palaeoenvironmental changes in temperature and precipitation regimes (pale yellow intervals and black bars on top) and major chronological periods of geomorphic evolution of Grande Terre (white, blue and red intervals), both based on the synthetic model of.