Secondary sympatry caused by range expansion informs on the dynamics of microendemism in a biodiversity hotspot

Abstract

Islands are bounded areas where high endemism is explained either by allopatric speciation through the fragmentation of the limited amount of space available, or by sympatric speciation and accumulation of daughter species. Most empirical evidence point out the dominant action of allopatric speciation. We evaluate this general view by looking at a case study where sympatric speciation is suspected. We analyse the mode, tempo and geography of speciation in Agnotecous, a cricket genus endemic to New Caledonia showing a generalized pattern of sympatry between species making sympatric speciation plausible. We obtained five mitochondrial and five nuclear markers (6.8 kb) from 37 taxa corresponding to 17 of the 21 known extant species of Agnotecous, and including several localities per species, and we conducted phylogenetic and dating analyses. Our results suggest that the diversification of Agnotecous occurred mostly through allopatric speciation in the last 10 Myr. Highly microendemic species are the most recent ones (<2 Myr) and current sympatry is due to secondary range expansion after allopatric speciation. Species distribution should then be viewed as a highly dynamic process and extreme microendemism only as a temporary situation. We discuss these results considering the influence of climatic changes combined with intricate soil diversity and mountain topography. A complex interplay between these factors could have permitted repeated speciation events and range expansion.

Figure 1. Chronogram reconstructed under BEAST and distribution of Agnotecous species.

On the chronogram, branches shown as thick black lines indicate posterior probabilities ≥0.95. When different from 0, degrees of sympatry are indicated below branches. Grey vertical bars indicate morphogenesis episodes and dry periods (after [66]).The marginally significant case of diversification rate shifts detected with apTreeshape is indicate by an arrow, and the distribution of yule estimator of diversification for all trees the genus and clade B2 is shown. Hypothetical areas (in km²) are given on the right of the taxa names, and species are colored and numerated according their respective distribution. On the maps, the distribution of ultramafic rocks and corresponding metalliferous soils is indicated in grey (after [9]).

Figure 2. Species-level log lineages-through-time (LTT) plot for Agnotecous (excluding infra-specific data).

Two minor increases in diversification rates around 3.5 Ma and 2 Ma are showing, but likelihood analyses reject the null hypothesis that diversification rates in Agnotecous have been constant through time.

Figure 3. Degree of geographical overlap plotted as a function of relative age in the genus Agnotecous.

Each point represents a node in the species phylogeny in Figure 1. Linear regression yielded low intercept (0.02170<0.5) and positive slope (0.01244), suggestive of allopatric speciation, but values are not statically significant (P-value = 0.462>>0.05).

Figure 4. Correlation between the proportion of cases showing zero overlap and the proportion of range overlap.

Using the Phillimore et al., 2008 method, allopatric speciation is suggestive but values are not statically significant (P-value = 0.2746>>0.05).

Figure 5. Relationship between areas and ages of species.

The negative relationship between area and age of species indicates a geographical range expansion.