Most of our research projects have a strong evolutionary flavour with a focus on historical biogeography, trait evolution and diversification dynamics. We assemble comprehensive phylogenetic/phylogenomic trees to answer key questions in evolutionary biology such as:

1/ What are the main mechanisms responsible for the current geographic ranges of species?

2/ Why are some clades more diverse than others in terms of species richness?

3/ Why is biodiversity unevenly distributed across the globe?

4/ Are some particular morphological traits, ecological associations or abiotic events driving the diversification of clades across the globe?

5/ How do lineages respond to global shifts in climate or landmass configuration?

Below are a few examples of studies we have published and that address these questions.

Gondwanan biogeography / Dispersal vs. vicariance hypothesis testing

Many clades comprise species distributed in parts of the ancient continent called Gondwana. These regions include Africa, Antarctica, Australia, India, Madagascar, New Caledonia, New Zealand and South America. Before the introduction of plate tectonics, it was believed that dispersal was the main driver of geographic range evolution in such clades. After the introduction of this model by Wegener, dispersalism was slowly replaced by vicariance hypotheses suggesting that the current distribution of clades in these regions was the result of the break-up of Gondwana. However, there is a new paradigm shift with the advent of molecular dating, because in many instances, clade divergence time estimates are inconsistent with the timing of the geological events, suggesting in turn that dispersal is the main mechanism at play.

Examples of clades supporting the West Gondwana vicariance hypothesis Overview of clades for which the West Gondwana vicariance hypothesis was suggested based on molecular dating and/or model-based biogeographical estimation/reconstruction. (Toussaint et al. 2017 / J Biogeogr)

We use phylogenies of widespread clades in the Austral and Indian region to test these two hypotheses (dispersal versus vicariance). Using comprehensive and robust phylogenies, coupled with careful fossil-based molecular dating, we suggested interesting empirical cases for both scenarios. In Hydrophilini water beetles, an origin in West Gondwana is hypothesized (see above), while in Laccobiini water beetles, an origin in East Gondwana is supported by our data and analyses (see below).

Summary of groups that have been suggested as evidence of India-Madagascar or India/Seychelles vicariance. (Toussaint et al. 2016 / Biol J Linn Soc)

In other instances, while a Gondwana origin seems obvious based on current distribution of lineages, we find a surprising result. It is the case in Hydrobiusini water beetles, where the main lineages are distributed from Australia to South Africa and South America. Yet, the divergence times and ancestral range estimation in this clade suggest that long-distance dispersal was the main mechanism shaping the evolution of ranges in this group.

Fine-scale archipelagic and mountain biogeography

At a finer scale, the evolution of geographic ranges is closely tied to the geological evolution of mountain ranges and the assembly of archipelagoes. We have focused on groups that are endemic to these regions to understand the link between orogenies and lineage diversification. The main idea is to test different models that include or not geological information to see if the latter have a better explanatory power than naive models. To do so, we have focused for many years now on the Southeast Asia archipelago. This region is very complex, presenting many islands of different origins and sizes.

Phylogeny of the New Guinean Exocelina beetle radiation and paleotectonic / orogenic evolution of the island. Diversification occurred as the island was being formed in the past 10 million years, by allowing peripatric and allopatric speciation. (Toussaint et al. 2014 / Nat Comm)

In some cases, using densely sampled dated phylogenies, we can dissect the time frame of diversification and test its correlation with orogenic processes. Using such an approach, we were able to suggest that New Guinean Exocelina diving beetles were so diverse in this region of the planet because they seem to have continuously diversified since the first mountain build-up in New Guinea about 10 million years ago (see above). In other cases, the diversification of some clades seem to have been shaped by the assembly of the Indo-Australian archipelago, although classical zoogeographic boundaries were repeatedly crosses. This is the case in Charaxes (Polyura) butterflies, that have taken advantage of the archipelagic orogeny to colonize Pacific islands and back-colonize the heart of the archipelago in the latest stages of their diversification.

Divergence times and historical biogeography of Charaxes (Polyura) butterflies at different geographical scales. (Toussaint & Balke 2016 / J Biogeogr)

Trait-driven diversification dynamics

Some clades are more diverse than others, and some clades are unique because of particular traits. Understand the link between trait evolution and diversification is a key challenge to get insights into biodiversity assembly through space and time. To address this question, we develop comprehensive phylogenies and use different likelihood models to test whether some traits seem to influence speciation and/or extinction rates through time. For instance, we tested whether the evolution of subterranean lineages in diving beetles would be associated with different diversification regimes. Our results suggest that the evolution of the genus Paroster followed a diversity-dependent pattern characterized by high initial speciation rates that decreased over time. This rapidly lead to a carrying capacity close to the extant species richness meaning that the genus is currently near equilibrium. Given the extent of morphological changes the genus experienced during its evolution, these results are in support the hypothesis of an adaptive radiation in groundwater ecosystems. In the same study, we estimated massive extinction in the genus Sternopriscus, possibly driven by the late aridification of the Australian continent in the Miocene and Plio-Pleistocene.

A picture of a subterranean diving beetle, Paroster byroensis (Dytiscidae). (Credit: Chris Watts)

Lineages-through-time (LTT) plots for the Australasian Hydroporini and genera Paroster (a) and Sternopriscus (b). (Toussaint et al. 2015 / Syst Biol)

Abiotic drivers of diversification dynamics

Our planet has dramatically changed during the past dozens/hundreds of millions of years. Landmasses have changed in terms of position, size and elevation, fostering intense volcanic activity, archipelagic formations, and mountain buildup. The climate also oscillated considerably, thereby influencing bathymetry, ice cover and ecosystem turnovers. Disentangling the impact of these major shifts on the diversification of clades is possible by studying dated phylogenies and using likelihood models that try to test if speciation and extinction rates vary in relation to these abiotic factors. Several methods exist, and the field is rapidly developing, which makes this kind of studies exciting. For instance, we have shown that swallowtail birdwing butterflies presented high extinction rates during periods of elevated sea level and global warming in the Indo-Australian archipelago.

Evaluation of the effect of environmental changes on diversification processes in the evolutionary history of birdwing butterflies. (Condamine et al. 2015 / Sci Rep)