Phylogenetic conservatism of species range size is the combined outcome of phylogeny and environmental stability

Aim: To test for the phylogenetic conservatism of geographic range size and to explore the effect of the environment on this potential conservatism. Location: The western Tethys Ocean and its surroundings (present-day Europe, the Middle East and North Africa) during the early Pliensbachian (Early Jurassic). Methods: Using 104 localities and 1,765 occurrences of ammonite species, we estimated geographic range sizes using the extent of occurrence and the latitudinal range. The phylogenetic conservatism of range sizes was tested using Moran's I index which measures phylogenetic autocorrelation, and Pagel's lambda which indicates whether a phylogeny correctly predicts covariance patterns among taxa on a given trait according to a Brownian evolution model. We conducted these analyses for two neighbouring provinces with contrasting environmental features (Mediterranean and Northwest European). We also explored scale effects by considering the whole western Tethys and two temporal resolutions (chronozone and sub-stage). Results: A marked difference in phylogenetic signal is observed between Mediterranean (MED) and Northwest European (NWE) species; the range size of MED species is more frequently phylogenetically conserved than that of NWE species. No phylogenetic conservatism of species range size is observed during the last chronozone of the early Pliensbachian which is characterized by numerous palaeoclimatic and palaeoenvironmental changes. Main conclusions: Species range size may be partly determined by phylogeny, but this phylogenetic conservatism is modulated by spatio-temporal environmental stability. The phylogenetic signal of species range size may be labile through time within the same lineage and may differ between contemporaneous species of the same group. This lability stems from the fact that species range size results from a complex interplay of intrinsic and extrinsic factors.