Quantification of the degree to which ecological niches change over evolutionary timescales is important for deepening our understanding of evolutionary and ecological processes. Niche conservatism is the retention of niche-related ecological traits between closely related species thus going against ecological divergence during speciation. Niche conservatism is expected in cases of allopatric speciation. We present a new null biogeographic model to test for niche conservatism and divergence (the RTR significance test), which we combine with a novel metric for quantifying multidimensional overlap (the MO metric).The RTR null model comprises many thousands of replicates generated by randomly translocating and rotating the set of occurrence records for two populations (e.g., sister species) while maintaining the spatial configuration between all occurrences within each replicate. For each replicate we calculate niche overlap using the MO metric. This novel approach enables us to test whether the observed niche overlap is more or less than expected by chance given the environmental conditions present in the study area. We test the performance of our approach to real sister taxa with current allopatric distributions: two pairs of crested newts in Europe and four pairs of lemurs in Madagascar.
Results/Conclusions:
We find evidence for niche conservatism in both pairs of crested newts. We also find one case of niche conservatism and one of niche divergence in the group of lemurs, while two remaining pairs had non-significant niche overlap. While niche conservatism in 3 out of 6 species pairs analysed supports the hypothesis of the retention of niche related traits through time during allopatric speciation, the findings of niche divergence and non-significant cases suggest other factors other than vicariance may played a role in the speciation of those sister taxa.
Our results demonstrate that both the RTR significance test and the novel metric of niche overlap are consistent with evolutionary theory and are suitable methods to test for niche conservatism and divergence. We expect that the methods will prove useful for addressing a broad set of questions relating to ecological niche evolution and speciation, particularly for restricted-range species for which few known occurrence records are available and when applied to bigger phylogenies.