Niche diversification of Mediterranean and southwestern Asian tortoises

Abstract

Background: Tortoises of the genus Testudo are widely distributed throughout the Mediterranean region and southwestern Asia. However, the evolutionary mechanisms of diversification in this genus are still poorly understood.

Methods: In this study, we assessed the evolutionary patterns in the climate niches of five species and 11 subspecies of the genus Testudo using ecological niche models and evaluated the niche overlap based on species phylogenetic distances.

Results: The ecological models indicated that most species differ in their climate niches, but show overlap, with gradual transitions at range boundaries. As expected, the ecological divergence among subspecies was lower than that among species. Evaluation of the phylogenetic signal indicated that climate niches have been weakly conserved, but sister species also show high evolutionary divergence.

Keywords: Ecology; Evolution; Glacial cycles; Refugia; Reptiles; Testudo.

Figure 1. The study region and species sites, with superimposed terrain elevation (grayscale).

Red circles, Testudo graeca; green squares, Testudo hermanni; yellow hexagons, Testudo horsfieldii; orange stars, Testudo kleinmanni; blue triangles, Testudo marginata.

Figure 2. Phylogenetic relationships among species and subspecies of the genus Testudo, based on estimated times of molecular divergence.

In T. graeca the names in red represent the western group of subspecies and in blue the eastern group. Clockwise: Testudo marginata (blackish specimen), T. graeca nabeulensis, T. graeca whitei, T. hermanni hermanni, T. horsfieldii, T. kleinmanni.

Figure 3. PCA triplot showing (A) the position of each species of the genus Testudo and (B) the position of the subspecies (above) and T. graeca western-eastern subclades (below), based on data obtained from bioclimatic variables.

Red circles, Testudo graeca; green circles, Testudo hermanni; yellow circles, Testudo horsfieldii; orange circles, Testudo kleinmanni; blue circles, Testudo marginata. The red arrow indicates an increase in temperature; the blue arrow indicates an increase in rainfall.

Figure 4. Results of the Maxent models representing the niche of Testudo species, obtained from climatic variables.

The red colour indicates greater environmental suitability, and the blue colour reduced suitability. (A) Testudo graeca; (B) Testudo hermanni; (C) Testudo horsfieldii; (D) Testudo marginata.

Figure 5. Regions most likely to represent stable climate refugia throughout the Pleistocene-Holocene periods (787 ka to 0 ya).

The yellow colour indicates a higher probability (high suitability for at least five time periods), and the blue colour indicates high suitability for less than two time periods. (A) Testudo graeca; (B) Testudo hermanni; (C) Testudo horsfieldii; (D) Testudo marginata.