Relative roles of Neogene vicariance and Quaternary climate change on the historical diversification of bunchgrass lizards (Sceloporus scalaris group) in Mexico Robert W. Bryson Jr. a,⇑ , Uri Omar García-Vázquez b , Brett R. Riddle a a School of Life Sciences, University of Nevada, Las Vegas, Las Vegas, NV, USA b Museo de Zoología Alfonso L. Herrera, Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito exterior s/n, Cd. Universitaria, México 04510, Distrito Federal, Mexico article info Article history: Received 7 June 2011 Revised 13 October 2011 Accepted 19 October 2011 Available online 31 October 2011 Keywords: Biogeography Divergence dating Mexico Mitochondrial DNA Phylogeography Sceloporus Trans-Mexican Volcanic Belt abstract Neogene vicariance during the Miocene and Pliocene and Quaternary climate change have synergistically driven diversification in Mexican highland taxa. We investigated the impacts of these processes on genetic diversification in the widely distributed bunchgrass lizards in the Sceloporus scalaris group. We searched for correlations between timing in diversification and timing of (1) a period of marked volca- nism across the Trans-Mexican Volcanic Belt in central Mexico 3–7.5 million years ago (Ma) and (2) a transition to larger glacial–interglacial cycles during the mid-Pleistocene. From our phylogenetic analyses of mitochondrial DNA we identified two major clades that contained 13 strongly supported lineages. One clade contained lineages from the two northern sierras of Mexico, and the other clade included lineages associated with the Trans-Mexican Volcanic Belt and Central Mexican Plateau. Results provided support for Neogene divergences within the S. scalaris group in response to uplift of the Trans-Mexican Volcanic Belt, a pattern observed in several co-distributed taxa, and suggested that Quaternary climate change likely had little effect on diversification between lineages. Uplift of the Trans-Mexican Volcanic Belt dur- ing specific time periods appears to have strongly impacted diversification in Mexican highland taxa. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Emerging patterns of historical diversification in the Mexican highlands suggest mixed responses in co-distributed taxa to past geological and climatic events despite a presumed shared history in the same region (Sullivan et al., 2000; León-Paniagua and Morrone, 2009; Bryson et al., 2011a). Neogene vicariance in the Miocene and Pliocene heavily influenced lineage divergences in some taxa (e.g., Castoe et al., 2009), while Quaternary climate change triggered increased diversification in others (e.g., Bryson et al., 2011b). The synergistic effects of Earth history and glacial– interglacial cycles, coupled with the complex topography of Mexico, appear to have produced a myriad of species-specific re- sponses. Reconciling a common pattern of lineage diversification in Mexican highland taxa has proven difficult (Flores-Villela and Martínez-Salazar, 2009). Ancient development over 30 million years ago (Ma) of most of the major mountains in Mexico (Ferrusquía-Villafranca and González-Guzmán, 2005) probably pre-dates diversification of the extant highland-adapted species. Neogene formation of the Trans-Mexican Volcanic Belt (TVB), however, undoubtedly affected both the timing and tempo of diversification in many montane spe- cies. This volcanic chain of mountains is one of the predominant geographical features of Mexico, and its geological development has been posited as a primary contributor to the biogeographical histories of numerous taxa (Mulcahy et al., 2006; Bryson et al., 2011c). Uplift of the TVB created new geographical barriers and montane habitats, but also linked previously isolated highland bio- tas (Anducho-Reyes et al., 2008). The complex geological history of this mountain range (Ferrusquía-Villafranca, 1993; Gómez-Tuena et al., 2007) unfortunately makes accurate dating of vicariant events presumably responsible for divergences among co-distrib- uted taxa difficult. A recent revision on the past two decades of re- search on the origin of the TVB (Gómez-Tuena et al., 2007) suggests four major volcanic episodes during the Neogene formed most of the range. Two of these episodes (Fig. 1), one around 10–19 Ma and one about 3–7.5 Ma, resulted in major mountain formations that almost completely subdivided Mexico. Estimated mean diver- gence dates in several taxa distributed on or near this region are largely congruent with these two periods (Table 1). In particular, numerous divergences are remarkably similar at around 4–7 Ma. These dates suggest that the marked period of uplift around 3– 7.5 Ma may have had a comparatively stronger effect on lineage diversification than other periods of uplift. 1055-7903/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ympev.2011.10.014 ⇑ Corresponding author. Address: School of Life Sciences, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4004, USA. E-mail address: brysonjr@unlv.nevada.edu (R.W. Bryson Jr.). Molecular Phylogenetics and Evolution 62 (2012) 447–457 Contents lists available at SciVerse ScienceDirect Molecular Phylogenetics and Evolution journal homepage: www.elsevier.com/locate/ympev