Award Date


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Biological Science

Number of Pages



Historical environmental change is thought to have played an important role in the diversification of the biota of western North America. Many patterns of diversification have been associated with glacial-interglacial cycles of the latest Pleistocene. Recent evidence on diversification patterns in small mammals suggests a response to older environmental change, especially the uplift of the western North American cordillera; To evaluate the relative influence of old and recent historical environmental change on lineage diversification, mitochondrial DNA molecular phylogeographies of Phrynosoma douglassi (the short-horned lizard) and Phrynosoma platyrhinos (the desert horned lizard) were analyzed. Both species are widespread and have relatively old histories in western North America; P. douglassi and P. platyrhinos demonstrated multiple scales of lineage diversification representating responses to relatively old and recent historical environmental change. Deep levels of divergence in P. douglassi were structured geographically among Kuchler Physiographic regions and were temporally concordant with the late Tertiary uplift of the western North American cordillera. Significant phylogenetic gaps between regions suggest long-term barriers to gene flow among regions. P. platyrhinos demonstrated a deep divergence between the southern Sonoran Desert and the remainder of the species' range. With the assumption of 2% divergence per million years, this divergence roughly corresponds to the Bouse Embayment, a large lake incursion that divided areas east and west of the Colorado River during the Pliocene. Assuming a roughly consistent rate of molecular evolution, both species exhibit divergence depths within regions indicative of response to Pleistocene glacial-interglacial cycles; The depth of lineage diversity in P. douglassi and P. platyrhinos was concordant with a model of habitat change during the last glacial maximum. Regions that maintained large patches of suitable habitat during the last glacial maximum had deep levels of divergence whereas regions that lost large areas of suitable habitat had shallow levels of within-region divergence; The phylogeographic patterns of P. douglassi and P. platyrhinos, and results of simulated habitat change, do not support a model of mass habitat shift to the south in response to glaciation. Rather, results of this study suggest that suitable habitats and populations persisted in many areas within the current ranges of both species ranges during the last glacial maximum, although the distribution of P. platyrhinos was more limited.


Desert; Divergence; Diversity; Douglassi; Horned; Lizard; Lizardphrynosoma; Patterns; Phrynosoma; Phylogeography; Platyrhinos; Short

Controlled Subject

Molecular biology; Ecology; Paleoecology; Zoology

File Format


File Size

7680 KB

Degree Grantor

University of Nevada, Las Vegas




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