Assessing the Geological and Climatic Forcing of Biodiversity and Evolution Surrounding the Gulf of California

The biota of the lands has had a restless place and has endured displacements, inundations, extinctions, and has been forced into migrations with the coming and going of the sea, with the submergence or emergence of mountains, and with the concomitant changes of local climate. Close study of the plant and animal life, when directed by a correlating intelligence, should reveal a course of evolution, expressed jointly by plant and rock, hardly equaled in plant geography.

—Howard Scott Gentry (1949:82)

Introduction

For almost a century the Baja California peninsula (Peninsula), Gulf of California (Gulf), and broader Sonoran Desert region (figure 1) have drawn geologists and biologists alike to study its unique physical and evolutionary processes (e.g., Wittich 1920; Darton 1921; Nelson 1921; Johnston 1924; Beal 1948; Durham and Allison 1960). The challenge remains to untangle the long, intricate, and at times enigmatic geological and climatological histories that have shaped the high levels of endemism and biodiversity observed in the region today (Van Devender 1990; Grismer 2000; Riddle et al. 2000).

Evolutionary theory argues that areas of endemism are generated through increased speciation rates or an unusual capacity to sustain species whose populations go extinct elsewhere. Areas with such high levels of unique biodiversity also demand conservation effort to preserve the underlying evolutionary processes and mitigate the extinction risk posed to species with limited ranges (Myers et al. 2000). Endemism rates

Figure 1.

Physiographic map of the Gulf of California–Salton Trough–Sonoran Desert regions of western North America. Outline of Sonora Desert in white. Towns: E-Ensenada, SF-San Felipe, G-Guaymas, SR-Santa Rosalía, SI-San Ignacio, A-Alamos, L-Loreto, LP-La Paz, LC-Los Cabos. Places: SGP-San Gorgonio Pass, SS-Salton Sea, BC-Ballenas Channel, PE-Punta Eugenia, TV-Las Tres Virgenes volcanoes, RC-La Reforma Caldera, PM-Punta Mita. Islands: IAG-Isla Ángel de la Guarda, IT-Isla Tiburón, ITM-Islas Tres Marías. Marine Basins: UTB-Upper Tiburón basin, GB-Guaymas basin, EGB-East Guaymas basin, AB-Alarcon basin. All geographic analyses and maps use base map elevation data from the Shuttle Radar Topography Mission (SRTM) [http://srtm.usgs.gov/].

for reptiles and plants reach 54% and 30%, respectively, along the Peninsula (Grismer 2002; Brusca et al. 2005; Riemann and Ezcurra 2005; Munguía-Vega 2011; Rebman and Roberts 2012), and the Gulf is one of the world’s marine diversity hotspots (Roberts et al. 2002). Yet, after almost a century of investigation into the geology, climate, and biology of this region, questions of timing and causality between these perspectives remain. In this article we attempt to unite the wealth of knowledge that remains divided along disciplinary lines, bridge their perspectives, and guide future study. To do so we include examples from the emerging field of geogenomics, in which large-scale genetic data inform geological hypotheses (Baker et al. 2014). The interdisciplinary nature of this effort is founded on the belief that through a synthetic approach incorporating plate tectonics, fossils, climate, ecology, and genetics we can better answer the long-standing questions about the physical history and origins and patterns of biodiversity surrounding the Gulf of California.

Reviews with varying scopes and emphases have summarized previous geological and biological work (Case and Cody 1983; Atwater and Stock 1998; Helenes and Carreño 1999; Case et al. 2002; Oskin and Stock 2003a; Riddle et al. 2000; Hafner and Riddle 2005; Lindell et al. 2006; Riddle and Hafner 2006). This contribution presents geological and climatological processes with the biological patterns they are hypothesized to create, under an explicit discussion of the timescales on which these phenomena occur. Geological processes are organized into three tiers (figure 2, table 1) based on the typical duration of the process. First-order processes fundamentally shape the landscape and take the longest to occur (>5 million years [Myr]). These processes involve plate tectonics...