A scientific team co-led by University of Oregon researcher Rebecca Dorsey is searching for evidence that geological processes played a key role in a long-recognized biological mystery on the Baja California peninsula.
Plant and animal life, involving dozens of species, are separated from the northern and southern parts of the peninsula not only by miles but, most curiously, by their genetics.
With a $2.6 million National Science Foundation grant, researchers from four U.S. universities and Mexican collaborators, under the Baja GeoGenomics consortium, are exploring three theories. They are looking at geology, rainfall patterns and climate cycles to see how they may have shaped the evolution and biodiversity of life there.
"Biologists say, 'Something had to cause the pattern of divergence,'" said Benjamin Wilder, director of the University of Arizona's Desert Laboratory and a co-principal investigator on the project.
The geology theory is that a physical barrier, possibly an ancient seaway about three to six million years ago, created a low pass that separated species along the long narrow peninsula between the Pacific Ocean and the Gulf of California.
Last November, Dorsey's team, working east of San Ignacio, found tidal deposits that appear to record such a barrier. The project is the first to use geological data to test the theory, which is popular in the biological literature, Dorsey said.
"We can confidently interpret the deposits as tidal, based on distinctive sedimentary structures, a high degree of sorting, marine burrows and the presence of fossil mangrove roots," said Dorsey, a professor in the Department of Earth Sciences. "These deposits are interbedded with basaltic sediment and lava flows that came from nearby cinder cones previously dated at 3 to 4 million years old."
Tidal flats are uninhabitable by the plants and animals that display the previously recognized genomic divergence, Dorsey noted.
"We have preliminary findings that support the cross-peninsular seaway hypothesis, but with an unexpected tidal-flat twist that will require a lot more work to understand how this depositional system behaved, how long it lasted and how much of the peninsula was covered," she said.
Dorsey and UO postdoctoral researcher Mike Darin are working with a team led by Brian Hausback at California State University, Sacramento, using geologic mapping, stratigraphy, sedimentology, fault studies, volcanology and age-dating in the area of the discovery.
In addition, researchers from the University of Arizona and Arizona State University are using biological and genomic data to test theories related to rainfall patterns and climate cycles of the Baja peninsula.
In previous research, a Dorsey-led team redefined scientific thinking on how the Colorado River made its path to the Gulf of California. Tectonic development and faulting in the gulf played a role in the evolution of the Baja peninsula, she said.
Global climate fluctuations over the past several million years could have created dynamic changes in the landscape, Wilder said. Organisms may have retreated up and down mountainsides, finding pockets where they could survive as the climate swung between cool glacial periods and warm interglacial periods.
The third hypothesis suggests that rainfall patterns created differences in genetic expression within species. The northern half of the peninsula is rainy in winter and dry in summer. In the south, the opposite is true. The rainfall differences may have isolated species through misaligned timing in reproduction or other mismatched adaptations that led to the genetic patterns observed today.
Researchers also will test the genomes of six species that run the length of the peninsula and create models to test their distribution against their predictions. The species are the pitaya agria cactus and brittlebush, the black-tailed brush and Baja California spiny lizards, and two small mammals, the packrat and the Merriam's kangaroo rat.
"Ten years ago, we couldn't have done a project like this. We can sequence almost anything now, and it's relatively cheap," said Greer Dolby, an assistant research professor at Arizona State University and a co-principal investigator. "And now there's a big focus on data and how you integrate it."
The research is part of the emerging field of geogenomics, which was coined in 2014 to describe the use of large-scale genomic data to address geological questions.
"One of the coolest things about this project is that it's all about the biologists learning how the geologists see it, and we're trying to embed the geologists in the biology," Wilder said. "That approach has the great potential to unlock a history of evolution that has been elusive up to now."
