Evolution: Not as Long as You Think

The paper, “Contemporary Evolutionary Divergence for a Protected Species following Assisted Colonization,” is based on research by Dr. Craig Stockwell, a James A. Meier associate professor in biological sciences at North Dakota State University, Fargo, with Dr. Michael Collyer, Western Kentucky University, Bowling Green, Ky., as lead author, and Dr. Jeffrey Heilveil, SUNY College, Oneonta, N.Y.

Study results raise the question of whether current biological conservation practices should take into account the possibility of contemporary evolution, to increase the chances of species survival from extinction.

Although studies of threatened species typically focus on factors associated with extinction risk, this published study reports a case of contemporary evolution for a recently introduced population of protected White Sands pupfish (Cyprinodon tularosa) in southern New Mexico.

To hedge a bet against extinction of a species, conservation biologists may colonize certain species in an effort to protect them. Such refuge populations are considered “genetic replicates” that could be used for future re-colonization in the event of a catastrophe in the species’ native site.

The study by Stockwell, Collyer and Heilveil shows divergence in body shape in an approximately 30-year-old refuge population of the protected White Sands pupfish (Cyprinodon tularosa). The result is a body-shape mismatch with its native environment, reflecting a case of contemporary evolution (over a 30-year period).

“Darwin, instead of visiting the Galapagos Islands, could have visited the western United States and deduced the same patterns of speciation following isolation by studying the various pupfish species,” Stockwell points out. “Today, most pupfish species are threatened with extinction. As a consequence, pupfish are routinely transplanted to new habitats to reduce their risk of extinction.”

Because pupfish populations have been isolated over both pre-historic and contemporary time scales, they provide an ideal opportunity to evaluate the mode and tempo of evolutionary divergence. Two native populations of the White Sands pupfish were isolated in Salt Creek (a saline creek) and Malpais Spring (a relatively fresh water spring) about 3000 to 5000 years ago. In the 1970s, two new populations were established, with one population introduced to another saline creek, Lost River, and the other to another spring, Mound Spring.

Stockwell previously used molecular markers to show that both of these introduced populations descended from the native Salt Creek population. Subsequently, Collyer’s dissertation research focused on applying geometric morphometrics to study body shape variation within and among native and recently established pupfish populations. The fish from the spring populations were found to be deep bodied, whereas the fish from the salty creeks were slender. “These body shapes make evolutionary sense because salt increases the density of water, giving a selective advantage to fish with slender bodies compared to fish with deep bodies,” said Stockwell.

The current paper published in PLoS provides geometric morphometric analyses to evaluate fish raised in a common garden study, demonstrating that body shape is heritable and thus, the divergence reflects adaptive evolutionary divergence over two different time scales.

These research findings suggest that the Mound Spring population is a poor “replicate” for the native Salt Creek population. In addition, the observed morphological divergence of pupfish, instead of taking millennia, could theoretically have happened soon after the populations were isolated, maybe within a few decades.

“Studies concerning threatened species often focus on factors promoting extinction risk, but our study shows that a common management practice such as assisted colonization can have observable evolutionary impacts within a few decades,” Stockwell explains. “This is important because such evolution may result in refuge populations that are actually mal-adapted to their native habitat.”

Research funding included U.S. Department of Defense Legacy Resource Program Grant no. DACA87-00-H-0014, an EPA-STAR North Dakota Experimental Program to Stimulate Competitive Research grant to Dr. Stockwell, and a U.S. Environmental Protection Agency Science to Achieve Results fellowship to Dr. Collyer. Dr. Heilveil is a former postdoctoral student of Dr. Stockwell and Dr. Collyer, a Ph.D. student advised by Dr. Stockwell.

More information:
“Contemporary Evolutionary Divergence for a Protected Species following Assisted Colonization”

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022310

“The Rate of Evolution,” Interview with Dr. Craig Stockwell
http://old.globalpublicmedia.com/the_rate_of_evolution_an_interview_with_dr_craig_stockwell
“Contemporary evolution meets conservation biology”
http://sciencewatch.com/dr/fmf/2008/08julfmf/08julfmfSockwl/