Marine snail study suggests conservation efforts should move beyond genetic diversity

May 30, 2001

A study of climate-induced evolutionary change in a California intertidal snail suggests that conservation plans for protecting endangered or threatened species should not focus exclusively on genetic diversity. The research is funded by the National Science Foundation (NSF).

In the study, detailed in the June 1 issue of Science, biologists at the University of California, San Diego, and Louisiana State University measured the genetic diversity of populations of Acanthinucella spirata, a common marine gastropod from San Francisco to San Diego. They discovered that the snail's genetic diversity, the most commonly used gauge of a population's health, is highest in the Los Angeles area and lowest in the northern part of its range.

"This novel study provides a new angle to research on genetic/morphologic diversity," says Rich Lane, director of NSF's geology and paleontology program, which funded the research. "It has been thought that morphologic variation is greatest in populations where genetic diversity is highest. While that may be logical, here is a case where it is not true. The bottom line is that greater care and investigation may be needed in choosing locations of biological reserves, and in considering environmental and ecological decisions."

Measurements of genetic diversity now greatly influence decisions on where to locate protected reserves, since the most genetically diverse populations are assumed to be better able to withstand environmental changes. But while the snails around Los Angeles and southward are genetically more diverse than those to the north, the researchers discovered that they are less diverse morphologically. This is due to the presence of a different shell form, or morphology, in some populations in the northern part of the range that is not found in the Los Angeles area or in regions to the south.

"There's a big difference in the morphology-in the shape and size of the shells-between the northern and the southern populations," says Kaustuv Roy, a biologist at UCSD who conducted the study with Michael E. Hellberg, a biologist at LSU, and Deborah P. Balch, a former student at UCSD. "Looking at the coast today, there's a dramatic decline in genetic diversity in this species going from south to north," he adds. "But the northern regions have a greater amount of morphological diversity. So, which do you use to make conservation decisions-genetic diversity or morphological diversity? Here's a case where the two come into conflict."

During many previous ice ages, the cooling of the planet caused localized extinctions of many populations of marine and terrestrial organisms near the northern limits of their ranges. When the earth warmed, some of the southern populations re-colonized the previously occupied northern regions. This is what the scientists believe happened to Acanthinucella spirata, because the snail's genetic diversity is highest around Los Angeles and declines sharply northward. The decline in genetic diversity suggests that the northern part of the range of this marine snail was re-colonized by a small number of individuals from the south, resulting in genetically homogeneous populations to the north.

The researchers obtained their estimates on when this new morphological type arose by calculating the changes over time in a protein they used as a "molecular clock." They also confirmed from their fossil collections that this second distinct morphology was not present along the California coast before this time period.
Photographs of the two forms of Acanthinucella spirata available at:
Credit: Kaustuv Roy, UCSD

National Science Foundation

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