Environment's Effects on Evolution of Survival Traits
Advances in studying genes mean that scientists in evolutionary developmental biology or “evo-devo” can now explain more clearly than ever before how bats got wings, the turtle got its shell and blind cave fish lost their eyes, says University of Massachusetts Amherst evolutionary biologist Craig Albertson.
He recently won a five-year, $625,000 Faculty Early Career Development grant from the National Science Foundation (NSF) to study the evo-devo of jaws in cichlid fish, tropical freshwater relatives of the tilapia. These highly adaptable cousins of sunfish, usually medium-sized and looking a bit like perch, have a phenomenal ability to undergo evolutionary change. They’ve developed 1,000 new species in Lake Malawi, Africa, over the past million years, a faster pace than usual for other vertebrates in a similar period.
The NSF grant is the foundation’s most prestigious award in support of junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of both.
Through evo-devo studies, scientists now know that much biodiversity is due not only to differences in genes, but to changes in how and when genes are expressed, says Albertson. They also now recognize that genes interact with each other and the environment in development to determine phenotype, or an animal’s observable traits.
“This carries Charles Darwin’s ideas forward to a new level, to previously unconsidered sources of variation that can affect the evolution of traits. One in particular is phenotypic plasticity, the idea that different patterns of variation will be produced in different environments,” Albertson says. “They’re obviously doing something right, from an evolutionary perspective, in a very dynamic environment, Africa’s Rift Valley.”
Lake Malawi water levels have fluctuated up to 300 meters in the past 2 million years, providing everything from clear fresh water to an oxygen-poor soup high in salt, alkali or sediment, for example, but cichlids continue to adapt and survive.
Jaws are a good marker of adaptation because they are linked to survival, and the jaws of Lake Malawi cichlids can change rapidly to take advantage of new food resources. For example, open-water feeders have long jaws to snatch free-swimming, mobile prey, while bottom-feeders tend to have short, stout jaws for scraping algae from rocks. It is clear that these differences in jaw type are genetically determined, but Albertson wants to find out how much is also determined by the environment.
In studies he started at Syracuse University before coming to UMass Amherst in 2011, Albertson and colleagues are working to identify the set of genes responsible for determining jaw shape in both “normal” and “extreme” environments. They are taking a genetic mapping approach, using hybrids from a cross between two species differing subtly in jaw length. To begin, they raised an initial group of hybrids on an algae-based flake food, which is very easy for the fish to eat, Albertson explains. This population will be used to map genetic determinants of jaw shape under “normal” conditions.
Next, the biologists split the resulting hybrid families and reared them on two different diets, an algae-based diet spread on lava rocks, requiring fish to scrape to feed. These fish developed shorter jaws to accomplish this. The other treatment involved the same food, this time ground and sprinkled on the water surface. These fish had to suck food out of the water column; they developed longer jaws as they became more efficient at this task.
“The idea is that these two conditions should be similar to those in early Lake Malawi, when fish first arrived from surrounding rivers," Albertson said. "Presumably the ancestors of Lake Malawi cichlids all looked the same, but some went up to suction feed while others went down to scrape, and plasticity produced fish with different jaw lengths. By re-creating this scenario and mapping the genes that underlie these environmentally induced shape differences, we hope to learn about the genetic interactions that were the first step in producing the 1,000-plus species in the lake today.”
One key question he and his colleagues want to answer is whether the same set of genes are involved in developing crushing jaws and sucking jaws under normal and extreme environments.
“We don’t know if patterns of plasticity will affect patterns of evolution," Albertson. "We may see a different genetic response to distinct mechanical stimuli. But if we do recover a common set of genes under both normal and extreme conditions, it would substantiate key theories with respect to plasticity’s role in evolutionary change.”
Albertson also plans to reach out to high school science teachers with workshops to offer new perspectives on genetics and evolution.
“Evolution has been taught largely the same way for decades now,” Albertson said. “But today evo-devo has produced a much more detailed understanding and appreciation for fine points of evolution. We now know, for example, the genetic and developmental processes responsible for how the bat got its wings and how the whale lost its hind limbs. These are compelling examples of evolutionary change that should resonate with students. It is a really exciting time to be an evolutionary biologist!”