ASU Research E-Magazine: Adapt or Die?

Difference is the spice of life, and possibly the best insurance of survival for a species. Variations among individuals mean at least a few will be ready for hard times.

Natural selection is the grim reaper, or so the story goes. Kill or be killed. Only the strong survive. Nature, red in tooth and claw. Survival of the fittest. Indeed, biologists believe that natural selection is the process that adapts creatures great and small to their environments.

During his famous voyage aboard The Beagle, English naturalist Charles Darwin spent lots of time observing the creatures of the Galapagos Islands, isolated volcanic bits of land located 600 miles off the coast of Ecuador. The islands proved to be the perfect natural laboratory for studying evolution. Natural selection, as Darwin noted, was the process that evolved the beaks of groups of Galapagos' finches into different forms best suited for cracking seeds or snatching insects.

Despite Darwin's observations, and many others made by biologists over the past 150 years, nagging questions remain for Kimberly Hughes and other scientists. If natural selection winnows out the less strong, the less adapted, the malformed, and the undernourished, why then are individual organisms within a species different from each other at all? Why is it that natural selection has not eliminated variation completely?

“I’m interested in the maintenance of variation in both natural populations and laboratory populations,” says Hughes, an assistant professor of life science at Arizona State University’s West Campus. Hughes has studied fruit flies, mice, and now, guppies.

Hughes is spending the current academic year working at the University of California, Riverside. Her research is dedicated to determining why so much genetic variation exists in guppy populations. Colleagues working at the UC Riverside laboratories are known for their expertise in guppy research. They have generated a lot of preliminary information that is useful to Hughes.

“Male guppies have tremendous genetic variability in a couple of fitness-related traits, specifically, in size, maturity, and color pattern. We think this is important,” Hughes says.

Hughes is investigating the impact of frequency dependence on fitness. She explains: “For example, if small males are at an advantage when they’re rare, but at a disadvantage when they’re common, and if the same is true for large males, then that’s a very powerful mechanism for maintaining genetic variation.”

Hughes also tests this same hypothesis on male guppy color patterns. Sometimes being exotic can be a powerful siren call. A lone brunette in a room full of blondes often turns more heads than when other brunettes are present.

To some extent, testing these ideas represents something of a professional risk. In the past, similar kinds of experiments tried on fruit flies once suggested positive results, but later investigation and evaluation failed to find repeatable results.

“There was a rash of these types experiments in the 1950s, the 1960s, and even later,” Hughes says. “Then scientists began to pay more critical attention to how these experiments were done and it appeared there were a lot of methodological problems. To date, nobody has done these experiments the way I’m proposing they be done.”

During the past few years, Hughes has applied her knowledge of population genetics to the study of aging in fruit flies. She discovered that as the flies aged, they died for more and more different reasons. The result is consistent with evolutionary theory.

To get her results, Hughes used a few scientific tricks and a lot of statistical analysis. She bred fruit flies to have approximately 40 percent of the same genetic material. She observed the flies and recorded their exact time of death. Using that information, she was able to calculate exactly how much the time of their death was due to genetics, and how much was due to the environment.

In simple terms, what she found was that each population of fruit flies accumulated genetic mutations that tended to be lethal to individuals at advanced ages. As the populations aged, the flies expressed more and more genetic variation.

The findings appear consistent with human life spans. For example, on the average, teenagers die for only a few reasons: auto accidents, suicides, and rare diseases. But as humans age, we find ourselves susceptible to far more numerous ailments. The killers mount in number and variety: heart disease, cancer, stroke, diabetes, Alzheimer’s disease. The list goes on.

Natural selection did not cull our unfortunate susceptibilities to such diseases and disorders because they did not prevent our ancestors from producing us. In the case of such human disease, variation in a population does not have any particular purpose at all. It arises simply because, like a Xerox copied too many times, the genetic record becomes a little blotchy and gray.

Hughes’ interest in genetic variation has extended to issues in conservation. As human habitats expand via suburban sprawl, and wildlife habitats contract, the number of mates a wild population has to choose from becomes smaller and smaller. The potential for inbreeding increases.

Each individual human being is genetically imperfect. Each of us carries six or seven lethal recessive genes. Under most circumstances, this does not matter, because we have two full sets of genetic material, one set from our mother, and one from our father. Lethal recessives only express themselves when there are two identical sets. When inbreeding occurs, the potential for such a match goes up considerably.

Consider that any individual’s total genetic material is 50 percent the same as that of the person’s parents and brothers or sisters. The material is 25 percent the same as that of first cousins.

What is true for us is true for wildlife; the fewer mates a creature has to choose from, the smaller the genetic variation will be in the resulting population will be. It also means that the possibility increases of mating with an individual that possesses matching lethal recessive genes.

The results of Hughes’ work with mice on this regard is not encouraging. She and her coworkers took field mice and inbred them, producing apparently healthy offspring. However, when these mice were released back into the field, their fatality rate was much higher than mice which had not been inbred.

Her findings raise discouraging questions about the capability of zoos to maintain healthy populations of wild animals for possible later reintroduction to their native habitats.

Hughes is looking to the future. She has begun a collaboration with fellow ASU West zoologists Timothy Craig and Joanne Itami. The husband-wife team conducts ecological research work with salt bush and insects which parasatize the plant common to Arizona’s Sonoran Desert.

Hughes is helping her colleagues analyze the genetic variation present in these organisms. When she returns to ASU West for the Spring 1996 semester, Hughes will begin teaching a new course on techniques of wildlife conservation. The intent is to share her knowledge of population genetics and help to put that information to needed practical use.—John Svetlik