The Phenotypic Plasticity of Death Valley's Pupfish
Desert fish are revealing how the environment alters development to modify body shape and behavior
After a visit to Death Valley, the desert wanderer Edward Abbey wrote that "the first impression remains a just one. Despite variety, most of the surface of Death Valley is dead … a land of jagged salt pillars, crackling and tortured crusts of mud, sunburnt gravel bars the color of rust, rocks and boulders of metallic blue naked even of lichen." For the most part, that description is accurate. As one of the world's harshest desert regions, Death Valley is a land of eroding badlands, scorching alluvial fans, and barren flats of mud and salt.
Even maps of the Death Valley region of California and Nevada allude to the region's sweltering temperatures. Furnace Creek, the Funeral Mountains, Dante's View, the Devil's Golf Course—the names of its geological features are omens of an extreme landscape. And, in those extremes, Death Valley delivers. Death Valley holds concurrent titles of being the lowest, driest and hottest location in North America. The valley's floor dips 86 meters below sea level and is the lowest location in the Western Hemisphere. In an average year, Death Valley receives only 5 centimeters of precipitation, and temperatures soar routinely above 49 degrees Celsius during summer.
Yet hidden in remote corners of Death Valley live the desert pupfishes—several related species that survive in an archipelago of permanent water habitats scattered in a sea of desert. Death Valley's pupfishes inhabit isolated springs, streams and marshes that are remnants of the region's milder climate less than 20,000 years ago. Since that cooler and wetter time, pupfishes in this region have evolved from a common ancestor into nine closely related species and subspecies, with each taxon living in full geographic isolation from the others. Death Valley's pupfishes are thus a little like the well-known Darwin's finches of the Galapagos Islands, in that they offer an opportunity to watch the process of evolution in action.
In the case of pupfish, variations in body shape and behavior play the role that beak sizes played for Darwin. The body shape, behavior and even the brain of a pupfish is flexible so that its development is influenced by the environmental conditions experienced during early life. This flexibility is termed phenotypic plasticity, and studies by myself and colleagues are uncovering how it plays a key role in shaping the phenotypic diversity of Death Valley's pupfishes.
You might be familiar with phenotypic plasticity in other animals. Many social insects have a division of labor within the colony where individuals have phenotypes specialized for certain tasks. In the honeybee, for example, one female serves as the colony queen and all other females act as workers. Phenotypic plasticity determines whether a female develops into the queen or worker phenotype; the diet received by a female during the first days of larval life determines which phenotype she expresses later in life. Many vertebrates also show phenotypic plasticity. Such plasticity may be a gradual phenotypic response as an individual develops or a rapid, reversible shift in behavior as an adult.
As research on phenotypic plasticity accrues, it is becoming clear that almost all traits show some plasticity. My own studies on the physiological underpinnings of variation in body shape and behavior, however, are revealing something unexpected. Patterns of phenotypic variation among pupfish populations are generated in part through the plastic developmental responses of fish to their ecologically distinct habitats. This finding suggests that what we consider population- or species-specific phenotypes can be malleable and depend in part on the development of that population in the unique habitats where they live. And when the habitats change, species' phenotypic characteristics may quickly follow.