Developmental Plasticity as a Means of Adaptation
The concept of adaptation is one of the organizing principles of evolutionary biology and refers broadly to changes in organismal structure, function, or behavior that improve survival or reproductive success. Genetic adaptation more specifically refers to the process by which gene variants (alleles) that code for beneficial traits become more common within a population’s gene pool through the mechanism of natural selection. Although natural selection is a powerful mode of adjustment at the population level, many environmental changes occur more rapidly than can be efficiently dealt with by changes in gene frequency, which require many generations to accrue. To cope with more rapid change, human biology includes additional, more rapid adaptive processes (Kuzawa 2005; Lasker 1969). The swiftest ecological fluctuations (e.g., fasting between meals or the increase in nutrients that our bodies need when we run) are handled primarily via homeostatic systems, which respond to changes or perturbations in ways that offset, minimize, or correct deviations from an initial state. Operating not unlike a thermostat, which maintains a constant temperature by turning the furnace on and off, homeostatic systems modify physiology, behavior, and metabolism to maintain relatively constant internal conditions despite fluctuations in features like ambient temperature, dietary intake, and physical threat. The distinctive features of homeostatic systems include their rapid responsiveness, their self-correcting tendencies, and the fact that the changes they induce are reversible.
Some environmental trends are chronic enough that they are not efficiently buffered by homeostasis, but also do not persist for long enough for substantial genetic change to occur. As such, organisms may not rely on homeostasis and natural selection to adjust biological strategies to these intermediate timescale trends. A simple example illustrates how a sustained change in experience might overload the flexible capacities of a homeostatic system if this was the only means available to help the organism adapt (Bateson 1963). In this case, imagine an individual that has recently moved to high altitude where oxygen pressure is lower, resulting in an elevated heart rate that increases blood flow and thus the rate that oxygen-binding red blood cells pass through the lungs. By engaging a homeostatic system (heart rate), the body has activated a temporary fix to help compensate for the low oxygen pressure. However, this is only a short-term solution that comes with a cost: not being able to increase heart rate further if the need arises, like when fleeing from a predator. Thus, the homeostatic strategy of elevating heart rate may work for short-term acclimation, but is a poor means of coping with chronic high-altitude hypoxia.
Over longer time spent at high altitude, additional biological adjustments ease the burden on the heart, such as increasing the number of oxygen-binding red blood cells in circulation. However, individuals who grow and develop at high altitude exhibit an even better strategy for coping with hypoxia. They grow larger lungs, a developmental response that increases the lung’s surface area for oxygen transfer, thus obviating the need for temporary and more costly short-term adaptations (Frisancho 1993). This change in developmental biology is an example of developmental plasticity, which allows organisms to adjust biological structure on timescales too rapid to be dealt with through natural selection, but too chronic to be efficiently buffered by homeostasis (Kuzawa 2005).
These mechanisms can be viewed as allowing the organism to fine-tune structure and function to match the needs imposed by their idiosyncratic behavioral patterns, nutrition, stress, and other environmental experiences that cannot be “anticipated” by the genome (West-Eberhard 2003). Unlike homeostatic changes that are transient, growth and development occur only once, and thus plasticity-induced modifications tend to be irreversible once established. In this sense, developmental plasticity is intermediate between homeostasis and natural selection in both the phenotypic durability of the response and the timescale of ecological change that it accommodates.
