Adaptation is studied as an empirical science

Adaptation is studied as an empirical science

Biologists today are giving a great deal of attention to the question of how to obtain data that will guide a decision on whether or not a trait is an adaptation. Sometimes the biosphere presents a “natural experiment” that permits scientists to observe evolution taking place over multiple generations in a natural setting. Scientists cannot depend entirely on such natural experiments to study adaptation, because the natural experiments are uncommon and may not speak to questions of greatest interest. Nonetheless, a natural experiment may provide particularly useful insights into adaptation because it may allow the adaptiveness of a trait to be judged from all the angles that matter. Industrial melanism is a famous phenomenon—with which you are likely familiar from your study of general biology—that exemplifies a natural experiment for adaptation. Melanism refers to a genetically coded dark body coloration. Industrial melanism is an evolutionary increase in the frequency of melanism in a population of animals living in an environment modified by human industries. A species of moth in the industrial regions of England has two genetically determined color states: light and dark. The moths were predominantly light-colored prior to the industrial era, when light-colored lichens covered the tree trunks on which they rested during the day. With increasing industrialization, the lichens on the trees were killed by pollutants, and soot from factories darkened the tree trunks. Within 50 years the moth populations in the industrial areas became predominantly dark-colored because, from generation to generation, genes for dark coloration increased in frequency. Studies demonstrated that on dark tree trunks, the dark-colored moths were less likely than light-colored ones to be seen by avian predators. From the direct observation of this natural experiment, we can say the following: In an environment impacted by industrial pollution, dark coloration became common in the moth populations by way of natural selection because it increased an individual’s likelihood of survival in comparison with the available alternative coloration. Dark coloration thus met all the standards of our formal definition of adaptation and could be judged, based on evidence, to be an adaptation to the sooty environment. Usually biologists are not able to observe evolution in action in this way. Thus, to study adaptation empirically, they must adopt other approaches. Several techniques have been developed—or are being developed—to study the question of adaptation when nature fails to provide an ideal natural experiment.