Biologists have a long history of arguing about the relative importance of organisms and environments in driving evolution. Do organisms passively respond to their environments or actively shape them? Is the environment just a filter that removes the least fit organisms from each generation, or also a source of new traits? The most recent incarnation of these debates focuses on developmental plasticity, a developing organism's sensitivity to environmental inputs. All organisms are plastic to some degree. Many can change their sex, morphology, and behavior in response to their environments. The question for biologists is, does the widespread presence of developmental plasticity mean new traits can originate in response to new environmental conditions, and prior to genetic changes? According to one influential hypothesis, plasticity is a significant source of novel traits in evolution. I make three arguments regarding this plasticity-first hypothesis. First, that it has revisionary implications for how we understand what biological inheritance is. Second, that confirming the hypothesis will require biologists to shift their methodological priorities and make use of a richer set of resources, especially formal modeling and experimental evolution. Third, that the ongoing debate about the hypothesis is exemplar of a fascinating and misunderstood type of scientific controversy called a relative frequency controversy. Some philosophers consider these controversies a waste of time, but I provide an account on which they are an important and productive component of scientific practice.
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