How the Adaptation Got its Start

Most of us are understandably amazed by the marvelous adaptations of plants, animals, and even fungi and microbes: acacias that dutifully supply nectar and grow thorny homes for the ants that protect them from herbivores (Janzen 1966), anglerfish that tempt their prey into striking range with a lure inhabited by glowing bacteria (Leisman et al. 1980), parasitic fungi that drive their doomed insect hosts up blades of grass to better distribute their spores, and bacteria that can easily survive in boiling water thanks to their heat-resistant proteins (Stetter 1999). Such features are undeniably impressive—but narrowly focusing on their current function ignores what is often the equally compelling story of their evolutionary history. In an article in this issue (Becoming Modern Homo sapiens), Ian Tattersall (2009) addresses a concept that gets at the evolutionary history of currently useful traits: exaptation, a feature that performs a function but that was not produced by natural (or sexual) selection for its current use. Tattersall’s discussion focuses on the human capacity for symbolic thinking, but the idea applies broadly throughout the living world. Here, we will dig into the concept of exaptation, find out why it is important, and see how we can use the concept in understanding the evolutionary history of traits. What is Exaptation?