Traditional definitions of Darwinian fitness emphasize reproductive design and adaptability to environment. The “energetic definition of fitness” emphasizes the distribution of energy among the following activities: food acquisition, metabolism, growth, reproduction, and survival.
Coinciding with the energetic definition of fitness, it appears that evolution is actually slowing snails down, leaving them more energy for other tasks. A BBC News article noted, “It is the first time that evolution has been shown to select for [lower metabolism] in individuals of any species.” Why would nature select for such a trait? According to evolutionary biologist Roberto Nespolo, “Animals that spend less energy [on maintenance] will have more surplus for survival and reproduction.”
Nespolo is currently conducting studies on this topic, and has found “significant directional selection on metabolism.” His team measured standard metabolic rate – the “minimal amount of energy an animal requires to stay alive” – by gauging the amount of carbon dioxide produced by each snail at rest. In a sample of 100 snails, Nespolo found that after seven months, the surviving snails had a 20% lower metabolic rate than those who failed to survive.
Even still, Nespolo’s team must now answer the question: “is having a slow metabolism linked to moving slowly?” The relationship between these two variables is critical to understanding what’s being selected for and snail energetics.