Countergradient Selection for Rapid Growth in Pumpkinseed Sunfish: Disentangling Ecological and Evolutionary Effects

Ecologists often assume that the evolutionary response to an environmental gradient will be the same as the phenotypic response. However, under some circumstances the evolutionary response to a gradient may be opposite of the ecological response, a phenomenon known as “countergradient variation.” We had previously predicted that countergradient selection has occurred in pumpkinseed sunfish in response to a resource gradient created by competition with bluegill sunfish. Because competition only occurs when pumpkinseed are small, rapid growth has evolved to allow passage through the competing size classes as quickly as possible. We performed a common-garden experiment using juvenile pumpkinseed derived from lakes with bluegill and from lakes without bluegill. Pumpkinseed were raised in cages without bluegill, or with a low density of bluegill, or with a high density of bluegill. Bluegill significantly decreased growth for all pumpkinseed, following the predicted ecological gradient. However, pumpkinseed derived from lakes with bluegill grew faster than those from lakes without bluegill under all treatments, following the predicted evolutionary response.