COUNTERGRADIENT SELECTION FOR RAPID GROWTH IN PUMPKINSEED SUNFISH: DISENTANGLING ECOLOGICAL AND EVOLUTIONARY EFFECTS.

JEFFREY DAVID ARENDT [1]

Department of Biological Sciences, Binghamton University (SUNY), Binghamton, New York 13902-6000 USA

Abstract. 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, pumpkins eed derived from lakes with bluegill grew faster than those from lakes without bluegill under all treatments, following the predicted evolutionary response.

Key words: bluegill; common-garden experiment; countergradient selection; ecological vs. evolutionary responses; interspecific competition; intrinsic growth rate; Lepomis gibbosus; Lepomis macrochirus; pumpkinseed sunfish.

INTRODUCTION

Explaining phenotypic variation is one of the primary goals of ecologists. Variation in a trait among species or among populations within a species is usually associated with environmental gradients such as moisture, light, temperature, or nutrients. An interesting phenomenon that has the potential to complicate standard ecological interpretations of phenotypic variation is countergradient variation (defined by Levins [1969]; reviewed by Conover and Schultz [1995]). Countergradient variation appears when the evolutionary response to an environmental gradient is to compensate for the usual phenotypic effect of that gradient. For example, Levins (1969) described a case of countergradient variation in Drosophila body size with respect to an altitude/temperature gradient. In the wild, flies from a cool, high-altitude site were larger than those from a warmer, low-altitude site. This is the usual phenotypic response in Drosophila to temperature (e.g., Atkinson 1994, Partridge et al. 1994), and the typical genetic response across a latitudinal cline (Partridge et al. 1994). However, when raised under common laboratory conditions, Levins found that flies from his warm site were larger than those from the cool site. Thus, genetic differentiation in body size was the opposite of the phenotypic gradient. Most subsequent examples of countergradient variation have also dealt directly or indirectly with temperature gradients, especially the effect of temperature on growth rate and adult body size (Conover and Schultz 1995).

Nutrient availability is another important gradient associated with phenotypic and genetic variation in growth rate. To date, most examples of genetic adaptation to nutrient levels have reflected cogradient variation. That is, organisms adapted to low nutrient levels evolve to grow slowly (but efficiently) even when nutrients are made abundant (Grime and Hunt 1975, Case 1978, Chapin 1980, Arendt 1997a). By contrast, Arendt and Wilson (1997) identified a situation in which low resource levels could select for a rapid intrinsic growth rate, provided there is a size-determined niche shift from a low-resource niche to a high-resource niche. This situation is illustrated by the competitive interaction between pumpkinseed sunfish (Lepomis gibbosus) and bluegill sunfish (L. macrochirus). Pumpkinseed and bluegill are common in lakes and ponds throughout the northern United States and southern Canada. During the last glaciation of North America, freshwater fish were confined to refugia that did not freeze. The refuge for pumpkinseed is thought to have been the Saint Lawrence River, while for bluegill it…