MORPHOLOGICAL EVOLUTION MEDIATED BY BEHAVIOR IN THE DAMSELFLIES OF 2 COMMUNITIES

Behavior can play a mediating role in determining the selective pressu res that influence the evolution of morphological structures. To exami ne this, I quantified patterns of morphological variation among larvae of Enallagma damselfly species (Odonata: Coenagrionidae) that use dif ferent behaviors to avoid the major predators found in each of two com munities, lakes with and without fish. Specifically, I quantified the sizes and shapes of the abdomens and caudal lamellae (used for swimmin g) and legs for three species from fishless lakes and six species from lakes with fish. A preliminary cladistic analysis indicates that spec ies within each lake type are not members of a single clade, which sup ports the conclusions of previous odonate taxonomists. Previous studie s have shown that species in fishless lakes are very active, running a nd swimming frequently and at high rates of speed in the absence of pr edators, and they avoid their primary predators, large dragonflies, by swimming. These species have the widest abdomens, the largest caudal lamellae relative to overall body size, and the longest legs of the sp ecies studied, which should make them powerful swimmers and runners. F urthermore, species in fishless lakes are morphologically very similar to one another and differ greatly from fish-lake species, although ea ch is more closely related to species in fish lakes. In contrast, spec ies from lakes with fish move very slowly and infrequently in the abse nce of predators and do not attempt to evade attacking predators. Howe ver, despite their behavioral similarity, large interspecific variatio n in morphology exists among the fish-lake species, and the only morph ological patterns were differences associated with membership in the t wo primary clades identified in the cladistic analysis. A modification of Felsenstein's (1985) method of evolutionary contrasts which allows character change to be isolated along single branches is introduced a nd is used to reconstruct the evolutionary histories of these characte rs. This analysis suggests that large increases in caudal lamella size , abdominal segment lengths and widths, and leg length accompany speci ation events associated with habitat shifts from fish-lakes to fishles s lakes. Following habitat shifts selection pressures exerted by drago nfly predation apparently favored swimming as an escape tactic, which mediated selection pressures onto morphologies used in swimming to inc rease swimming performance; morphological patterns in extant species r eflect this adaptation to a new environment. Mechanisms by which behav iorally mediated selection could have accelerated evolutionary dynamic s following founder events are discussed.