Does selection by sheet webs maintain polymorphism in midge (Diptera: Chironomidae) body size at the University of Michigan Biological Station?

Abstract

Natural selection should favor the most fit form of a trait and result in the loss of all other forms, yet polymorphism, the existence of two or more forms of a trait in a population, is quite common. There are a few ways polymorphism can be maintained, including disruptive and directional selection. When two or more predators favor different forms of the same trait in a prey species, polymorphism can be maintained. It has been observed that orb webs preferentially capture midges with smaller body sizes at the University of Michigan Biological Station (UMBS) (Johnson et al. 2002). Despite this directional selection, body size among midges at UMBS remains polymorphic We investigated the possibility that sheet webs, a different type of web, are preferentially capturing larger midges i.e. that sheet webs select against smaller midges) . To test this hypothesis, we compared means and variances for thorax length, thorax width, and the ratio of thorax length to width for midges caught by sheet webs vs. uncaught (free) midges. We did this separately for two midge species, tentatively called A and B, in two sites on the UMBS campus: Blissville and Manville. We found that evidence of directional selection acting on Species B midges in Manville (length: p=0.005, width: p=0.005, ratio: p=0.28). However, like orb webs, sheet webs favored larger midges. We also found evidence of disruptive selection acting on Species B midges in both Blissville (length: p=0.020, width: p=0.008, ratio: p=0.007) and Manville (length: p=0.004, width: p=0.000016, ratio: p=0.48). We conclude that disruptive selection, but not directional selection, may explain the persistence of polymorphism in midge body size at UMBS, particularly if the total mortality caused by sheet webs exceeds that caused by orb webs. Alternatively, as yet unidentified factors may play a role in the maintenance of this polymorphism.