Spider predation as a selective pressure on midge body size.

Abstract

The adaptationist paradigm suggests that organisms today are primarily a result of natural selection. However, fixed and neutral traits, as well as genetic drift, often undermine the assumed pervasiveness of natural selection. For many species, body size is polymorphic and non-neutral, permitting natural selection to favor certain body sizes. Two midge (Diptera: Chironomidae) species, A and B, from UMBS were studied to determine if spider predation exerted non-random mortality on midges with respect to thorax length and width. And if so, whether webs with different cell size exerted different selective pressures. We predicted that intermediate thorax sizes would be selected against (disruptive selection). However, for Species A thorax length and width were significantly smaller for midges caught in webs than for uncaught midges. Thorax length and width for Species B were also significantly smaller for caught midges (length: p<0.001; width: p=0.003). These results suggest that directional selection, not disruptive selection, is favoring longer and wider midges. A potential explanation is that larger midges are better able to avoid or escape from webs. For Species A, small and medium celled webs all appear to exert directional selection for greater length (p<0.001) and width (p<0.007), while larger webs exerted no selective preference on width (p=0.153). Similarly, for Species B, medium and large-celled webs appear to exert directional selection for greater length (p<0.007) and width (p<0.033). However, smaller-celled webs do not appear to exert a selective pressure on length (p=0.263) or width (p=0.831). Smaller webs may negate the size advantages of larger midges, effectively capturing a random sample of the midge population and exerting no overall selective pressure.