Evolution of Net-Spinning Caddisflies: A Hypothetical Mechanism for the Reproductive Isolation of Conspecific Competitors

Abstract

Net-spinning caddisflies construct capture nets with a wide range of mesh sizes, although the ancestral net-spinner presumably spun large-meshed nets while inhabiting high current velocity microhabitats. Thorp proposed that the evolutionary diversification of mesh sizes resulted from the competitive displacement of some net-spinners into lower flow microhabitats where less water and therefore less food passed through their nets. As a result, smaller meshes were selectively advantageous because they captured smaller, more abundant food items. Although competition is often strongest between conspecifics, Thorp did not present a mechanism by which reproductive isolation, and ultimately speciation, would be achieved between the competitively inferior and superior individuals. One such mechanism may have been temporal isolation. Because the initially large-meshed nets of the competitively inferior caddisflies would have been inefficient in the lower flow microhabitats, these individuals would have theoretically grown slower and emerged as adults later than the competitively superior individuals, which in turn would have led to reduced gene flow between the two populations. Although temporal isolation may not have been their final reproductive isolating mechanism, it may have opened the door to the evolution of other isolating mechanisms.