Wing dimorphism as an adaptive strategy in water‐striders (Gerris)

Several hypotheses have been suggested to account for the adaptive significance of the different wing morphs in water-striders (Gerris, Heteroptera). Stability and isolation of population sites should favour short-wingedness; increased rates of population extinction should increase the fitness of the long-winged individuals. Further, if the populations are often resource (food) limited, dimorphism may be optimal. Combinations of other selective pressures can also produce local dimorphism, which need not be optimal — dimorphism can result from mixing of individuals from different population sites. The term morphism cycle is coined to express a cyclical change: when a region (comprising a great number of population sites) is initially colonized, long-wingedness is favoured, but short-wingedness becomes more advantageous after the colonization phase. However, if the populations become totally short-winged, they probably face a relatively high risk of extinction, and the cycle may begin anew. The ecological genetics of the Finnish water-striders (nine species) is discussed in connection with the numerous predictions suggested by the original cluster of wing-dimorphism hypotheses.