Reproductive Isolation and the Potential for Character Displacement in the Damselflies, Calopteryx Maculata and C. Aequabilis (Odonata: Calopterygidae)

Abstract

Waage, J. K. (Division of Biological and Medical Sciences, Brown University, Providence, Rhode Island 02912) 1975. Reproductive isolation and the potential for character displacement in the damselflies, Calopteryx maculata and C. aequabilis (Odonata: Calopterygidae). Syst. Zool. 24:24-36.-Potential reproductive isolating mechanisms between sympatric Calopteryx maculata (Beauvois) and C. aequabilis Say are examined and evidence is presented for the reinforcement of species discrimination during pair formation and for a selective basis for changes in female wing pigmentation in the context of reproductive isolation between these species. The species are sympatric across the northeastern United States and southeastern Canada. In the area of sympatry C. aequabilis shows nearly complete ecological and temporal overlap with C. maculata. Mechanical barriers to interspecific copulations are apparently absent and no hybrids are known despite several observed interspecific copulations. Visual discrimination based on female wing pigmentation is identified as a major component in the reproductive isolation between C. maculata and C. aequabilis. C. maculata males discriminate between their females and C. aequabilis females by responding to the females with the darker of two alternative wing colors. Differences in wing pigmentation between sexes and among Calopteryx species are accentuated and displayed during pair formation and courtship. There is experimental evidence for the reinforcement of C. maculata male species discrimination ability in the area of sympatry with C. aequabilis. Experiments also indicate a selective disadvantage against dark winged sympatric C. aequabilis females. Coupled with the fact that sympatric C. aequabilis females have more lightly pigmented wings than allopatric ones, this finding provides a potential example of character displacement in the context of reproductive isolation between these species. [Reproductive isolation; damselflies; character displacement.] Of particular interest in the study of speciation is the origin of effective barriers to gene exchange between sympatric or parapatric populations of closely related (cognate) species. Whether these barriers result from genetic divergence during the allopatric stage of speciation, or from selection in the context of reproductive isolation during sympatry has been the subject of debate among systematists since the exchanges of Darwin and Wallace on the evolution of sterility (Dobzhansky, 1937, 1958; Fisher, 1930; Mayr, 1959). The opposing views are partially resolved (Littlejohn, 1969; Mecham, 1961) by noting that postmating incompatibility must normally arise as an incidental byproduct of divergence, either during the allopatric stage of speciation, or, less likely, during sympatry when there are sufficient restrictions to gene flow. The question then becomes: do premating isolating mechanisms also originate as incidental byproducts of divergence, or are they selected for in the context of reproductive isolation? Since these are not mutually exclusive alternatives, the question is more one of degree rather than kind. For example, a premating isolating mechanism might evolve between two species A and B in sympatry and incidentally function between one of the species and a third species C at a later time or in another part of its range (Littlejohn, 1969). The isolating mechanism would then be of incidental origin in the context of A and C or B and C, even though its origin was a consequence of selection in the context of isolation between A and B. A striking feature of the five North American Calopteryx species is the difference in wing pigmentation between sexes and among species (Howe, 1917; Walker, 1953).