The Proliferation of Cryptic Species in Chrysoperla Green Lacewings through Song Divergence

Abstract

Green lacewings of the economically important genus Chrysoperla produce vibratory signals with their abdomens during courtship and mating. These signals or are complex and species-specific, and must be exchanged in a reciprocal manner between the male and female of a courting pair before copulation will take place. Such a system of communication could have evolved in response to sexual selection, selection from species isolation, or both. Sexual selection requires reasonably high variance among individuals in reproductive success, manifested as a polygamous mating system. The sympatric sibling species Ch. plorabunda and Ch. downesi, taken as representative of the genus, exhibit relatively low polygamy of females but extreme polygamy of males. However, because of irreversible sperm depletion in males, effective lifetime ability to produce offspring is approximately the same in both males and females, resulting in relatively low variance in reproductive success regardless of sex. Consequently, sexual selection alone is probably insufficient to account for the evolution and differentiation of the observed songs. On the other hand, experiments testing choice by females of conspecific versus heterospecific songs and courtship partners demonstrate clearly that calls are of the utmost importance in the reproductive isolation of closely related, potentially interfertile species. Other laboratory studies reveal that the basis for call differences between species like Ch. plorabunda and Ch. downesi is polygenic yet still simple, indicating that complete reproductive isolation and hence specification in the genus need not involve much genetic change. Thus it is not entirely unexpected that in certain regions, such as the mountains of western North America and central Europe, exist several physically indistinguishable but acoustically unique song morphs of the carnea-plorabunda-downesi complex that seem to be reproductively isolated from their often sympatric relatives only by their calling behavior. I suggest here that small, chance alteration in the genes controlling call patterns can catalyze specification in such lacewings by preventing normal individuals from dueting and copulating with mutants. Sexual selection probably facilitates the process of divergence by assuring that individuals prefer partners with extreme and therefore more attractive calls. Thus, species proliferation within the complex can occur without obvious adaptive or historical cause.