Abstract

Optimal life histories are the result of fitness trade-offs that balance the costs and benefits of particular trait combinations. Trade-offs, represented by the genetic correlations between fitness characters, can constrain evolution by impeding the rate at which traits evolve independently. Wing dimorphisms, in which one morph (micropter) has reduced wings and cannot fly while the other (macropter) possesses fully developed wings and can fly, occur within several major orders of the Insecta (Orthoptera, Hemiptera, Homoptera, Coleoptera). A substantial amount of evidence indicates that a phenotypic trade-off exists between morph and fecundity in wing-dimorphic insects. However, to have evolutionary significance trade-offs must be genetically based. A preceding study (Roff, 1994a) conducted on the sand cricket, Gryllus firmus, documented a negative correlated response in fecundity to selecting on the proportion macropterous. The present study demonstrates that direct selection for decreased fecundity in G. firmus produces a correlated increase in the proportion macropterous. The observed symmetrical correlated response indicates that the genetic constraint acts both ways. From the direct response, the realized heritability of fecundity was estimated to be 0.200 $\pm$ 0.0496. The demonstrated correlated response supports the hypothesis of a genetic basis for the phenotypic trade-off between migratory ability and reproduction commonly observed within wing dimorphic insects. Thus, fecundity is constrained from evolving independently of the proportion macropterous.