A major unresolved issue in biology is why phenotypic and genetic variation is sometimes continuous, yet other times packaged into discrete units of diversity, such as morphs, ecotypes, and species. In theory, ecological discontinuities can impose strong disruptive selection that promotes the evolution of discrete forms, but direct tests of this hypothesis are lacking. Here we show that Timema stick insects exhibit genetically-determined color morphs that range from weakly to strongly discontinuous. Color data from nature and a manipulative field experiment demonstrate that greater morph differentiation is associated with shifts from host plants exhibiting more continuous color variation to those exhibiting greater coloration distance between green leaves and brown stems, the latter of which generates strong disruptive selection. Our results show how ecological factors can promote discrete variation, and we further present results on how this can have variable effects on the genetic differentiation that promotes speciation.

These data were generated to investigate how color variation in host-plants is selecting for the evolution of discrete morphs in Timema stick insects. To do so, we measured Timema and host-plants reflectance and coloration. To estimate the strength and shape of selection we respectively transplanted 60 and 60 Timema individuals onto two host-plant treatments (first treatment: combination of Ceanothus sp. and Adenostoma sp., AC treatment; second treatment: Cercocarpus sp., MM treatment). Within each treatment, we released 20 individuals with green coloration, 20 individuals with a melanistic coloration (grey, red, or brown), and 20 individuals with a coloration intermediate between green and melanistic. We marked all individuals on the ventral side using a Sharpie pen to recognize them from naturally occurring insects. We collected the surviving individuals two days after their release and counted them, allowing us to estimate survival of the different color categories in the different treatments.

none