Data from: Selection on a genetic polymorphism counteracts ecological speciation in a stick insect

The interplay between selection and aspects of the genetic architecture of
traits (such as linkage, dominance, and epistasis) can either drive or
constrain speciation. Despite accumulating evidence that speciation can
progress to “intermediate” stages—with populations evolving only partial
reproductive isolation—studies describing selective mechanisms that impose
constraints on speciation are more rare than those describing drivers. The
stick insect Timema cristinae provides an example of a system in which
partial reproductive isolation has evolved between populations adapted to
different host plant environments, in part due to divergent selection
acting on a pattern polymorphism. Here, we demonstrate how selection on a
green/melanistic color polymorphism counteracts speciation in this system.
Specifically, divergent selection between hosts does not occur on color
phenotypes because melanistic T. cristinae are cryptic on the stems of
both host species, are resistant to a fungal pathogen, and have a mating
advantage. Using genetic crosses and genome-wide association mapping, we
quantify the genetic architecture of both the pattern and color
polymorphism, illustrating their simple genetic control. We use these
empirical results to develop an individual-based model that shows how the
melanistic phenotype acts as a “genetic bridge” that increases gene flow
between populations living on different hosts. Our results demonstrate how
variation in the nature of selection acting on traits, and aspects of
trait genetic architecture, can impose constraints on both local
adaptation and speciation.