Non-parallel morphological divergence following colonization of a new host plant

Abstract

Divergent ecological selection may diversify conspecific populations evolving into different niches, and may lead to speciation if gene flow is ceased, e.g, due to reinforcement leading to character displacement. Adaptation to the same niche is expected to be parallel. Whether selection against maladaptive hybridization in secondary sympatry also results in parallel divergence in traits that are not directly related to the ecological niches remains an empirical challenge, although such character displacement can be decisive for if ecotypes develop into new species. Here, we use a host shift in the phytophagous peacock fly Tephritis conura, with both host races represented in two geographically separate areas East and West of the Baltic Sea to investigate convergence in morphological adaptations. We asked i) if there are consistent morphological adaptations to a host plant shift and ii) if the response to secondary sympatry with the alternate host race is parallel across contact zones. We found surprisingly low and variable, albeit significant, divergence between host races. Only one trait, the length of the female ovipositor, which serves an important function in the interaction with the hosts, was consistently different. Instead, co-existence with the other host race significantly affected the degree of morphological divergence, but the divergence was largely driven by different traits in different contact zones. Thus, local stochastic fixation or reinforcement could generate trait divergence, and additional evidence is needed to conclude whether divergence is locally adaptive.