Morphological measurements of two host specialists of the dipteran Tephritis conura, both in sympatry and allopatry

Adaptation to new ecological niches is known to spur population
diversification and may lead to speciation if gene flow is ceased. While
adaptation to the same ecological niche is expected to be parallel, it is
more difficult to predict whether selection against maladaptive
hybridization in secondary sympatry results in parallel divergence also in
traits that are not directly related to the ecological niches. Such
parallelisms in response to selection for reproductive isolation can be
identified through estimating parallelism in reproductive character
displacement across different zones of secondary contact. 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 between host races. 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.