Nature rarely presents opportunities to observe the evolution of new species in real-time. For most taxa it is often difficult to determine the historical causes of phenotypic and genetic differentiation as it is challenging to disentangle the role of selection from other historical evolutionary processes. Model systems of contemporary rapid ecological adaptation, such as the red-shouldered soapberry bug (RSSB; Jadera haematoloma) provide a unique glimpse into the evolutionary events of the past. Here we harness a complex sympatric set of host associated populations of RSSB in Texas to test for the same signatures of ecological divergence that made this species a textbook example of rapid adaptation in an allopatric set of populations in Florida. We ask the question: Does rapid ecological adaptation become a barrier to gene flow in its self? One of the classical rapid ecological adaptations of the RSSB that has been documented in Florida is a change in insect beak length. This changes in the phenotypic expression of mouthparts facilitates feeding on the different sized host seeds. We use similar measurements of a range of ecologically important phenotypes collected at 31 sites across Texas from 3 geographically overlapping host plants ranges to test for ecological adaptive traits.
Results/Conclusions
Similar to Florida, insect beak lengths are closely correlated with host plant seedpod size. Shifts from the native host to a recently introduced invasive host in the last 60 years has facilitated a 0.4 mm increase in mean beak length of insects associated with the invasive host (ANOVA, F(1, 901)=9.568, p<0.01) . Unlike the relatively large seed and thin seed case of the native host plant, the invasive seed is small and found deep inside a bulbous seedpod. Mismatches of insect beak length and host leads to reduced survival. This phenotypic shift in beak size thus, demonstrates local ecological adaptation in the face of gene flow and indicates seed morphology as the likely agent of selection. Additionally, 15% of the insects that are associated with the newly colonized invasive host were found to be flightless, in contrast to 57% of the ancestral native host associated insects. This flightless wing morphology is positively related to fecundity while negatively related to dispersal. The relationship between wing morphology and host association is significant χ2(2, N=901)=88.97, p<0.001. The combined effect of locally adaptive beak-lengths and discrepancies in dispersal ability between host-associated populations could present significant reproductive barriers to gene flow.