Positive FDS as an agent of convergence and divergence
Positive FDS is the evolutionary force that drives mimicry inHeliconius butterflies (Chouteau et al., 2016). It is the result of local predators learning through experience to avoid the aposematic signals of the most common unpalatable prey. Müllerian mimicry posits that unpalatable prey will benefit by sharing similar aposematic signals thereby allowing them to share the cost of training the local prey population. As we saw in our study, local pFDS can be a strong evolutionary force that can vary over relatively short distances. Within Heliconius populations, pFDS will drive mimics to a local optima color pattern that often varies little within or between species. In our study, this is clearly seen in the French Guiana red morphs whose hindwing rays are near perfect copies of the hindwing rays of H. erato , the most common Heliconius in French Guiana with a red rayed color pattern. In contrast the blue H. doris are co-mimics of H. sara , which do not have blue rays , but rather a blue iridescence that extends broadly from the proximal region of the forewings (Figure 1). Correspondingly, the shape of the blue rays ofH. doris are starkly different from the shape of the red rays. Therefore, not only has pFDS driven a difference in hindwing color, but also the shape of the color pattern. This is further seen in Panama, where again the red H. doris morphs are shaped differently than those in French Guiana, where the red ray shape is a near perfect to red co-mimics in Panama (Figure 1). This variation in color pattern shape exemplifies the power of pFDS to drive convergence (or advergence) within local populations.
In contrast to our results indicating local advergence, the difference in color pattern shape between regions demonstrates the ability of pFDS to drive divergence within species. Across its range, H. dorispresently exhibits divergent red color pattern morphs, that in French Guiana were distinguishable by the local predator community.
In Panama, the local predators attacked the models much less and did not show evidence that they distinguished between the divergent red morphs. We suggest this may be a result of more generalized avoidance in the Panama populations, which would result in less attacks on all morphs, as we saw. It is worth noting that this site in Panama, near the Gamboa Smithsonian Tropical Research Station, has been the location of manyHeliconius studies (live and model based) over the years (Seymoure et al., 2018; Dell’aglio et al., 2016; Finkbeiner et al., 2014; Merrill et al., 2012), and nearby there has been many anthropogenic changes to the environment that collectively could have influence on the local predator community.
We found that even at small regional scales, selection on mimetic warning patterns differs depending on local predator communities. Although Müllerian Mimicry theory predicts mimicking species to achieve monomorphism in color patterns overtime, we demonstrate that H. doris maintains a balance of multiple warning colors across its range. Further, we find that the same selective forces acting to maintain the balanced polymorphism, also drive divergence in warning coloration across its range. These results highlight the complex nature of pFDS and the impacts it has on interspecific variation of mimetic warning colorations. Collectively, our study as well as other model studies such as those of color polymorphisms in H. numata(Chouteau et al., 2016) and Peruvian dart frogs (Chouteau & Angers, 2011), have demonstrated that pFDS can simultaneously be an agent that both limits and facilitates diversification of mimetic traits.