Hybridization constrains the evolution of mimicry complexes in woodpeckers

The evolution of interspecific mimicry does not always result in perfect resemblance between mimics and models. Differences between members of a mimicry complex can be explained by genetic or developmental constraints. Alternatively, imperfect mimicry might be the outcome of a trade-off between multiple selective pressures. In this study, we explored the evolutionary conflict between mimicry and hybridization in woodpeckers. Based on the selective trade-off hypothesis, we expected that mimicry complexes will start to evolve once the constraint of maladaptive hybridization is relaxed. Hence, we predicted limited overlap in the divergence times between hybridizing species pairs and members of a mimicry complex. This prediction was supported by clear tipping point in the probability of hybridization and mimicry at ca. 9 million years of divergence. Around this timepoint, the probability of hybridization approaches zero while the probability of belonging to a mimicry complex increases. This finding is only correlational and remains to be confirmed in other taxonomic groups. Nonetheless, our results suggest a selective trade-off between evolving interspecific mimicry and avoiding maladaptive hybridization in woodpeckers.

Methods
We compiled a database of hybrid woodpeckers using the Serge Dumont Bird Hybrids Database (http://www.bird-hybrids.com/) and the Handbook of Avian Hybrids of the World (McCarthy 2006). For each hybrid record, we tracked down the original source and assessed its reliability. In addition, we compiled a list of woodpecker species that belong to a mimicry complex based on a literature search in Web of Science, using the search string “woodpecker* and mimic*”. For each pair of woodpecker species, we obtained divergence times from TimeTree version 5 (Kumar et al. 2022). To ensure consistent comparisons, most divergence times were based on the phylogenetic analyses of Dufort (2016), except for four species pairs in the genus Melanerpes (where we followed Navarro-Sigüenza et al. 2017).