Chase-away evolution maintains imperfect mimicry despite rapid evolution of mimics

We studied a brood parasite-host system to test the fundamental hypothesis that deceptive mimics evolve to resemble models, selecting in turn for models to evolve away from mimics (“chase-away evolution”), and whether such reciprocal evolution maintains imperfect mimicry over time. We quantified the pattern complexity of host and parasitic eggs; complexity of egg patterns predicts egg rejection in the host species. Over only 50 years, parasites evolved towards hosts and hosts evolved away from parasites. This resulted in no detectible increase in mimetic fidelity. Our results reflect rapid adaptive evolution in wild populations and show that chase-away evolution in models can counteract even rapid evolution of mimics, resulting in the persistence of imperfect mimicry. Methods Eggs were photographed in linearised RAW format, in shade with a Nikon D90 camera with a 60 mm Micro-Nikkor lens. Complexity was quantified exactly as described in Dixit et al., 2022 (Visual complexity of egg patterns predicts egg rejection according to Weber's law. Proc. R. Soc. B. 289: 20220710. https://royalsocietypublishing.org/doi/10.1098/rspb.2022.0710).