Egg Color Polymorphism in Brood Parasites and Their Hosts: Adaptation and Evolution
Polymorphism refers to the diversity of clearly distinct phenotypes within biological populations, and it is one of the extraordinary phenomena in biodiversity that is well known in predator–prey interactions. In avian brood parasitism, coevolution between parasites and hosts can also cause evolution of polymorphism because the egg traits of both parties are subject to selection causing mimetic parasite eggs to evolve as a response to host egg rejection, which in turn selects for the evolution of egg polymorphism in hosts, which further promotes egg polymorphism in parasites. Here we review previous studies and demonstrate that (1) egg polymorphism is an adaptation and counteradaptation in both hosts and parasitic cuckoos, (2) egg polymorphism has fitness consequences for both hosts and cuckoos, and (3) egg polymorphism in the cuckoo-host system changes temporally and spatially. Egg polymorphism can evolve as an anti-parasite strategy in cuckoo hosts, and it provides effective defenses to dramatically reduce the success of cuckoo parasitism. However, compared to hosts, egg polymorphism in cuckoos is simply a result of frequency-dependent selection, but not an effective counteradaptation to increase the success of cuckoos. If we consider each cuckoo species as a global population, egg polymorphism is common in most cuckoos. However, egg polymorphism in hosts is much more complex since it changes temporally and varies spatially among populations. It may be explained by the history of the interaction between cuckoos and their hosts and the intensity of selection linked to cuckoo diversity.
KeywordsAvian brood parasitism Coevolution Egg color Egg polymorphism Frequency-dependent selection
M. Soler and D. Sherry provided constructive criticism and helpful suggestions. This work was supported by the National Natural Science Foundation of China (Nos. 31672303 and 31260514 to CY, 31272328, 31472013 and 31772453 to WL) and the Program for New Century Excellent Talents in University (NCET-13-0761).
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