"Homeostatic hitchhiking": a mechanism for the evolutionary retention of complex adaptations.

The complexity of organismal organization channels and accommodates novel genomic and developmental modifications. Here, I extend this perspective to suggest that emergent processes that dominate homeostasis-co-option, re-use, and recombination of accumulated elements-can create configurations and dependencies among these elements that strongly reduce the number of evolutionary steps needed for the evolution of precise novel adaptations. Evolutionary retention and environmental matching of such configurations are further facilitated when they include elements of homeostasis that are responsive to particular environmental cues. I apply this perspective to the study of evolution of sex-biased egg-laying in birds, a phenomenon that combines precision, complexity, context-dependency, and reversibility. I show that homeostatic hitchhiking can overcome the main difficulty in the evolution of this adaptation-the perceived necessity of de novo co-evolution of oogenesis, sex-determination, and order of ovulation in each environmental context-something that would require unrealistic expectations of evolutionary rates and population sizes and is not a desirable outcome for a process that needs to retain substantial environmental sensitivity. First, I explain the rationale behind the homeostatic-hitchhiking hypothesis and outline its predictions specifically for studies of sex-bias in order of egg-laying. Second, I show that a combination of self-regulatory and emergent processes and ubiquitous re-use of conserved growth factors make oogenesis particularly amendable to homeostatic hitchhiking. Third, I review empirical evidence for this mechanism in the rapid evolution of adaptive sex-biased order of egg-laying that accompanied colonization of North America by the house finch (Carpodacus mexicanus).