Meeting Abstract
Phenotypic trait integration strongly controls the strength and direction of natural selection. Traits covary due to genetic/developmental factors (e.g. two traits influenced by one gene) or functional factors (e.g. two traits of the same biomechanical apparatus). Historically, these underlying factors of trait integration have been very difficult to tease apart, but understanding the relative contribution of these factors is important, because they determine how much of an effect selection can have on an animal’s phenotype. Studies on integration determinants in vertebrates have been largely limited to mammal skulls and jaws. Studying the integration within and among limb elements of flying and flightless birds provides an opportunity to more fully understand sources of phenotypic evolutionary constraint in a very different taxonomic group. Birds have become flightless many times, offering natural repetition of this transition. Intriguingly, despite independent evolutionary routes to flightlessness, flightless birds share similar skeletal limb element proportions and an affinity for sexual dimorphism of body size. In this study, I compared shared traits of flightless birds to the patterns of trait integration seen in the limbs of flightless and flying birds to distinguish genetic/developmental from functional factors of trait integration for the first time. The results show the limb integration patterns of flightless birds diverge from a shared pattern of flying birds. This suggests a functional cause of integration (flight) rather than conserved, intrinsic mechanisms that constrain the phenotype of all bird limbs regardless of flight ability. Once biomechanical pressures of flight have been released in flightless birds, skeletal traits can reorganize into new patterns of integration, which can impact how phenotype responds to sexual selection.
