During the evolution of the direct ancestors of mammals, a striking transformation happened: the reduction, migration and incorporation of several bones of the jaw joint into the middle ear. It has been proposed that this transformation was driven by selective pressure for hearing, for chewing, or for the decoupling between these two functions.

In 2020, I suggested a novel hypothesis: this striking transformation has also increased the "evolvability" (capacity for adaptive evolution) of the ear and its associated sensory functions in mammals. Indeed, the incorporation of several jaw bones into the mammalian ear has considerably increased its genetic, regulatory, and developmental complexity. This, in turn, has increased the evolutionary degrees of freedom for an independent adaptation of the different functional units of the ear, thus contributing to the evolutionary success and adaptive diversification in the vast diversity of ecological and behavioural niches observed in past and extant mammals.

In my previous research, I have found evidence of the high evolvability of the mammalian ear, illustrated by the multiple adaptive convergences of inner ear morphology. However, one aspect of ear evolvability remains unclear: by coopting bones from the jaw, the mammalian middle ear also coopted genetic and developmental pathways initially optimised for chewing function, and that still partially remain involved in jaw development. My hypothesis is that this evolutionary history of the mammalian ear impacts its ability to vary and respond to selection for hearing, i.e. ear evolvability is constrained by inherited developmental pathways adapted to another function.

I will test this hypothesis by comparing the intraspecific variational properties of the morphology of the middle ear ossicles in mammals and their biologically homologous ear and jaw bones in birds, with a focus on adults, by high-resolution 3D imaging and cutting-edge multivariate methods.