As I study functional morphology for my thesis, I think about this subject an unusual (or unhealthy) amount. And there's one thing that's been hurting my head for the last few days.
That is: how does physical stress as a response to function, e.g. biting reaction force and its skeletal response, work as selective pressure on morphology? In other words, what are the mechanisms behind functional adaptation as seen by morphological change through time?
Skeletal response to mechanical stresses occur during the lifetime of an individual. These are ontogenetic developments that are not the same as primary ontogenetic development, as shared by all members of the species (predestined by the genome), but secondary responses brought about by extrinsic factors. So surely, any changes acquired as a direct response to extrinsic mechanical stresses, must be acquired traits, thus aren't passed down to the next generation.
My question therefore is, how do these acquired traits get passed down to the descendants? We could trace functional adaptations through the evolutionary history of certain animals, e.g. the increasing robusticity in tyrannosaur skulls. Thus, it is quite possible that skeletal response to extrinsic forces, whether it be to withstand higher biting stresses or increased muscle forces, are effectively being passed down. Otherwise, taking structural stresses and strains of an animal and linking it in with evolution is completely pointless - we can see how extrinsic stresses affect ontogeny but not evolution.
Or is it that, every single member of derived tyrannosaurids go through this ontogenetic change independently within their individual lifetimes as a response to certain extrinsic pressures and consequently ending up with the same adult morphology? So in other words, is it that Tyrannosaurus wasn't genetically predestined to have more robust skull morphology than Daspletosaurus but rather led significantly more rough lifestyles? This basically means that responses to extrinsic factors are not passed down but acquired separately in every generation, and independently in every single individual within their respective lifetimes.
Or perhaps, the more responsive individuals fare better and naturally get selected for, thus increasing the mean response level in the population, eventually producing a descendant population with significantly higher responses to the same extrinsic stresses than the ancestral population. So, perhaps it is this responsiveness that are hereditary? In the case with tyrannosaurs, Tyrannosaurus possibly may have had higher response levels to extrinsic forces than Daspletosaurus which led to increased skull robusticity during ontogeny.
Or, from basic theory of evolution, is there a background variation in predestined skull robustness that some just so happens to perform better under extrinsic forces than others in a typical tyrannosaur lifestyle and that they are naturally selected for, and given enough time the population mean skull robustness shifts towards increased robustness in derived tyrannosaurs, most notably in Tyrannosaurus? In this case, skeletal response to extrinsic factors are not passed down but those that perform better to those factors are selected for from a population with varied degrees of responses.
I don't know the answer. I wonder if anyone has an answer...