Constraints versus adaptations as contending evolutionary explanations of morphological structure : The giraffe (Giraffa Camelopardalis) head and neck as a heuristic model

摘要

The current study uses the head and neck of giraffe (Giraffa camelopardalis)udas a model for tracking the course of evolutionary change. Gould (2002) has arguedudthat there are three main avenues of evolutionary change that result in the genesis ofudnew morphologies. These are phylogenetic constraints, structural or allometricudscaling laws of form, and specific unique adaptations. It is well known that theudunique characteristic of the giraffe is its extremely long neck and yet, it only hasudseven cervical vertebrae. To study the neck the vertebral body lengths of differentudaged giraffes were measured to determine the contribution of the cervical vertebrae toudthe total vertebral column. The vertebrae of several extant ungulates as well as thoseudof fossil giraffids were used as a comparison with the giraffe. CT scans were used onudseveral giraffe skulls to study the extent of the frontal sinus in the giraffe in an attemptudto explain why the giraffe evolved such a large frontal sinus. The vertebral columnsudand skulls of several ungulates, including the okapi (Okapia johnstoni) were also usedudto compare with the results obtained from the giraffe. Immunohistochemistry wasudused to study the medulla and spinal cord sections of the giraffe to determine if theudlocation and size of the nuclei remained unchanged to the basic ungulate orudmammalian plan in spite of the unusually long neck, or if this long neck led toudchanges in the nuclei found in those regions. The results of these stains were alludcompared to the published literature available. Although more studies need to beudconducted on other ungulates to conclusively determine why giraffe have evolved audlong neck, overall the results showed that the anatomy giraffe head and neck remainedudtrue to the basic mammalian plan, with very little changing in terms of it morphology.udThe giraffe brain and spinal cord also resembled that of a typical ungulate. Thisudleads to the conclusion that constraints and allometric scaling laws of form play audgreater role than previously thought in the evolution of extreme morphologies.