Natural selection drives the evolution of both form and function, yet the biophysical world can influence and/or constrain evolutionary pathways. For example, the convergent fusiform body shape of actively swimming aquatic organisms is most likely the common evolutionary solution to the influences of hydrodynamic drag. In terrestrial vertebrates, the effects of gravity may have influenced the interactions of body size and shape, with cardiovascular morphology and function. Specifically, in long bodied animals (snakes), in which large hydrostatic columns exists, the evolution of the cardiovascular system may have been influenced by organismic features (length of animal), behavior (climbing and non-climbing), and habitat occupied (terrestrial, aquatic). A previous study (Lillywhite, H., Amer Zool, 1987) found the heart in terrestrial and arboreal species of snakes was located near the head, while in aquatic species the heart was located near the middle of the animal's body. Based on the concept that the vertebrate heart must do additional work to overcome gravity, the anterior heart in arboreal species served to reduce the hydrostatic blood pressure when these animals adopt vertical postures during climbing. A recent study (Gartner et al, Physiol Biochem Zool, 2009) analyzed a new data set of 155 species from five major families of snakes. This analysis indicated that heart position is influenced both by gravity as well as a strong phylogenetic signal.