The vertebral column is an important adaptation in the evolution of the subphylum Vertebrata and even the morphological feature that gives the clade its name. However, different vertebrate classes vary in the number of regions that make up their vertebral column and how many individual vertebrae make up each region. The mammalian vertebral column is made up of five regions, more than most other vertebrates, but they do not vary as much in the number of vertebrae in each region. Several factors, such as locomotion and habitat, have been proposed as reasons behind the limited variation in mammalian vertebral numbers. In my dissertation, I use the vertebral formulae of a variety of mammalian species to predict the formula of the ancestral mammal as well as to determine which of several ecological factors are linked to vertebral number variation. I found the ancestral mammalian precaudal vertebral formula to be seven cervical, thirteen thoracic, six lumbar, and three sacral vertebrae. I also found aquatic locomotion to be a predictive factor for changes to mammalian vertebral number. Additionally, I examine three genes that have been found to be associated with changes in vertebral number in one or more regions of the mammalian vertebral column to determine how those genes are related to changes in vertebral formulae throughout the class Mammalia. All three genes showed evidence of purifying selection, indicating evolutionary pressure against changes to amino acids. Comparing the gene trees to a species tree for the same species found minimal changes, also indicating selective pressure against changes in these genes. When I reconstructed the ancestral sequences for the genes, it revealed at least one region of each gene that was retained by many species, potentially indicating regions to examine further. My work provides more pieces of the puzzle of why mammalian species do not vary in vertebral number as much as other vertebrate species.
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