The actin-cross-linking protein spectrin is a prominent component of the 
membrane cytoskeleton. Spectrin is a tetramer of two antiparallel 
alphabeta-dimers which share a unique and ancient gene structure. The 
alpha-spectrin and beta-spectrin genes are composed primarily of tandemly 
repeated 106-amino-acid segments, each of which forms a triple alpha-helical 
coiled coil. Both the genes and the repeats themselves are homologous. The two 
genes are thought to be the result of a gene duplication event, and each gene is 
the product of duplications of the 106-amino-acid repeats. In this work we 
compare the process of molecular evolution across the repeated segments of the 
alpha- and beta-spectrin genes. We find that the alpha-spectrin segments have, 
for the most part, evolved in a homogeneous fashion, while considerable 
heterogeneity is found among beta-spectrin segments. Several segments with 
unique known functions are found to have evolved differently than the others. On 
the basis of heterogeneity of the evolutionary process, we suggest that at least 
one repeat has a unique function that has yet to be documented. We also present 
new statistical methods for comparing the evolutionary process between different 
regions of DNA sequences.