  Secondary structure elements of many protein families exhibit differential
conservation on their opposing faces. Amphipathic helices and beta-sheets by
definition possess this property, and play crucial functional roles. This type
of evolutionary trajectory of a protein family is usually critical to the
functions of the protein family, as well as in creating functions within
subfamilies. That is, differential conservation maintains properties of a
protein structure related to its orientation, and that are important in
packing, recognition, and catalysis. Here I define and formulate a new concept,
called the selection moment, that detects this evolutionary process in protein
sequences. A treatment of its various applications is detailed.
