The measurement of the characteristics of natural selection

A new index for the intensity of natural selection is proposed. This index is designed to measure the strength of the selection process itself rather than the effect of the process operating on a particular population for a particular period of time. However a new index for the effect of selection is also proposed.The indices can be used when the population being acted on is divided into classes with all the individuals in a class having the same susceptibility to the selection process. The fitting of susceptibility functions is discussed for cases where the division into classes is based upon the values of a quantitative variable.