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Introduction
Natural selection is the process of preservation of genotypes among organisms that increases their chances of survival, procreation and multiplication from generation to generation at the expense of less advantageous ones. Evolution often occurs as a consequence of this process. Natural selection may arise from differences in (1) survival, (2) fertility, (3) rate of development, (4) mating success, or (5) any other aspect of the life cycle. All such differences result in natural selection to the extent that they affect the number of progeny an organism leaves.
The problem of measuring the intensity of natural selection directly in human populations was solved by Crow (1958) with his 'index of opportunity for natural selection'. This enabled change in fitness to be measured using specific birth and death rates. Thus 'natural selection' is the relative probability of survival and reproduction of the genotype. Only when a certain variability of this fitness is present in a population does natural selection occur. In human populations the relative reproductive success of different genotypes depends on the total number of offspring that each couple leaves to the next generation (Jacquard, 1969). However, natural selection does not only operate through differential fertility; the existence of differential survival of descendents up to reproductive age also determines biological fitness (Reddy et al., 1987; Lasker & Kaplan, 1995). Crow's index of opportunity for natural selection merely sets an upper limit on the amount of selective opportunity that can exist in any particular population; it would only measure actual selection if the heritability of fitness were complete and if birth and death rates were totally determined by genetic factors (Jorde & Durbize, 1986).
Despite this limitation, Crow's index has been widely used in human populations, because it allows indirect quantitative estimation of the selection inherent in the evolutionary process simply on the basis of the demographic statistics of birth and death rates (Terrenato et al., 1979; Hed, 1987; Reddy & Chopra, 1990; Kapoor et al., 2003; Alfonso-Sanchez et al., 2004). The study of secular trend, a covariate of Crow's index, and its implications provides a new dimension to understand the evolution and process of transition going on in a population. No such study has been...