Opportunity for natural selection among the Indian population: secular trend, covariates and implications.

Crow's index is widely used for indirect quantitative estimation of natural selection using birth and death rates. The present investigation is based on 179 studies among 144 different endogamous communities belonging to nineteen states and six geographical regions of India, categorized into six social groups. These studies appeared in 33 different years over six decades (1956 to 2007). The secular trend in Crow's index (I(t)) and its mortality and fertility components (I(m) and I(f)) shows a gradual decline in I(t) and radical shift in the relative contributions of I(m) and I(f). Before 1990 the opportunity for natural selection was mainly determined by differential pre-reproductive mortality (I(m)), whereas after 1990 it has been determined by differential fertility (I(f)). To find out the covariates of I(t), I(m) and I(f) sixteen socio-demographic variables were considered, and nine were found to be significantly correlated with I(t): total dependency ratio, decadal growth rate 1991-2001, young age dependency ratio, crude death rate, total fertility rate, child mortality rate, under-5 mortality rate, old age dependency ratio and decadal growth rate 1981-1991. On the basis of multivariate stepwise regression analysis, female literacy emerged as one of the most important predictors of I(t). The declining trend of I(t), Im and I(f) shows that the Indian population is passing through the demographic transition.