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Abstract
Background: Socio-economic transition with time offers us a way to understand the dynamics of evolution by studying the change in selection pressure.
Aim: The present study aimed to explore the effect of temporal change in socio-economic conditions on the selection pattern of Sonowal Kachari, a major tribal population of Assam, north-east India, over a period of 31 years (from 1975 to 2006). It also attempts to locate the present position of the tribe in the demographic transition model on the basis of the prevalent selection pattern.
Subjects and methods: Data on differential fertility and differential pre-reproductive mortality were collected from 155 unrelated Sonowal Kachari post-menopausal women from Dibrugarh, Upper Assam. Opportunity for natural selection was calculated on the basis of the indices put forward by Crow (Ic) and Johnston and Kensinger (Ij). To compute the opportunity for natural selection, different parameters such as index of selection due to infant mortality (Im), index of selection due to fertility (If) and index of selection due to embryonic mortality (Ime) were taken into consideration. Standard statistical techniques were applied wherever relevant.
Results: The different components of selection were found to be higher among the rural population than those in the urban one. Without considering the effect of regional variability, the Crow's index (Ic) of opportunity for natural selection was found to decrease with time from 0.364 in 1975 to 0.244 in 2006. The Johnston and Kensinger index (Ij) also decreased from 0.541 in 1975 to 0.284 in 2006. Irrespective of the methodology, the mortality component was found to decrease considerably with time without marked reduction in the fertility component.
Conclusion: The study, within its limitations, suggests that at present the Sonowal population is experiencing a selection relaxation due to the temporal improvement of socio-economic conditions. By comparing the fertility and pre-reproductive mortality components it can be argued that the population may have been passing through the initial period of post-transitional stage, which may increase the segregation load in the population.