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Indian Journal of Human Genetics, Vol. 9, No. 2, July-Dec, 2003, pp. 69-73 Selection intensity among Kshatriyas an endogamous population of Andhra Pradesh B. Dharani Priya, P. Veerraju, T. Venkateswara Rao Department of Human Genetics, Andhra University, Visakhapatnam
- 530003,India.
Code Number: hg03014 Opportunity for natural selection has been studied among the women of Kshatriya caste population from rural and urban areas of north coastal Andhra Pradesh. The urbanrural difference for different demographic characteristics is not significant. The contribution of fertility component for both Crow's index and Jonston and Kensinger's index is more. The present study is compared with other Andhra populations. The decreased mortality differentials in Kshatriyas suggest that this component was directly affected by the better socio-economic level and reflects on the population's transitional phase. Key Words: Selection intensities, Fertility, Mortality, Urban, Rural. Introduction The measurement of natural selection involves the chances of survival and reproduction of individuals that have different genotypes. It is identified by the fertility and mortality rates of different genotypes. The rate of evolution i.e; geometric rate of increase in the fitness depends on the available amount of genetic variation in fitness. In human populations the fitness of individuals can be estimated from their number of successful offspring.1 If there is no genetic variation in fitness there can be no differential selection and hence, in general, no significant evolutionary change in the genetic content of a population. According to Fisher's.2 "Fundamental theorem of natural selection", the rate of increase of fitness of any organism at any time is equal to its genetic variation in fitness at that time. In this context differences in fertility and mortality levels in various groups in a population are of great significance in understanding the structure of the population. Changing fertility and mortality patterns due to modernising forces leading to socioeconomic and demographic changes have a profound effect on the opportunity for natural selection in human populations.3 The people of India are socially organized into a number of distinct groups that are largely more or less endogamous and reproductively isolated. This nature provides a good opportunity to understand the dynamics of evolution. Several theories are developed by different population geneticists to study populations in this direction.2,4 Crow developed an index that facilitates quantitative estimation of selective pressure provided the reproductive pattern of a population is known.5 This index refers to the total amount of selection and consists of two components - the mortality component (Im) and the fertility component (If). It measures the proportion by which fitness would increase with specific birth and death rates if they were all selective and heritability of fitness were complete.5 However several factors such as physical, socioeconomic and cultural factors influence the fertility and mortality. Hence this index should be considered as the upper limit for the potential action of natural selection. Accordingly this index was renamed as "Total Index of Opportunity for Natural Selection".6 Later Johnston et al modified Crow's index incorporating prenatal mortality.7 But the index of Johnston and Kensinger has its own drawbacks and should be carefully interpreted. In the present study the total index of opportunity for natural selection is studied among the Kshatriyas of Andhra Pradesh an endogamous caste population hither to not touched in this aspect. The Kshatriya is a warrior community and compared to several other populations, a smaller one. It is mainly concentrated in few districts of Andhra Pradesh. Material and Methods A total of 1500 women were interviewed on different demographic aspects and related information from rural and urban areas of four districts of Andhra Pradesh, viz., Visakhapatnam, Vijayanagaram, East Godavari and West Godavari districts during the period of 1997-99. Among these 193 women in the rural area and 162 women in the urban area (total 355 women) attained menopause The index of opportunity for natural selection is studied among these women. The index of opportunity for natural selection is calculated by two methods suggested by Crow and Johnston et al.5,7 Crow's index takes into account only live births and postnatal mortality. In contrast to Crows index, the Johnston and Kensinger's index takes into account all pregnancies, prenatal and postnatal mortality. The following formulae are used to calculate the indices. Crow's Index I = Im + If / Ps Im = Pd / Ps Ps = 1 / Pd If = Vf / Where I = Index of total selection intensity. Im = Index of selection due to mortality Pd = Probability of deaths up to Prereproductive age Ps = Probability of survival up to reproductive age If = Index of selection due to fertility V = Variance due to fertility
Johnston and Kensinger's Index I = Ime + Imc / Pb + If / Pb X Ps Ime = Ped / Pb Pb = 1 Ped Imc = Pd / Ps Ps = 1 Pd If = V / Where I = index of total selection intensity Ime = index of total selection due to prenatal mortality Ped = probability to die before birth Pb = probability to survive till birth Imc = index of total selection due to postnatal mortality Pd = probability to die before reaching reproductive age Ps = probability to survive till reproductive age If = index of total selection due to fertility V = variance due to fertility
Pd and Ps are calculated based on Prereproductive deaths in the present study. Results The number of live births along with the mean and variance with among the Kshatriya women who attained menopause is shown in Table 1.The rural women have a maximum number of live births with mean 3.64. Among the urban women the mean number of live births is 3.56. The variance is more in the rural area (4.2816) than in the urban area (3.4059). Statistical constants with regard to fertility performance of the women who attained menopause are given in Table 2. The mean number of conceptions per women is almost the same in rural and urban areas. Total prenatal deaths are more in urban areas while pre-reproductive deaths are more in rural area. The mean number of surviving children is almost similar in rural and urban areas. The urban- rural difference with regard to all the demographic characteristics is not significant. Crow's index of opportunity for total selection and its components are given in Table 3. The total index for natural selection is lower in the urban area (0.3747) than in the rural area (0.4690). In both the areas the fertility component is more than the mortality component. It is 77.45% in the urban area while it is 76.42% in the rural area. Components of opportunity for selection based on fecundity characteristics are presented in Table 4.7 It is slightly higher in the rural area (0.5818) than in the urban area (0.5178). However, the percent of fertility differential's contribution to the measure of total selection is declined considerably than in the Crow's index. Index of opportunity for natural selection based on fecundity characteristics is higher (0.5538) than Crow's index (0.4263). An attempt is made to compare the present study with other studies from Andhra Pradesh (Tables 5 & 6). Discussion The excess of prenatal deaths among the Kshatriyas in the urban area which includes abortions and stillbirths, can be attributed to medical termination of unwanted pregnancies for which facilities are available in the urban areas. Moreover the stigma of getting aborted is less in the urban area and awareness of family planning also might have contributed to the excess of prenatal deaths in the urban area. Higher mortality component of Crow's index in rural areas suggests comparatively poor medical facilities and health care in that area. On the other hand the impact of better facilities of medical and health care is reflected in the higher percentage value of fertility component in the urban area. The index of Johnston and Kensinger is definitely an improved one over that of Crow. However it is having several draw backs as it considers the embryonic deaths. Often embryonic deaths go unnoticed, especially among the uneducated women in India. Nonidentification of such deaths, and their underreporting by the women, inspite of best efforts of the surveyors would unduly influence the other components of the index. Therefore this index should be viewed with caution. The component due to embryonic deaths is higher in urban area. The lower value of this component in the rural area may be due to underreporting. The component due to prereproductive deaths is more in rural area, and reflects poor medical and health facilities. The comparative study has revealed several interesting points. The total index of opportunity for Natural Selection (Crow's index) has increased along with decrease in social hierarchy. It is lower among the Brahmins and gradually increased among the Kshatriyas, Vysyas and Sudras in that order. It is higher among the agricultural and artisan castes as well the scheduled castes and tribes indicating increased role for Natural Selection as the caste hierarchy decreased. Considering the components of the index, the mortality component is lower in the upper castes than in the lower castes and tribes. This can be attributed to education, awareness and better facilities of medical and health care in these communities. Among the tribes the total index is uniformly high with a higher mortality component in all the populations. One interesting observation in the comparative study is the changing trend of the components of the index in two different studies among the Brahmins of the same area over different periods.8,9 Srikumari observed a higher mortality component of 0.1270 and a lower fertility component of 0.180.8 When compared to the study of Sitalaxmi which reported a mortality component of 0.0396 and a fertility component of 0.3484.9 This change occurred over a period of about 20 years. The operation of selection through mortality in the earlier study is shifted to fertility in the later study over time. There is also a slight increase in the total index indicating enhanced fitness of the population. Studies of the same populations as seen above at two different periods of sufficiently large interval would throw valuable light on the micro-evolutionary aspects with respect to fitness through mortality and fertility. It is relevant here to mention the study of Ulizzi et al and Terranato et al.10,11 They have investigated the change of selection opportunities with a changing environment of Italy over a century and demonstrated the relevance of socioeconomic and cultural changes to the evolution of selective patterns of our species. Summary and Conclusion The Kshatriyas of Andhra Pradesh from urban and rural areas are studied regarding mortality and fertility aspects. Prenatal deaths are more in the urban area while pre reproductive deaths are more in the rural area. Higher mortality component of Crow's Index of opportunity for total selection may be the result of poor medical facility and health care in this area. A comparative study of this index in Andhra Population showed that the index is inversely proportional to the caste hierarchy. Two studies of Brahmin population of the same region over different periods with twenty years interval indicated a shift in the operation of selection through mortality in the first study to fertility in the later study which may be due to improved conditions of medical care and health. Acknowledgements The first author is thankful to GATE-UGC for financial support to carryout the above work in the Department of Human Genetics, Andhra University, Visakhapatnam. References
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