Opportunity for natural selection among some selected population groups of Northeast India

BACKGROUND: Selection potential based on differential fertility and mortality has been computed for seven population groups inhabiting different geographical locations of Northeast India. MATERIALS AND METHODS: Crow’s as well as Johnston and Kensinger’s index have been used for the present purpose. RESULTS AND CONCLUSION: Irrespective of the methodology, the total index of selection was found to be highest among the Deoris followed by the Kaibartas. The lowest selection index was found among the Oraon population. If the relative contribution of fertility and mortality components to the total index is considered to be multiplicative, it is observed that in all these communities the fertility component exceeds that of mortality component, which may indicate initiation of demographic transitional phase in the selected populations with the betterment of healthcare and socioeconomic condition within the last few decades.


Original Article
measures the proportion by which fitness would increase with specific birth and death rates if they were all selective and the heritability of fitness were complete. [4] But, in practice, the genetic component in differential fertility and mortality is not the sole factor, as the reproductive outcome of an individual and or/population is a result of the interaction of variety of sociocultural factors that affect the fertility and mortality behavior of a particular population. Thus, the index only sets an upper limit for the potential action of natural selection and is accordingly renamed as opportunity for natural selection. [5] It can be divided into two components, i.e., selection due to differential fertility and selection due to differential mortality. It is more descriptive than analytical [4] and can lay out the facts that can be derived from vital statistics. Though several authors have pointed out the difficulties in using and interpreting the index, [6,7] Crow's index is considered to be one of the most feasible means of inferring selection as it is difficult to measure selection directly in human population. As such this is the index mostly used worldwide. Later, the index was modified by Johnston and Kensinger [8] to include prenatal mortality.
In the present study, the total index of natural selection is worked out to focus on the opportunity of natural selection among seven endogamous population groups of Northeast India. The population groups thus studied have different ethnocultural histories and have different socioeconomic backgrounds.

Material and Methods
The materials for this study consisted of 196 women belonging to seven different ethnic communities of DOI: 10.4103/0971-6866.69328

Introduction
Natural selection is one of the major evolutionary forces that bring about changes in the gene frequencies in a population through the action of differential fertility and mortality over generations. Differential fertility and mortality are the basic events of natural selection, which operate singly or jointly to determine the fitness of a particular population in a given environment. [1] Crow [2] devised an index that facilitates quantitative estimation of the selective pressure, given the reproductive pattern of a population. This is a generation analog of Fisher's [3] 'Fundamental Theorem of Natural Selection' and Northeast India. Seven villages, where the studied population groups live homogenously, were visited.
A detailed demographic account of the investigated villages was collected . A total of 72 Deori, 27 Manipuri   Meities, 24 Garo, 19 Oraon, 15 Kaibarta, 16 Assamese Muslim, 23 Tai-Khamyang evermarried women who had completed their fertility period were interviewed to get the relevant information for this study. An interview schedule was designed and was used to get the detailed reproductive history of these women. The information was later checked with their husbands and elderly members of the family. Crow's [2] as well as Johnston and Kensinger's [8] indices were applied to find out the opportunity for natural selection. The formulae that are used are shown below.
Crow's index can be computed by using his formula (Crow, 1958) and the modified one that includes the prenatal losses (Johnston and Kensinger, 1971). The computed procedures are given below.
Crow's index: where I is the index of total selection intensity, I m the index of selection due to mortality, P d the probability of deaths up to prereproductive age, P s the probability of survival upto reproductive age, I f the index of selection due to fertility, V the variance due to fertility, and X is the mean number of live births.
Johnston and Kensinger's index: where I is the index of total selection intensity, I me the index of total selection due to prenatal mortality, P ed the probability to die before birth, P b the probability to survive till birth, I mc the index of total selection due to postnatal mortality, P d the probability to die before reaching reproductive years, P s the probability to survive till reproductive age, I f the index of selection due to fertility, V the variance due to fertility, and X is the mean number of live births. P d and P s are calculated based on prereproductive deaths.
A brief descriptive account of the studied population groups has been provided below.

Deoris
The Deoris constitute one of the distinct tribes of Assam. Assam, Tripura, and Uttar Pradesh. [9] The Meities are described as the Kuki-chin section of the Tibeto-Burman stock by Chatterjee. [10] Though the general features of the Meities are Mongoloid, there has been a great diversity of features among them, some of which approach Aryan type. [11] The data from this population group were

Assamese Muslims
In Assam, Muslims constitutes the second largest of the mass. As per the historic records, the present day Assamese Muslims are the descendents of three different Muslim groups. [12] One of them is known as 'Garia', as they claim to have come from Gaur, the ancient Muhammadan capital of Bengal. [13,14] The second one is known as 'Maria' who are said to be the descendents of prisoners captured when Turbok, a pathan general, was defeated and killed in 1532 AD. [15] The third section is known as 'Sayeds' or 'Daon', who trace their ancestry from Azan Fakir, a 17 th century Muslim saint who came from Baghdad. All the three sets form endogamous groups among themselves and prefer to marry among themselves. The people for this present study belonged to the Garia sect and were concentrated homogeneously in the Uppor Nazira Muslim village under Sibsagar district. The village is 12 km away from Sibsagar town.

Tai-Khamyang
Tai group is one of the significant elements that Agriculture is their main source of livelihood; however, they are also found to engage themselves in other public as well as private jobs.

Results
The mean number of live births along with its variance and other relevant parameters to detect the selection intensity among the seven population groups is shown in Table 1 The fertility index is found to be the highest among the Deoris (0.179) and lowest among the Oraons (0.087).

The indices of selection intensity as per Johnston and
Kensinger in the seven populations have been shown in Table 3. It also indicates a pattern similar to that of Crow's index in the relative occurrence of selection pressure in different population groups. The highest opportunity of natural selection is found among the Deoris (0.546) and the lowest among the Oraons (0.266). The embryonic mortality index is found to be highest among the Oraons The intergroup comparisons for embryonic and postnatal mortality estimates based on standardized normal deviate [16] have been shown in Tables 5 and   6, respectively. It is evident from the tables that Oraon population varies significantly with Deoris, Garos, Assamese Muslims, and Tai-Khamyangs, with respect to embryonic mortality. The intergroup differences are,    Z score values for intergroup variability with regard to postnatal mortality reveal that the Deori population varies significantly with all the populations except Kaibartas while the differences are not significant between the other population groups.

Discussion
The computed mortality and fertility components and the index of the total selection intensity as per Crow (1958) indicate a relatively low index of total selection among the studied populations of Northeast India. Recently, Reddy and Chopra [17] have reported that selection index (I) in the Indian population ranges between 0.258 and 2.250. Thus, the present findings are inclined more toward the lower limit of Indian range.
This finding goes in conformity with several other studies done recently in different parts of India. [18][19][20][21][22] As far as the contribution of fertility and mortality components to the total index (Johnston and Kensinger) is concerned, it is observed that fertility component contributes more to the total index than the mortality component in all the population groups. A similar observation has also been made by Sikdar [23] among the other different population groups of Northeast India in recent years. Reddy and Chopra, [17]  with stable fertility component (Sikdar) [23] . However existence of variability in the selection index within these populations reflects variability in the accessibility of various healthcare measures among them. Z = (X 1 /n 1 ) − (X 2 /n 2 )/√p (1 − p) (1/n 1 + 1/n 2 ), where X 1 and X 2 are the characteristics 'A' in n 1 and n 2 samples of the population N1 and N2, respectively, and P = (x 1 + x 2 )/(n 1 + n 2 ); *Significant at 5% level Z = (X 1 /n 1 ) − (X 2 /n 2 )/√p (1 − p) (1/n 1 + 1/n 2 ), where X 1 and X 2 are the characteristics 'A' in n 1 and n 2 samples of the population N1 and N2, respectively, and P = (x 1 + x 2 )/(n 1 + n 2 ); *Significant at 5% level