Introduction
Man is the product of two interdependent courses of evolution -biological and cultural. This complex matrix of culture and gene has made human population survive since, its evolution and to evolve continuously to adapt to the changing environmental regime. Human adaptation is a very significant and extremely fascinating process because it is largely different from other related as well as non-related biological species. It is no doubt that largely, the emergence of culture in the course of human evolution has been determining biological evolution; however, it is a misconception that cultural progress has nullified biological evolution. Penrose [1] rightly states that although our species-Homo sapiens is freed from many habitational or spatial constraints; however, the species is not completely released from the evolutionary forces and factors, particularly the influences of natural selection. Ever since Charles Darwin published his remarkable work, The Origin of Species by Means of Natural Selection in 1859, even today natural selection is considered the most important mechanism to maintain and pattern the variety of organic life. The processes of adaptation and natural selection are two sides of the same coin because a population adapts to ecology due to its variation in the genotype (that is inherited) and modification in the culture (transmitted in a non-genetic way). Most of the empirical studies demonstrate that adaptation never occurs in abstract but only in relation to specific habitational pressures and challenges or selective pressures. [2] Natural selection operates mainly through two means, differential fertility, and differential BACKGROUND: Numerous literature on the migration of Mishings point out to the fact that the Mishing and the Minyong are two culturally and linguistically cognate tribes that co-existed in the same ecology in the hills of Arunachal Pradesh. The Mishing tribe after migration, now inhabits flood-prone areas of Brahmaputra valley of Assam. AIM: The study aims to measure the adaptation process of these two cognate tribes inhabiting two different ecologies at present: Hills and plains by calculating the index of selection intensity by Crow's and Johnston and Kensinger's formulae.
MATERIALS AND METHODS:
The reproductive histories of 77 Mishing mothers of completed fertility inhabiting a flood affected village of Assam and 74 Minyong mothers inhabiting a hilly village of Arunachal Pradesh are selected. RESULTS AND DISCUSSION: The Minyongs show higher average fertility than the Mishings. The proportion of embryonic death is higher, and child death is lower among the Mishings (0.1661; 0.1623) than the Minyongs (0.1319; 0.2238). The index of selection due to mortality component is contributing more toward the total index of selection in both the tribes. CONCLUSION: The contribution of mortality component is sizeable to the total selection like many other tribes of North-East India. Higher proportion of embryonic deaths among the Mishings infers that the causes are mostly biological whereas, the higher proportion of child deaths among the Minyongs infers that the causes are mostly socio-cultural.
MATERIALS AND METHODS:
Key words: Adaptation, cognate, minyongs, mishings, natural selection mortality. Differential fertility refers to the differences in the contribution made to the next generation by individuals due to their inequalities in reproductive performance.
MATERIALS AND METHODS:
Differential mortality operates on individuals prior to the reproductive age and represents that class of individuals who cannot contribute biologically to the next generations.
MATERIALS AND METHODS:
With regard to selection, the most important variable to consider is Darwinian fitness or simply fitness, which is a measure of capacity to survive and to leave descendants or offspring. Capacity to survive is the phenomenon of adaptation at the biological level and cultural level in human groups. A class of individuals that can survive to the reproductive age contributes differentially to the next generations. However, it is probable that natural selection operating through differential mortality is less important among modern human populations where differential fertility appears to be the effective agent. Very significantly, this transition occurred due to improved quality of life and use of modern healthcare among the present populations. It is pertinent to note here that the contribution of mortality is sizeable as well as prominent to the total selection intensity among some less developed populations including in India.
MATERIALS AND METHODS:
Crow [3] devised an index of total selection intensity (I) to quantify the influence of natural selection inherent in an evolutionary process. The index assumes that the reproductive differentials are due to genotypic differences and more importantly, fitness is completely heritable. The index is divided into two components: I m -due to mortality prior to the reproductive age and I f -due to differences in reproductive performance among women who have reached reproductive age. As the reproductive outcome of a population is the result of the interaction of a variety of socio-cultural factors as well as genetic factors, therefore, the index sets an upper limit for the potential action of natural selection and is renamed as an opportunity for natural selection. Since this index covers mortality only at the postnatal age, Johnston and Kensinger [4] 
MATERIALS AND METHODS:
where I is the index of total selection intensity, I m the index of selection due to mortality, I f the index of selection due to fertility, P d the proportion of premature deaths up to pre-reproductive age (i.e., deaths before 15 years of life), P s the proportion of survivors up to the reproductive age and above (1 − P d ), V f the variance in number of live-births due to fertility, and X is the mean number of live births per woman aged 45 years and above.
MATERIALS AND METHODS:
Johnston and Kensinger's index of total selection intensity is as follows:
MATERIALS AND METHODS:
where I is the index of total selection intensity, I me the index of selection due to embryonic mortality, I mc the index of selection due to child mortality, P ed the probability to die before birth, P b the probability to survive till birth, I f the index of selection due to fertility, P d the proportion of premature deaths up to pre-reproductive age (i.e., deaths vary from author to author), and some scholars [5, 6] have referred that this displaced group is the present Mishing community of Assam. The folk-tales and folk songs also reveal the fact that they were hill inhabitants [7] As per the census report of 1991, [8] the population of the Mishings in the state was 467,790, which constituted around 16.3% of its total tribal population of the state. According to 2001 census, [9] the population of Mishings is increased to 587,310, which is 17.8% of the total scheduled tribe 
A Brief Ethnographic Account of the Two Populations

The Mishings
The Mishings are erstwhile a hill tribe of Arunachal Pradesh [10, 11] and has become second most populated scheduled tribe (plains) of Assam. Ethnically, they are Mongoloid and belong to the Indo-Tibetan linguistic group. Their ethno-linguistic affinity with the Adis is striking. [7, 11] Traditionally, Mishings were hunting, fishing and gathering tribe, and as a plains tribe in Assam besides cultivation, they also resort to fishing; however, hunting is rare. They construct their houses in raised platforms about feet above the ground. the Indo-Tibetan border. [12] In spite of the difficult terrain with rivers and hills, like the Mishings of Assam, they build permanent houses on wooden piles at a height of one to ten feet above the ground with split bamboo. 
Materials and Methods
The demographic data on the reproductive histories completed their fertility. [13] In the present study, the indices of opportunity of the natural selection are calculated by applying both the original formula of Crow and modified formula of Johnston and Kensinger, which takes into account the embryonic deaths. Table 1 depicts the parameters used in calculating the indices of opportunity for natural selection according to Crow's formula and Johnston and Kensinger's formula. The average number of live birth per mother among the Minyong population is found to be 6.58 ± 0.33, which is much higher than the Mishing mothers (6.00 ± 0.31) are. However, the proportion of According to Reddy and Chopra, [14] it is significant to note that the index of selection due to fertility (I f ) is much lower than the lower limit of the range of Indian populations in both the populations. It is also pertinent to note here that the opportunity for selection (calculated using Crow's formula and Johnston and Kensinger's formula) in the present populations operates primarily through differential mortality rather than the differential fertility that is very often observed in many Indian populations. The values of "I" is the highest among the Singpho of Arunachal Pradesh (1.020); [15] among the North-East Indian populations so far studied.
Results and Discussion

The indices of selection intensity by Crow's formula
However, in Assam, the Sonowals have the highest value of "I," followed by the Mishings of Assam (present study) according to Crow's index. Nevertheless, if the value of "I" is considered according to Johnston and Kensinger's index, then the Mishings of the present study show the highest value. Except the Bodo-Kacharis of Assam, the index of selection due to mortality component is contributing more to the total index of selection intensity.

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