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- People owe their differences to genetic and environmental factors. Both these interact so that the degree of manifestation of genetic factors depends on the environment. To a greater extent such interaction has played an important role in producing differences between people which, in turn has allowed human life to exist in a wide range of environments than would have been possible without this variety. The question of significance of the persistence of this variety has been much debated. Explanations have been put forth in reference to natural selection which ensures that the members of the species conform to the best type. However, the complexity of environments and relation to organisms are of paramount importance which has ultimately resulted into polymorphic states.
- As a gene may influence many characters and a character may be influenced by many genes, the facts of heredity are very complicated. Man is a world-wide species. The original roots of racial diversity must have been partly in the production of types suited to particular conditions. However, man has tended to maintain and magnify, for many reasons, superficial differences in features. For example, skin colour is believed to reflect racial differences. Human communities remain distinct partly because they contain individuals whose genes endow them with different physical capacities. The pattern of human life is not based upon communities composed of identical individuals. In fact we have varied capacities, motivations, satisfactions and hence opinions.
- Since man has been able to control the conditions around him, the question of what constitutes his 'environment' assumes a new aspect. The extremes of some climatic conditions may stimulate the inhabitants to find ways to isolate themselves from heat or cold. Living in centrally heated and air conditioned buildings has its own hazards for pharynx and respiratory system. This may set up selection pressures which are slightly different than others who are subjected to the normal hazards of climate. Infectious disease constitutes another environmental factor. Naturally, there are differences between people in the pattern of their resistance to infection. Thus, environment constitutes a very wide range which refers to the sum total of social, economic, moral, intellectual and all other patterns of learned behaviour which influence the genes along with the behaviour pattern of human beings. This however is always coupled with all the external physical forces that affect the life of an organism.
- Both genetic and environmental factors influence most of our characteristics. Some categories can be recognised which include elements of both heredity and environment. These may broadly be categorised as:
(i) Differences due to accident
(ii) Differences due to adaptive response during the lifetime of the individual
(iii) Differences due to cultural influences. - Generally, people differ in power of their adaptation because of differences in the degree of the hereditary control which is not easily altered by environment. On the other hand, there are characteristics like those of brain which are subject to hereditary influences but greatly affected by the environment. The endocrine system, muscles or blood are intermediate between these extremes. Thus there are some features having greater while others having lesser capacity to adapt during development. These powers of adaptation are inherited which help in responding various organs to environmental influences.
- Human differences may arise due to accident. Some people are born a little wiser than others. Similarly children born to mothers who are affected by German measles during the early months of pregnancy suffer from serious defects in the heart, brain, bones and elsewhere. The features also differ in their susceptibility to such afflictions. The complication of pregnancy leads to a variety of clinical conditions 'ranging' from cerebral palsy through epilepsy, mental deficiency, behaviour disturbances and reading disturbances.
People depend on the environment for food, water, air, warmth and many other factors. They are greatly influenced by the climate. It is suggested that there is a high incidence of congenital malformations in areas containing igneous rocks with a relatively high radioactivity. - The relationship of climate and disease 4s a specially interesting and important area. Diseases due to prevalence of parasites and intermediate hosts in certain areas are major problem of homeostasis e.g., the prevalence of malaria, hookworms, yellow fever, dysentery, sleeping sickness and other tropical v conditions. Resistance to all these depend to a large extent upon hereditary factors and on the capacity to survive with parasites or worms. Naturally we depend on the special conditions and many organs of the body including brain enable an individual to tolerate his diseased condition. Selection certainly operates and that is why people inhabiting tropical countries differ from those of temperate zones (those zones are devoid of intermediate hosts and hence the diseases mentioned above are not prevalent).
- Some of the differences between humans are undoubtedly due to the fact that survival depends largely on the capacity to resist infection. .Sickle cell gene provides the best example where change of one amino acid in the haemoglobin reduces its value as an oxygen carrier. This is fatal when the individual is homozygous. However, in heterozygous condition it confers some protection against the malarial parasite. Possibly there are variations in the capacity to resist all infective diseases (by production of anti-bodies). Resistance to infection by diphtheria bacilli is partly the result of the presence of a hereditary factor. Similarly the familiar distribution of Poliomyelitis suggests the involvement of hereditary factors. Very little is known about the capacity to resist even the more ordinary infections.
- In addition to the above, every habitat imposes a particular climatic regime on man which varies widely and especially so because man can make appropriate adjustments with his habitat. Human body is sensitive to many ailments that go to make up a climate of the place. This sensitivity and responsiveness help to maintain homeostasis. Under the conditions of extreme heat or cold, the heat regulatory system acts to maintain the body temperature within certain limits. At high altitudes it is the respiratory system which keeps the pressure of oxygen and carbon dioxide of the body fluids adjusted within stable limits.
- In addition to the stresses imposed by thermal factors and the low barometric pressure (at high altitudes) other environmental hazards are provided by excess of short wave radiation including both ultraviolet light and ionizing radiations. The efficient adaptation of human body to climatic change is necessary for the attainment of bodily comfort, performance of physical work without undue fatigue performance of skilled work involving alertness and dexterity and attainment of normal growth and development.
- As regards response to heat, the total number of sweat glands varies in different individuals, though, striking differences between different racial groups do not seem to exist. In an investigation by Weiner, South African Bantus and Europeans were found to have very similar sweat (gland count. In Europeans the density of glands has a distribution in decreasing order as follows: it is maximum in the upper limb (dorsum of hand fore arm upper arm), followed by the lower limb (foot leg thigh) and then over the trunk, abdomen and thorax. In Indians and Africans, the gradients are similar.
- The complex changes that make up physiological acclimatisation to heat have been demonstrated in peoples of different races living in hot climate (in Nigerians, Chinese/Indians, Malayans living in Malay, Kalahari Bushmen, South African Bantus as well as in Europeans in the tropics and in hot deserts). This physiological acclimatisation enables man to carry out' his active life under a wide variety of hot conditions and makes it possible for our species to occupy very different kinds of tropical and equatorial habitats. It seems that-genetic selection of various bodily characters for life and diverse climate has been superimposed on this physiological plasticity.
- There are indications that acclimatisation to cold develops gradually. Increased tolerance to cold can be acquired. The Australian aborigines can sleep naked with little apparent discomfort at air temperatures of about 32oF and radiant temperatures of 50oF. These conditions could not be tolerated and shivering and discomfort were apparent under the same conditions amongst the investigators. Similarly, Bushmen of the Kalahari sleep in extremely cold conditions with only a slight protection of single covering and small fire. A high degree of cold tolerance can be acquired by Europeans and many Norwegians. Eskimos are stated to have a far greater tolerance to cold in the hands than White men. This ability of Eskimos to use the hands efficiently at low temperatures is associated with an increased blood flow. However, the role of thyroid, like the adrenal gland, is involved in cold adjustment.
- Thus, the available evidence points strongly to the existence of acquired cold tolerance in which physiological adjustments play a large part. Differences in the average physique of different populations are of significance in climatic adaptations as they conform to the ecological rules of Bergmann (1847) and Allen (1877). According to Bergmann's rule "within the polytypic warm blooded species the body size of a subspecies usually increases-with decreasing temperature of its habitat". Allen's rule states that "in warm blooded species there tends to be an increase in the relative size of the protruding organs such as ears and the tail with increasing temperature of the habitat". Several studies have demonstrated that human body size and shape tend to follow these rules. The mean body weight in hot regions is demonstrably lower than that in temperate and cooler climate. Roberts has shown that the lower limbs tend to be longer in hotter climates. The dimensions of the trunk also become less in hotter climates.
- The variation in the subcutaneous fat covering in different populations has not been adequately studied. American Negroes have a smaller range of skin fold thickness than American Whites. Possibly Eskimos have thicker fat covering than Negroes.
Owing to the differences in adult physique, growth pattern also shows some relationship to climatic variation. The growth period is prolonged and maturation somewhat delayed in warm periods. A relatively greater height per unit body weight is attained by a delayed skeletal maturation. Coon, Garn and Birdsell have emphasised that Mongoloid face exhibits features adapted to life in extreme cold, e.g., the reduction of brow-ridges and the frontal sinuses, flattening and widening of orbital regions to permit more fat padding and the reduction of nasal prominence. - Amerindians and Eskimos are not affected by cold in spite of their long periods of exposure to cold. Climatic variables have a high correlation with the shape of the nasal aperture. It has been suggested that the nose shape may be more concerned functionally with moistening the inspired air rather than with heat exchange. Perhaps because, of this reason narrow nose is found in both hot and cold deserts having excessive dryness). The activity of the cilia of the respiratory epithelium is more reduced by drying than by heating or cooling.
- The above discussion indicates some functional advantages of certain body characters and their regional occurrence in certain people. It seem that regional selection has proceeded on Darwinian lines and climatic differences may be because of genetic differences. On the basis of twin studies we know that the variations in body shape, size, fat deposition, growth pattern, etc., are all determined largely, by the genetic constitution than by purely environmental factors. On the other hand, certain genotypes owing to the multifactor recombination like that of nose shape or limb length and trunk length ratio remain unaffected on change of environment.
- It has also been suggested that exposure to high temperature during the growth period can result in morphological changes conferring higher resistance to heat stress. This capacity to make immediate responses may have resulted into rapid selection and establishment of appropriate growth patterns in some populations genetically. Each major racial group (Caucasoid, Mongoloid, Negroid or Australia) occupies a wide range of climate possibly because of the physiological acclimatisation and partly because of the existence of Bergmann and Allen body size differences and partly to technological adjustment to climatic conditions.
- Man's cultural adaptation has provided an additional safeguard. The very spread of man over contrasting habitats would have been impossible if biological adaptation was not supplemented by technological measures. The history of the technology of housing, heating, cooling and clothing suggests that housing demands a high insulation of walls for facing extreme cold demands. Similarly, roof and floor must have been adjusted to make the best use of space in warming appliances and to provide efficient ventilation.
- The winter habitation of Central. Eskimo and those of the Alaska and Greenland display a remarkable mastery of these principles. Man's technological understanding of hot climate had undergone much development by the beginning of the historical period. Humans have developed appropriate building methods to deal with the complex factors of hot, dry or humid conditions. Such methods are always related to physiological principles and standards of comfort and efficiency. Like housing we may examine clothing-including footwear and hand wear - in terms of heat exchange in different climates both hot and cold.
- The implications of heredity and environment in the ethnic differentiation of man are not easy to evaluate. However, the nature of the probable mechanism or processes involved in the evolution of, ethnic-groups of man can be appreciated if we postulate that originally the ancestral human population [which has genetically heterogeneous) inhabited a circumscribed region of the earth. However, gradually individual families or groups of families dispersed over great distances at different times from the original ancestral habitation. Some of these groups became geographically isolated from one another and remained as such for generations.
- These isolated groups must have been subjected to natural selection, mutation, isolation, genetic drift, hybridisation and social selection. We have already discussed all these factors in detail. However, the implications of natural selection in the context of the interaction of heredity and environment need to be considered again. Natural selection preserves through the action of environment such variations that arise and are beneficial to the organism under its conditions of life [such variations are said to be adaptive). Since natural selection neither has a purposeful design nor foresighted planning, therefore, its results are always relative to the particular environment in which the organisms are living and to their structures and habits. Adaptive traits, therefore, are adaptive only in relation to the environment where structure and function fit harmoniously.
- We know that the skin colour is an adaptive character, it has selective value. In human population darkly pigmented people are geographically distributed in high temperature, sunlight and humidity regions while lightly pigmented people are in regions of lower temperature sunlight-humidity conditions. The marked gradients shown by pigmentation confirm Gloger's rule which states that melanin pigmentation tends to increase in the warm and humid part of the species range. It seems that black pigmentation is certainly, promoted by high humidity and high temperature whereas aridity with high temperature promotes the formation of reddish, yellowish and reddish-brown pigment.
- Nose is another adaptive character. Thomson and Buxton, on the basis of their research work, have stated that air at higher temperatures is breathed more easily through a broad nose than the narrow one. Because a broad nose allows maximum exhalation of hot air from the lungs, therefore, in hot climates [where heat dissipation is essential) a broad nose will be at selective advantage. On the other hand, it has been argued that narrow nose represents an adaptation to cold climate where it is necessary that the temperature of the air must be adjusted to the proper warmth and humidity. Kenny has shown that most human desert dwellers have a slender nose which can provide cooling and saturation of the inspired air before its exposure to the respiratory surfaces. Weinger stated that the broad nose is correlated rather more significantly with external absolute humidity than with air temperature and humidity.
- It has further been observed that populations living in extremely cold regions, e.g., Arctic area, Siberia, Alaska and Greenland are relatively short and tend to be well padded with fat. Their surface area is less as compared to those who have been living in regions of high temperature. The latter naturally would radiate as much heat as possible while the former as little. Body weight is also correlated with mean temperature. Thus the high body weight is associated with high temperatures whereas medium body weight to more temperate regions.
- Accumulation of fat in cold climate provides insulting qualities which affect body heat loss. A decrease of surface is naturally advantageous in maintaining body height in the cold environment. A large body size thus serves to reduce heat radiation and a small body size increases it. These facts are in accordance with Bergmann's rule which in this context states that smaller sized members of a population are to be found in the warmer parts of the range and the larger sized members in the cooler areas.
- Thus it is clear that the action of natural selection varies under different conditions and the rate at which it operates depends upon the character or quality involved. It may, however, be emphasised that even the organisms possessing certain selective advantages certainly need the cooperation of their fellowmen in order to be preserved. In fact, cooperation is an integral part of the process without which the survival itself would be seriously, jeopardised. Thus, it is the cohesive effect of natural selection as represented by the cooperative aspect which ensures the perpetuation of the species or group. The 'Favoured Races' are preserved by cooperation. Cooperation has thus certainly played a role in the evolution of man and also in the interaction of heredity and environment.
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