Evolution of man is not confined only to the past. The mechanism that brings in changes from one generation to another through change is gene frequencies. Gene frequency is the number of gametes, that is, spermatozoa that carry the gene.
Hence, knowledge of these mechanisms is essential for us to understand and to predict future trends in the genetic composition of human populations. The limitations inherent in such attempts are threefold.
Que is that extrapolation is possible only from the present trends though human history has been shaped by unpredictable events. Such an assessment is also true for the biological history of man, which is now inextricably intertwined with cultural, social and political history as well as with the future development of human biology and medicine.
The second limitation is that revolutionary future scientific developments are expected from the present trend. The last one is that we assume in modern society, special attention will be given to health and medicine and also preventive medicine will gain importance.
These explorations are subject to human survival and social progress will continue in capitalistic and other societies.
The main forces that determine evolution are genetic drift^mutation, and selection when the population is completely isolated with no gene

flow as in the rigid caste system in India. The chance fluctuations of gene frequencies can lead to appreciable genetic differences between sub-populations.
In modern Western society, there is a tendency towards breaking down of isolates and increasing intermarriages between different populations. This trend will not get reversed in the future and new isolates of a small size will be formed.
Thus chance fluctuations (genetic drift) which have played an important role in human evolution, will almost cease to operate. If this trend in human breeding structure continues, a new species of man will not develop, since for the creation of reproductive isolation, a subgroup is absent.
Science fiction writers often discuss the creation of new human like species. The currently existing genetic variability coupled with selective breeding of man could not lead to such a new species. One would have to create novel genetic combinations with technology not available today with rigorous enforcement of selective breeding. These possibilities are remote.
But we can create new types of viruses by shuffling of DNA or RNA by using existing genetic variability.

Mutation

The two factors that cause changes in or shift gene frequencies are mutation and selection. Mutations and all chromosomal aberrations are unfavourable for the individual as well as for the population. Most chromosomal aberrations kill the zygote, during the embryonic development, a minority survive upto birth or still longer but affected patients suiter from severe malformations.
Gene mutation often leads to inherited disease or to defects in multifactorial genetic systems. A large fraction of point mutations lead to changes in the aminoacid sequences of proteins and causes no functional deficiency. Mutations that bestow advantage are very few.
Mutation rates of numerical


chromosome increase with the age of mother. Hence, change in maternal age will lead to corresponding alterations in the total incidence of such chromosome mutations.
In many modern societies, there is an increasing trend towards a decrease in the number of children per family leading to a concentration of child births within the age groups of lowest risk, that is women in 20's. It was calculated that this trend alone will lead to a decrease of Down's Syndrome by about 20-40 per cent. If the child bearing is postponed to a somewhat higher age, the trend is reversed. On the other hand, antenatal diagnosis is most efficient for early recognition of chromosomal abnormalities, which is available in most countries including India,
If all pregnant women above the age of 35 would use this procedure, a significant reduction of Down's Syndrome, depending on the age distribution of mothers, would be expected. Anomalies due to numerical or structural chromosome aberrations will become completely avoidable.
On the other hand, the rate of gene mutations increase with the age of the father. The total impact of the paternal age effect is low and the possibility of an old father to have a child affected with a dominant mutation is almost negligible.
Any conceivable increase of radiation exposure will enhance the mutation rate by a few per cent. On the other hand, with the increasing exposure to chemicals in modern life, a small increase ' of mutation rate is to be expected which will lead to an increase of structural and numerical chromosome aberrations and dominant X-linked hereditary disease.
Since cancer is caused by somatic mutations mostiy induced by environmental chemicals, increase in cancer is to be expected. We can also anticipate decrease of lung cancer due to reduced cigarette smoking. Selection
Natural selection is differential reproduction of genetically different individuals as a result of their endurance to environ, by chromosome aberrations. Also most patients with Down's and Klinefelter's Syndrome are not able to reproduce.
Natural selection has not changed for these conditions. But selection did show relaxation for some pathological traits with autosomal dominant or X-linked recessive modes of inheritance.
There are genetic diseases that have been maintained so far by genetic equilibrium between mutation and selection. One example is haemophilia (clotting disease) for which treatment is now available with the substitution therapy of factor VI which enables patients to lead an almost normal life.
Life expectancy and the chance to have children is greatly improved. We can predict the incidence of this condition for some generations.
Retinoblastoma is a malignant eye tumour of young children. The great majority of cases are sporadic though some are familial oases. Surgical removal of the eye^jfellowed by X-irradiation and light coagulation leads to the cure of 80 per cent unilateral and 50 per cent bilateral cases. About 60 per cent of all new mutations for inherited cases are bilateral and about 40 per cent unilateral.
The inherited retinoblastoma will be two or three times more common, several generations after successful medical therapy has been introduced. The frequency will change from the current frequency of 1 in 40,000 to 1 in 15,000.
Natural selection is effective when acting through differential structural and numerical chromosome aberrations and dominant X-linked hereditary disease.
Since cancer is caused by somatic mutations mostiy induced by environmental chemicals, increase in cancer is to be expected. We can also anticipate decrease of lung cancer due to reduced cigarette smoking. Selection
Natural selection is differential reproduction of genetically different individuals as a result of their endurance to environ, by chromosome aberrations. Also most patients with Down's and Klinefelter's Syndrome are not able to reproduce.
Natural selection has not changed for these conditions. But selection did show relaxation for some pathological traits with autosomal dominant or X-linked recessive modes of inheritance.
There are genetic diseases that have been maintained so far by genetic equilibrium between mutation and selection. One example is haemophilia (clotting disease) for which treatment is now available with the substitution therapy of factor VI which enables patients to lead an almost normal life.
Life expectancy and the chance to have children is greatly improved. We can predict the incidence of this condition for some generations.
Retinoblastoma is a malignant eye tumour of young children. The great majority of cases are sporadic though some are familial oases. Surgical removal of the eye^jfellowed by X-irradiation and light coagulation leads to the cure of 80 per cent unilateral and 50 per cent bilateral cases. About 60 per cent of all new mutations for inherited cases are bilateral and about 40 per cent unilateral.
The inherited retinoblastoma will be two or three times more common, several generations after successful medical therapy has been introduced. The frequency will change from the current frequency of 1 in 40,000 to 1 in 15,000.
Natural selection is effective when acting through differential