kindly explain 1)Genetic Drift, 2)Gene Flow, 3)Natural Selection......How is Blood Group determined in humans????

Genetic Drift: Genetic drift is the variation in gene frequencies within a population which can occur by chance. Sometimes without any reason some individuals may leave few more offspring than the other individuals. As a result the genes of that specific individual will automatically express maximum in the next generations. This is genetic drift. In a small and isolated population genetic drift may be an important mechanism of evolution.

Gene Flow: The transfer of alleles or genes from one population to another is called gene flow. It is also known as gene migration. Gene flow depends upon the mobility of the organisms. Animals are more mobile than plants. In plants gene flow occurs by transfer of pollens and grains by insects, animals, birds, wind or water.

Natural Selection: A brilliant naturalist Charles Darwin gave the theory of natural selection which means all species have evolved from a single ancestor over time. Some are able to survive, some are not. According to Darwin nature is soul responsible for this selection. The organisms with characters that are helpful for successful reproduction are selected by the nature and they are capable of exceeding their generations whereas the organisms which have unfavorable characters will not be selected by nature and cannot reproduce. Gradually they get extinct from the nature. 

Determination of human blood group:

Human blood group is controlled by alleles. It is determined by the presence or absence of specific antigens and antibodies in the blood. There are four types of blood groups – A, B, AB and O containing either of the two antigens (A and B) and two antibodies (a and b). Given chart may express the presents and absence of antigens and antibodies more clearly.

Blood groups










No antigens

Both a and b


Both A and B

No antibodies 



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 Natural Selection                                      Darwin and wallace proposed that natural selection is the mechanism by which new species arise from pre existing ones. The theory of Natural selection states that there is "struggle for existence" within a population and variation exists within all population. The continuous competition between individuals for environmental resources creates a 'struggle for existence' and this struggle makes sure that certain organisms would fail to survive or reproduce. This would eliminate less suited organisms and better adapted organisms would survive and pass on their traits to next generation leading to evolution.


Genetic drift refers to the fact that variations in gene frequencies within populations can occur by chance rather than by natural selection. Random genetic drift may be an important mechanism in evolutionary change in small or isolated populations. In a small population not all the alleles which are representative of that species may be present. Chance events such as the premature accidental death prior to mating of an organism which is the sole possessor of a particular allele from the population. For example, if an allele has a frequency of 1% in a population of 1 000 000 then 10 000 individuals will possess that allele. In a population of 100 only one individual would possess that allele so the probability of loosing that allele by chance is much greater. 

Just as it is possible for an allele to disappear from a population, it is equally possible for it to drift to a higher frequency simply by chance. Random genetic drift, as its name implies is unpredictable. In a small population it can lead to the extinction of the population or result in the population becoming even better adapted to the environment or more widely divergent from the parental. In due course, this may lead to the origin of a new species by natural selection. Genetic drift is thought to have been a significant factor in the origin of new species on islands and in other reproductively isolated populations.

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 Gene flow refers to the movement of alleles from one population to another as a result of interbreeding between members of two populations. The random introduction of new alleles into the recipient population and their removal from the donor population affects the allele frequency of both population and leads to increased genetic variation. Gene flow ensures that all population of a given specie share the same gene pool, i.e. it reduces variation between the two population. Geographical proximity increases gene flow and people interbreeding freely have less differences. Gene flow must be interrupted for the origin of new species.

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