According to Hardy- Weinberg principle, gene frequency remains constant from generation to generation and is stable, this is called genetic equilibrium. Gene flow is one of the factors affecting Hardy- Weinberg principle. The transfer of section of population to another place resulting in a change in gene frequencies in both old and new population is called gene flow. New genes and alleles are added to new population which are genetically different but can interbreed.

The organic remnants of the past life forms preserved on the sedimentary rocks in the earth is called fossils. They are the best and direct evidences of organic evolution. The age of fossils is calculated by radio-carbon dating and many other methods, which tells us about the era when the life form were alive. Fossils are of many types, like: Actual remains like the cryopreserved Woolly mammoth (of Siberia) or insects preserved in amber. Petrified plants and animals. Skeletal elements like bones, teeth, etc. of an animal. Moulds are the hardened encasements formed in the outer parts of organic remains which later decayed leaving cavities. Casts are hardened pieces of mineral matter deposited in the cavities of moulds.

According to Hardy- Weinberg principle, gene frequency remains constant from generation to generation and is stable, this is called genetic equilibrium. Gene flow is one of the factors affecting Hardy- Weinberg principle. The transfer of section of population to another place resulting in a change in gene frequencies in both old and new population is called gene flow. New genes and alleles are added to new population which are genetically different but can interbreed.