NCERT Class 10 The most important Basic concepts of chapter 9 (part-2)

The most Important points of Heredity and Evolution (part-2)

10. Variations occur from one generation to the other because of errors in DNA copying and because of sexual reproduction. These variations determine the survival of a species in the prevailing natural conditions. A species which has such variations as are favourable to it in the present natural conditions would survive while those with unfavorable variations would perish. Thus, ultimately only the species with favourable variations would be present. This is know as natural selection. A species which can adapt to the changing environment would survive. When, over several generations, a population of organisms becomes so different from its ancestors (due to accumulating variations) that they (the ancestors and present progeny) are not able to reproduce with each other, the population would constitute a new species. This phenomenon and the process through which new species are formed is known as evolution. Evolution is the generation of diversity and acting upon this diversity by natural selection.

11. Acquired and inherited traits: The traits which are acquired by an organism during its lifetime are acquired traits. These traits are not inherited as these are changes in the non-reproductive tissues of the organisms. Only the variations in the reproductive cells can be passed on to the next generation.

12. Origin of life on earth: Life originated on earth from inorganic elements and compounds such as Nitrogen, Carbon, Hydrogen, Methane, etc., Which are the constituent elements of proteins, Carbohydrates, etc. Under conditions of high temperature in the primitive earth environment, these inorganic elements combined to form various complex compounds such as the amino acids which are constituents of proteins.

13. Speciation: Geographical isolation of two populations of same species over several generations may result in genetic drift and the populations which once had similar genetic make up would become different. Further natural selection acts on these isolated populations differently. If these differences accumulated over generations become so prominent that the two populations cannot reproduce among each other (reproductive isolation), a new species is said to be formed.

14. The more characteristics two species have in common, The more closely they are related. Classification of species is a reflection of their evolutionary relationship.

15. Homologous organs: These are the organs which perform different functions but have similar structure and origin. For example, a wing of a bat, flipper of a seal, front leg of a horse and arm of a man perform different functions but have the same structural plan and origin. This makes it evident that these animals have a common ancestor.

16. Analogous organs: These are the one which have similar appearance and functions but have different structure and origin. For example, wings of a butterfly, of a bat and of a bird perform similar function of flying but are very different structurally.

17. Fossils: The preserved traces of living organisms are called fossils. The age of the fossil can be determined by detecting the ratios of different isotopes of the same element in the fossil material.