CBSE Class 10 Science 2017 · Paper 31/1/C · Question 23

Evolution is the gradual change in the inherited characteristics of populations of organisms over many generations (over millions of years), through which new species arise from pre-existing ones. How it occurs: (1) Variation: during reproduction, DNA copying is not perfectly accurate and sexual reproduction combines the DNA of two parents, so offspring differ slightly; many of these variations are inherited. (2) Natural selection: variations that help organisms survive and reproduce in their environment are passed on to more offspring and become common over generations, as green beetles hidden from crows became common in NCERT's example. (3) Genetic drift: in small populations, some variations become common or are lost purely by chance. (4) Over many generations these changes accumulate; when a population is split, for example by geographical isolation, the separated groups accumulate different variations until they can no longer interbreed, and a new species forms. Changes acquired during an individual's life are not inherited and do not contribute. Fossils as evidence: fossils are the preserved remains or traces of organisms that lived long ago. (1) They show that many organisms of the past were different from those living now, and that many have become extinct. (2) Their age can be found, relatively because in undisturbed rock layers a deeper fossil is older, and absolutely by radioactive dating of the fossil material; arranged by age, the oldest layers contain only simple organisms and more complex ones appear in later layers, showing that life changed gradually. (3) Some fossils show features of two groups: Archaeopteryx had feathers and wings like a bird but teeth, clawed fingers and a long bony tail like a reptile, which suggests that birds evolved from reptiles.