Evolution Class 12 Notes Biology Chapter 7 - CBSE


What are Evolution?

Evolutionary biology is the study of history of life forms on Earth, and the evolution of life on Earth.

Year Scientist Theory
/ Experiment
1927 Lemaitre Big Bang theory The universe expanded from explosion of a primordial, hot substance.
1924 - 1929 Oparin and Haldane Biochemical theory for the origin of life Simple organic molecules originated from inorganic precursors.
1952 Stanley Miller and Urey Synthesis of biomolecules by creation of similar conditions as primitive atmosphere on laboratory scale Amino acids were synthesised from ammonia, oxygen, and carbon dioxide inside specialised apparatus.

Theories Of Evolution

  • Darwin made observations on his sea-trip around the world aboard H.M.S. Beagle and concluded that all existing living forms share similarities among themselves and also with other life forms, which existed millions of years ago.
  • The evolution of life forms is a slow process and those life forms better fit in environments that leave more progenies i.e. reproductively fit. This is called natural selection.

Evidences Of Evolution

  • Fossils: They are the remains of plants and animals that lived millions of years ago and are now extinct.
  • Comparative anatomy and morphology:
  • (a) Homologous organs: All mammals share the same internal design of forelimbs but they perform different functions. This is called divergent evolution and the structures are called homologous structures (common ancestors).
  • (b) Analogous organs: The pair of organs is not anatomically similar, but performs the same function (e.g., the wings of butterflies and birds). This is called convergent evolution.
  • Adaptive melanism: In England, it was noted that before industrial revolution, the number of white-winged moths was more than that of dark melanised moth. After industrialisation, melanised moths increased in number. Probable reason was that after industrialization, the tree trunks became darker with deposits of soot and smoke and hence, the number of dark moths increased so that they can camouflage and protect themselves from predators.

Adaptive Radiation

  • Darwin during his voyage on Galapagos Island noticed that there were many varieties of finches in the same island and they were different from normal seed eating varieties to those that ate insects.
  • This process of evolution starting from a single point and radiating in different directions is called adaptive radiation.
  • The other example for this is the evolution of the Australian marsupials from a single ancestor. Placental mammals also exhibit similarities to their corresponding marsupial. Example: placental wolf and the Tasmanian wolf.

Biological Evolution & Mechanism Of Evolution

  • Lamarck observed that evolution occurs due to the use or disuse of particular organs or body parts. For example, giraffe have developed long necks as a result of attempts to eat high up on trees.
  • Darwin also observed that variations are inheritable and the species fit to survive the most, produces more offsprings.
  • According to Darwin, evolution took place by natural selection and the number of life forms depends upon their ability to reproduce and their life span.
  • Another aspect of natural selection is the survival of the fittest, where nature selects the individuals, which are reproductively more fit.
  • Branching descent and natural selection are the two important concepts of Darwin’s theory of evolution.

Mechanism Of Evolution

  • Darwin did not quite explain how evolution gave rise to different species of the same organism.
  • Mendel mentioned about inheritable factors, which influenced the phenotype of an organism.
  • Hugo de Vries based on his work on evening primrose suggested that variations occurred due to mutations.
  • Mutations are sudden, random and directionless while Darwin mentioned that variations are small and directional.

Hardy-weinberg Principle

  • The frequency of occurrence of alleles of a gene in a population remains constant through generations if disturbances such as mutations, non-random mating, etc. are not introduced.
  • Sum total of all allelic frequencies is 1.
  • Individual frequencies are represented as p and q such as in a diploid, where p and q represent the frequency of allele A and a.
  • The frequency of AA is p2, that of aa is q2, and that of Aa is 2pq.
  • Hardy-Weinberg equilibrium is affected by gene flow or gene migration, genetic drift (changes occurring by chance), mutation, genetic recombination, natural selection.

Origin And Evolution Of Man

Year Evolution Characteristics
15 million years ago Dryopithecus (ape-like) and Ramapithecus (man-like) Hairy and walked similar to chimpanzees
3 − 4 million years ago Man-like primates Not tall, but walked straight
2 million years ago Australopithecines, also called Homo habilis, lived in East Africa Used stone weapons and ate fruits; human-like
with brain capacity of 420-500 cc; not meat eaters
1.5 million years ago Homo erectus Brain capacity of about 900 cc; were meat eaters
1,000 − 40, 000 years ago Neanderthal man Brain capacity of 1400 cc; used hides
75, 000 − 10, 000 years ago Homo sapiens Brain capacity of 1350 CC.