Revision: Genetics and Evolution >> Evolution Biology Science (English Medium) Class 12 CBSE

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. 

Bipedalism is a major type of locomotion that involves movement on two feet instead of four.

Evolution occurring due to sudden, large mutations leading to the formation of new species in a single step is called saltation.

• Hardy–Weinberg’s principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.
• The total collection of all alleles in a population is called the gene pool.
• Genetic equilibrium means no change in allele frequencies over time.
• If p is the frequency of dominant allele and q is the frequency of recessive allele, then
  p + q = 1.
• Genotype frequencies are expressed as:
  p² (AA) + 2pq (Aa) + q² (aa) = 1.
• Any deviation from Hardy–Weinberg equilibrium indicates that evolution is occurring.
• The principle helps in detecting the role of natural selection and other evolutionary forces.

1. The theory of special creation claimed that all life forms were created as they are, diversity remained unchanged, and Earth was only about 4000 years old; these ideas were challenged in the 19th century.
2. Charles Darwin, based on observations from the voyage of H.M.S. Beagle, proposed that life forms have evolved gradually, many became extinct, and all organisms share common ancestors.
3. Darwin proposed natural selection as the mechanism of evolution, where organisms with better reproductive fitness survive and produce more offspring, leading to evolution over long geological time.

• Human evolution began from simple unicellular organisms, eventually giving rise to ape-like animals around 15–20 million years ago, during the late Miocene period.
• Carl Linnaeus classified modern man as Homo sapiens, under the order Primates, which includes monkeys and apes.
• Evolutionary changes such as bipedal locomotion, erect posture, increased brain size, reduced body hair, and the development of chin and forehead marked the shift from primitive apes to early humans.
• Fossil records—especially from Asia and Africa—reveal how early ape-like ancestors gradually evolved into gibbons, orangutans, gorillas, and eventually humans.
• Ramapithecus, an early ancestor known from jaw and tooth fossils, is considered a prehuman form that walked upright, initiating the journey toward modern humans.

• The universe originated about 13.8 billion years ago by the Big Bang, and Earth was formed about 4.5 billion years ago in the Milky Way galaxy.
• Early Earth had a reducing atmosphere with gases like methane, ammonia, hydrogen and water vapour; oceans formed as the Earth cooled.
• Life appeared about 4 billion years ago, initially in water, after the formation of oceans and the ozone layer.
• The theory of spontaneous generation was disproved by Louis Pasteur, who showed that life arises only from pre-existing life.
• Oparin–Haldane theory proposed chemical evolution of life; Miller’s experiment (1953) proved that organic molecules like amino acids could form from inorganic substances under early Earth conditions.