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

Abiogenesis is the concept that living organisms can arise naturally from non-living substances.

Autogenesis is another term for spontaneous generation, meaning self-origin of life from non-living matter.

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.

• Protobiogenesis - Origin of life on Earth; still unsolved despite advances in science.
• Special Creation - God/supernatural power created all life; no scientific proof.
• Panspermia (Cosmozoic) - Life came to Earth from other planets as spores; we can't explain the origin on those planets.
• Spontaneous Generation (Abiogenesis) - Life arose from non-living matter; disproved by Louis Pasteur.
• Biogenesis - Life always comes from pre-existing life (reproduction); can't explain the very first life on Earth.

• Purpose - Stanley Miller and Harold Urey provided the first experimental evidence in support of Oparin's chemical evolution theory.
• Apparatus - A sterilised and evacuated spark-discharge glass apparatus was used; CH₄, NH₃, and H₂ gases were pumped in the ratio 1:2:2 along with water vapour.
• Lightning Effect - Electric discharge carbon arc spark was used to mimic lightning; heating mantle and condenser simulated evaporation and precipitation.
• Result - After several days of continuous electric discharge, the gases interacted and condensed; the collected liquid turned brown and contained simple organic compounds like urea, amino acids, and lactic acid.
• Significance - The experiment strongly proved that simple molecules present in Earth's early atmosphere can combine to form organic building blocks of life.

• Adaptive Radiation - Process of evolution of different species starting from one point and radiating to other habitats in a given geographical area.
• Darwin's Finches - Best example of adaptive radiation; small birds observed by Darwin in the Galapagos Islands; originally migrated from the American mainland.
• How it Occurred - Original seed-eating birds adapted to different environmental conditions of different islands; evolved into many forms with altered beaks (e.g. insectivorous features).
• Australian Marsupials - Another example: many marsupial mammals in Australia evolved from a common ancestor through adaptive radiation.
• Significance - Provides strong evidence of evolution; shows how natural selection drives species diversification and the creation of new species from a single ancestor.

• 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.