Revision: Class 12 >> Evolution NEET (UG) Evolution

Biogenesis is the idea that life arises only from pre-existing life.

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.

• 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.

• Evolution is an orderly change from one life form to another, explaining changes in flora and fauna over millions of years.
• Evolutionary Biology is the study of the history of life on Earth, including the origins of life, Earth and the universe.
• Understanding evolution requires knowledge of the origin of life, biodiversity, Earth and the universe.

• The origin of life is a unique event; Earth formed about 4.5 billion years ago, and life appeared about 4 billion years ago.
• Special Creation Theory: all organisms were created by a supernatural power; religion-based, no scientific proof.
• Cosmozoic Theory (Panspermia): life came from other planets as spores; not accepted, as it doesn't explain life's origin elsewhere.
• Spontaneous Generation (Abiogenesis): life arose from non-living matter; later disproved.
• Biogenesis: life arises only from pre-existing life; explains continuity but not the first origin.
• Redi (1668): maggots appeared only in the uncovered meat jar, showing they came from fly eggs.
• Spallanzani (1767): sealed boiled broth showed no growth, proving microbes come from air.
• Pasteur (1861): swan-neck flask trapped airborne microbes, conclusively disproving spontaneous generation.

• Abiogenesis (Spontaneous generation) → life arose from non-living matter; disproved by Louis Pasteur.
• Biogenesis → life arises only from pre-existing life (accepted theory).
• Panspermia (Cosmic theory) → life came from outer space in the form of spores.
• Special creation theory → life created by supernatural power (God).
• Chemical evolution theory (Oparin & Haldane) → life originated from simple inorganic and organic molecules.
• The first life forms were non-cellular (like RNA, proteins), and later evolved into single-celled organisms.
• Life originated in water and gradually evolved into complex organisms over time.

• 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 is the evolution of different species from a common ancestor, radiating into different habitats and ecological niches in a given geographical area.
• Mechanism: common ancestor → spreads into new habitats → different environmental pressures → natural selection favours useful variations → distinct species form.
• Darwin's finches (Galapagos): an ancestral seed-eating finch evolved into many species with different beak shapes for different food sources.
• Australian marsupials: many marsupial species evolved from a common ancestor and adapted to different ecological roles.
• Convergent evolution: unrelated placental mammals and marsupials (e.g., the placental wolf and the Tasmanian wolf) evolved similar forms under similar environments.

• Convergent evolution occurs when unrelated organisms in isolated geographical areas develop similar traits due to similar environmental pressures.
• It results from more than one adaptive radiation occurring in isolated areas, leading to resemblance between unrelated species.
• Example - Australian Marsupials and Placental mammals evolved similar forms independently, e.g., Marsupial mole ↔ Mole, Tasmanian wolf ↔ Wolf.
• Similar environments drive similar adaptations in completely unrelated organisms living in different geographical areas.

• Organic evolution is the process by which simple organisms gradually gave rise to complex organisms over time.
• Lamarckism: evolution by use and disuse of organs and inheritance of acquired characters (e.g., a giraffe's long neck); not accepted today.
• Darwinism: based on branching descent and natural selection - organisms with useful heritable variations survive, reproduce more, and form new species over generations.
• Microbes vs higher organisms: new forms appear in microbes within days, but in higher organisms, it takes millions of years.
• Hugo de Vries's Mutation Theory: evolution occurs by sudden, random mutations (saltation), not by gradual variations as Darwin proposed.

• Organic evolution is the process by which simple organisms gradually gave rise to complex organisms over time.
• Lamarckism: evolution by use and disuse of organs and inheritance of acquired characters (e.g., a giraffe's long neck); not accepted today.
• Darwinism: based on branching descent and natural selection - organisms with useful heritable variations survive, reproduce more, and form new species over generations.
• Microbes vs higher organisms: new forms appear in microbes within days, but in higher organisms, it takes millions of years.
• Hugo de Vries's Mutation Theory: evolution occurs by sudden, random mutations (saltation), not by gradual variations as Darwin proposed.

1. Mutations & Speciation - Mutations in microbes lead to new phenotypes; over several generations, this can result in speciation (formation of new species).

2. Natural Selection - Favours heritable variations that improve survival and reproductive success in a population.

3. Causes of Variation - Mutation, recombination, gene flow, and genetic drift alter gene/allele frequencies in future generations.

4. Types of Natural Selection - Three types:

• Stabilising - Favours mean/average individuals (peak gets higher & narrower).
• Directional - Favours one extreme value (peak shifts one side).
• Disruptive - Favours both extremes (two peaks form).

5. Outcome of Selection - Natural selection causes populations to appear different over time due to the enhanced reproductive success of selected individuals.

• First Life Forms - 2000 mya; first cells appeared, some released O₂; single-celled → multicellular.
• Aquatic to Land - 500 mya invertebrates; 350 mya jawless fish; strong-finned fish moved onto land.
• Amphibians → Reptiles - Reptiles laid thick-shelled eggs; dominated Earth for 200 million years; giant ferns formed coal.
• Mesozoic Era - 200 mya some reptiles returned to water (e.g., Ichthyosaurs); T. rex dominated land (20 feet tall).
• Dinosaur Extinction - 65 mya, dinosaurs suddenly disappeared due to climatic changes or evolved into birds.
• Rise of Mammals - Shrew-like, viviparous, intelligent; replaced reptiles as dominant life forms.
• Continental Drift - South American mammals were overridden by North American fauna; Australian marsupials survived due to no competition.

• Origin: human evolution began in the Palaeocene epoch from a tree-dwelling, shrew-like ancestor.
• As forests declined, arboreal mammals were forced to adapt to life on land, an important driving force in human evolution.
• Humans are most closely related to gibbons, chimpanzees, and gorillas.
• Major changes: increase in brain size and cranial capacity, bipedal locomotion, opposable thumb, erect posture, and development of chin and forehead.
• Functional significance: free forelimbs and opposable thumbs aided tool use, while bipedal locomotion and stereoscopic vision improved movement and survival.
• Early ancestors: Dryopithecus (ape-like) and Ramapithecus (man-like) lived ~15 mya.
• Australopithecines (~2 mya) lived in East African grasslands, hunted with stone tools, but ate mainly fruits.
• Homo habilis - first hominid, brain ~650–800 cc, with little or no meat in diet; Homo erectus (~1.5 mya, Java, brain ~900 cc) probably ate meat.
• Neanderthal man (~1400 cc, 100,000–40,000 years ago) used hides for protection and buried the dead.
• Modern humans: Homo sapiens arose in Africa during the ice age, spread worldwide; cave art developed ~18,000 years ago (Bhimbetka, M.P.), and agriculture and settlements began ~10,000 years ago.