Biology: Class 12 — Chapter 6: Evolution

• The Big Bang Theory attempts to explain the origin of the universe as a singular, unimaginable physical explosion.
• As the universe expanded, the temperature came down, allowing hydrogen and helium gases to form and condense under gravitation, forming galaxies.
• Earth was formed about 4.5 billion years ago in the solar system of the Milky Way galaxy.
• Early Earth Conditions: There was no atmosphere. The surface was covered by water vapour, methane ($CH_4$), carbon dioxide ($CO_2$), and ammonia ($NH_3$) released from molten mass.
• UV rays from the sun broke up water into hydrogen and oxygen; lighter $H_2$ escaped. Oxygen combined with ammonia and methane to form water, $CO_2$, and other compounds. The ozone layer was formed.
• As the earth cooled, water vapour fell as rain to fill depressions and form oceans. Life appeared 500 million years after Earth's formation (almost 4 billion years ago).

• Panspermia: Early Greek thinkers believed units of life called spores were transferred to different planets, including Earth.
• Spontaneous Generation: The belief that life arose from decaying and rotting matter (like straw, mud, etc.). This was dismissed by Louis Pasteur.
• Pasteur's Experiment: Demonstrated that life comes only from pre-existing life using pre-sterilised flasks. Yeast in a closed flask did not produce life, whereas a flask open to air allowed new living organisms to arise from 'killed yeast'.
• Chemical Evolution: Proposed by Oparin of Russia and Haldane of England. They suggested that the first form of life arose from pre-existing non-living organic molecules (RNA, proteins, etc.) and that chemical evolution preceded organic evolution.

Aim: To recreate and test the conditions of early Earth in a laboratory scale to prove chemical evolution.

Apparatus/Setup: In 1953, S.L. Miller created an electric discharge in a closed flask containing $CH_4$, $H_2$, $NH_3$, and water vapour at 800°C.

Observation: He observed the formation of amino acids. Similar experiments by others yielded sugars, nitrogen bases, pigments, and fats.

Inference: Chemical evolution—the formation of organic molecules from inorganic constituents—was accepted as a plausible pathway for the origin of life.

• Theory of Special Creation: Conventional religious literature asserts that: (1) all living organisms were created as they are seen today, (2) diversity was always the same and will remain so, and (3) Earth is about 4000 years old.
• Darwinian Theory: Based on observations during a sea voyage on H.M.S. Beagle, Charles Darwin concluded that existing life forms share similarities to varying degrees and share common ancestors that existed millions of years ago.
• Natural Selection: Individuals with characteristics that enable them to survive better in natural conditions outbreed others. Darwin defined this fitness ultimately as reproductive fitness.
• Alfred Wallace: A naturalist working in the Malay Archipelago who independently arrived at similar conclusions regarding natural selection around the same time.

• Fossils are the remains of hard parts of life-forms found in sedimentary rock layers.
• Different-aged rock sediments contain fossils of different life-forms, showing that life-forms varied over time and certain organisms were restricted to specific geological time spans.

• Proposed by Ernst Heckel based on embryonic features common to all vertebrates (e.g., vestigial gill slits in human embryos).
• Disproved by Karl Ernst von Baer, who noted that embryos never pass through the adult stages of other animals.

• Homology: Similarity in anatomy/structure due to common ancestry, though they perform different functions. This represents divergent evolution.
  • Examples: Forelimbs of whales, bats, Cheetah, and humans (all share humerus, radius, ulna, carpals, metacarpals, and phalanges); thorns of Bougainvillea and tendrils of Cucurbita.
• Analogy: Anatomically different structures performing similar functions due to adaptation to a similar habitat. This represents convergent evolution.
  • Examples: Wings of butterflies and birds; eye of octopus and mammals; flippers of penguins and dolphins; sweet potato (root modification) and potato (stem modification).

• Biochemical Evidence: Similarities in proteins and genes performing similar functions across diverse organisms point to a shared ancestry.
• Anthropogenic Action (Industrial Melanism): In England, before industrialisation (1850s), white-winged moths outnumbered dark-winged moths on lichen-covered trees. Post-industrialisation (1920), soot covered tree trunks, and dark-winged (melanised) moths survived better due to camouflage against predators.
• Pesticide/Antibiotic Resistance: Excess use of herbicides, pesticides, or antibiotics has selected for resistant varieties of microbes and pests in a very short time scale, demonstrating evolution by human action.

• Darwin's Finches: On the Galapagos Islands, Darwin observed many varieties of finches that evolved from an original seed-eating ancestor into insectivorous and vegetarian forms with altered beaks.
• Australian Marsupials: A number of distinct marsupials evolved from an ancestral stock within the isolated Australian continent.
• Placental Mammals in Australia: Exhibit adaptive radiation matching corresponding marsupials (e.g., Placental wolf and Tasmanian wolf), which is an example of convergent evolution when multiple adaptive radiations occur in an isolated geographical area.

• The two key concepts of Darwinian Theory of Evolution are branching descent and natural selection.
• Lamarckism: French naturalist Lamarck argued that evolution was driven by the "use and disuse of organs" (e.g., giraffes elongating their necks to forage leaves on tall trees). This conjecture is no longer accepted.

• In the early 20th century, Hugo de Vries proposed the Mutation Theory based on his work on evening primrose.
• He believed that mutations—large, sudden differences arising in a population—cause evolution, rather than the minor, gradual variations described by Darwin.
• Key Differences:
  • Darwinian variations are small, gradual, and directional.
  • Mutations are random, directionless, and can cause speciation in a single step, termed saltation.

The sum total of all allelic frequencies is 1. For a diploid locus with alleles $A$ and $a$ having frequencies $p$ and $q$ respectively:

$$p^2 + 2pq + q^2 = 1$$

• $p^2$: Frequency of homozygous dominant individuals ($AA$)
• $2pq$: Frequency of heterozygous individuals ($Aa$)
• $q^2$: Frequency of homozygous recessive individuals ($aa$)

Any deviation from this equilibrium indicates that evolutionary change is occurring.

1. Gene Migration / Gene Flow: Transfer of alleles into or out of a population due to migration.
2. Genetic Drift: Random change in allele frequencies occurring by chance. If a small group drifts to form a new population, they become the founders, causing the founder effect.
3. Mutation: Introduction of new alleles due to random genomic changes.
4. Genetic Recombination: New combinations of alleles formed during gametogenesis.
5. Natural Selection: Leads to three possible outcomes on traits:
   • Stabilising: More individuals acquire mean character value (peak gets higher and narrower).
   • Directional: More individuals acquire value other than the mean character value (peak shifts in one direction).
   • Disruptive: Individuals at both peripheral extremes are favoured (two peaks form).

• 2000 mya (million years ago): First cellular forms of life appeared on Earth. Some developed the ability to release $O_2$.
• 500 mya: Invertebrates were formed and active.
• 350 mya: Jawless fish evolved; fish with stout and strong fins (lobefins) could move on land and go back to water. In 1938, a lobefin fish called Coelacanth was caught in South Africa, proving they were ancestors of modern frogs and salamanders.
• 320 mya: Sea weeds and few plants existed. Plants were the first organisms to invade land.
• Reptiles: Amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up in the sun.
• 200 mya: Reptiles dominated Earth. Some went back to water to evolve into fish-like reptiles (e.g., Ichthyosaurs). Giant land reptiles were the dinosaurs; the largest was Tyrannosaurus rex (about 20 feet high with dagger-like teeth).
• 65 mya: Dinosaurs suddenly disappeared from the Earth, possibly due to climatic changes or avian evolution.
• Mammals: The first mammals were small, shrew-like, viviparous, and highly intelligent. When reptiles declined, mammals took over the Earth.

• Dryopithecus & Ramapithecus (15 mya): Hairy, walked like gorillas and chimpanzees. Ramapithecus was more man-like, while Dryopithecus was more ape-like.
• Australopithecines (3-4 mya): Walked upright in East African grasslands, hunted with stone weapons, and essentially ate fruit. Height was under 4 feet.
• Homo habilis (2 mya): The first human-like hominid. Brain capacity was 650-800cc. They did not eat meat.
• Homo erectus (1.5 mya): Fossils discovered in Java (1891). Had a large brain capacity of around 900cc and probably ate meat.
• Neanderthal Man (100,000 - 40,000 years ago): Lived in near East and Central Asia. Brain size was 1400cc. Used hides to protect their bodies and buried their dead.
• Homo sapiens (75,000 - 10,000 years ago): Arose in Africa during the last ice age and moved across continents, developing into distinct races. Pre-historic cave art developed about 18,000 years ago (e.g., Bhimbetka, MP). Agriculture and human settlements started around 10,000 years ago.