EVOLUTION

Origin of Life

• Evolutionary Biology is the study of the history of life forms on Earth.
• The universe is approximately 13.8 billion years old, comprised of huge clusters of galaxies, stars, and clouds of gas and dust. The Earth is a tiny speck within this vast universe.
• The Big Bang theory explains the origin of the universe as a singular, huge explosion followed by expansion and cooling, leading to the formation of Hydrogen and Helium, which then condensed into galaxies.
• Earth’s Formation: Approximately 4.5 billion years ago, within the solar system of the Milky Way galaxy.
  • Early Earth had no atmosphere.
  • Water vapour, methane, carbon dioxide, and ammonia were released from molten mass.
  • UV rays from the sun broke water into Hydrogen (which escaped) and Oxygen.
  • Oxygen combined with ammonia and methane to form water, CO2, and other compounds.
  • The ozone layer was formed.
  • As it cooled, water vapour fell as rain, filling depressions to form oceans.
• Appearance of Life: Life appeared about 500 million years after Earth’s formation, roughly 4 billion years ago.

Theories on the Origin of Life

• Panspermia: Early Greek thinkers believed units of life, called spores, were transferred to different planets, including Earth. This idea is still favored by some astronomers.
• Theory of Spontaneous Generation: Believed life arose from decaying and rotting matter (e.g., straw, mud).
  • Louis Pasteur disproved this theory through careful experimentation, showing that life only comes from pre-existing life. In sterilised flasks, no life arose from killed yeast, unlike in flasks open to air.
• Oparin and Haldane’s Hypothesis (Chemical Evolution):
  • Proposed that the first life form originated from pre-existing non-living organic molecules (like RNA, protein).
  • Life’s formation was preceded by chemical evolution, the formation of diverse organic molecules from inorganic constituents.
  • Early Earth conditions: High temperature, volcanic storms, and a reducing atmosphere containing CH4 and NH3.
  • S.L. Miller’s Experiment (1953): Replicated early Earth conditions in a laboratory, creating electric discharge in a closed flask with CH4, H2, NH3, and water vapour at 800°C.
    • He observed the formation of amino acids.
    • Similar experiments by others led to the formation of sugars, nitrogen bases, pigments, and fats.
    • Analysis of meteorite content also revealed similar compounds, suggesting similar processes elsewhere in space.
  • This evidence led to the acceptance of chemical evolution as the first part of the conjectured story of life’s origin.

Early Forms of Life

• The first non-cellular forms of life may have originated 3 billion years ago. They were likely giant molecules (RNA, Protein, Polysaccharides) that reproduced their molecules.
• The first cellular forms of life probably originated around 2000 million years ago, likely as single-cells in a water environment.
• The concept of abiogenesis, where the first life form slowly arose through evolutionary forces from non-living molecules, is widely accepted.

Evolution of Life Forms - A Theory

Theory of Special Creation

Conventional religious literature proposes:

• All living organisms (species) were created as they are today.
• Diversity has always been and will remain the same.
• The Earth is about 4000 years old.
• These ideas were strongly challenged in the nineteenth century.

Charles Darwin’s Theory of Natural Selection

• Based on observations during his sea voyage on the H.M.S. Beagle.
• Key Conclusions:
  • Existing life forms share varying degrees of similarities with each other and with life forms from millions of years ago.
  • Many past life forms are now extinct; new forms have arisen at different times.
  • There has been a gradual evolution of life forms.
• Natural Selection (Mechanism of Evolution):
  • Every population has built-in variations in characteristics.
  • Individuals with traits that enable them to survive better in natural conditions (climate, food, physical factors) will outbreed others.
  • Fitness refers ultimately to reproductive fitness.
  • Those better fit in an environment leave more progeny, survive more, and are thus selected by nature.
  • Alfred Wallace independently reached similar conclusions around the same time.
• Common Ancestry: All existing life forms share common ancestors, which were present at different geological periods.
• Earth’s Age: The geological history closely correlates with biological history, concluding that Earth is billions of years old, not thousands.

What are the Evidences for Evolution?

1. Paleontological Evidence (Fossils)

• Fossils are the remains of hard parts of life-forms found in rocks.
• Rocks form sediments, and different-aged sedimentary layers contain fossils of different life-forms that died during the sediment’s formation.
• Some fossils resemble modern organisms, while others represent extinct organisms (e.g., Dinosaurs).
• Studying fossils in sedimentary layers shows that life-forms varied over time, and certain forms were restricted to specific geological time-spans, indicating new life forms arose at different times.
• Fossil ages can be calculated using radioactive-dating.

2. Embryological Support (Disproved)

• Ernst Heckel proposed embryological support based on common embryonic features in vertebrates (e.g., vestigial gill slits behind the head in all vertebrate embryos, functional only in adult fish).
• However, Karl Ernst von Baer disproved this, noting that embryos never pass through the adult stages of other animals.

3. Comparative Anatomy and Morphology

• Compares similarities and differences among living organisms and extinct ones to understand shared ancestry.

Homologous Organs (Divergent Evolution)

• Definition: Structures with similar anatomical structure but performing different functions due to adaptations to different needs.
• Indicates Common Ancestry.
• Examples:
  • Forelimbs of whales, bats, cheetah, and humans (all mammals) have similar bone patterns: humerus, radius, ulna, carpals, metacarpals, and phalanges.
  • Vertebrate hearts or brains.
  • Thorn of Bougainvillea and tendril of Cucurbita (plants).

Analogous Organs (Convergent Evolution)

• Definition: Structures that look alike and perform similar functions but are not anatomically similar.
• Result from similar habitats leading to the selection of similar adaptive features in different groups of organisms towards the same function.
• Examples:
  • Wings of a butterfly and a bird.
  • Eye of an octopus and a mammal.
  • Flippers of Penguins and Dolphins.
  • Sweet potato (root modification) and potato (stem modification).

4. Biochemical Similarities

• Similarities in proteins and genes performing a given function among diverse organisms provide clues to common ancestry, paralleling structural similarities.

5. Artificial Selection (Anthropogenic Action)

• Humans have selectively bred plants and animals for agriculture, horticulture, sport, or security, creating diverse breeds (e.g., dogs) from the same group.
• This demonstrates that if humans can create new breeds in hundreds of years, nature could achieve similar changes over millions of years.

6. Industrial Melanism (Natural Selection in Moths)

• Observation in England (Biston betularia moths):
  • 1850s (before industrialization): More white-winged moths were observed on trees than dark-winged (melanised) moths.
    • Trees were covered with white-coloured lichens, providing camouflage for white moths. Dark moths were spotted by predators.
  • 1920 (after industrialization): The proportion reversed; more dark-winged moths were found in the same area.
    • Tree trunks became dark due to industrial smoke and soot, camouflaging dark moths, while white moths were easily spotted by predators.
• Explanation: Predators spot moths against contrasting backgrounds. Lichens indicate unpolluted areas. Moths that could camouflage themselves survived.
• In rural, unpolluted areas, the count of melanic moths remained low.
• This illustrates that in a mixed population, those better-adapted survive and increase their population size, though no variant is completely wiped out.

7. Resistance to Herbicides, Pesticides, and Antibiotics

• Excessive use of herbicides, pesticides, antibiotics, or drugs leads to the selection of resistant varieties/organisms/cells in a much shorter time scale (months or years).
• These are examples of evolution by anthropogenic action.
• Evolution is a stochastic process based on chance events in nature and chance mutations, not a directed process.

What is Adaptive Radiation?

• Definition: 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).
• Examples:
  • Darwin’s Finches (Galapagos Islands): Darwin observed diverse small black birds. He conjectured that many varieties evolved on the island from an original seed-eating form, with altered beaks enabling them to become insectivorous and vegetarian finches.
  • Australian Marsupials: A number of different marsupials evolved from an ancestral stock, all within the Australian island continent.
• Convergent Evolution: When more than one adaptive radiation occurs in an isolated geographical area (representing different habitats), leading to “similar” forms.
  • Example: Placental mammals in Australia also exhibit adaptive radiation, with each variety appearing similar to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial).

Biological Evolution

• True evolution by natural selection began when cellular forms of life with metabolic differences originated on Earth.
• Essence of Darwinian theory: Natural selection.
• Rate of Evolution: Linked to the life cycle or life span.
  • Microbes (fast-dividing) can evolve new species within days due to selective pressure from changing conditions.
  • For animals like fish or fowl, this takes millions of years.
• Fitness: Refers to an individual’s ability to survive better under new conditions. It is based on inherited characteristics and has a genetic basis.
• Key Concepts of Darwinian Theory: Branching descent and natural selection.
• Lamarck’s Theory: Proposed evolution driven by the use and disuse of organs (e.g., giraffes developing long necks by stretching). This conjecture is no longer believed.
• Evolution as Process or Result: The world we see is a success story of evolution.
  • Describing the story of the world: Evolution is a process.
  • Describing the story of life on Earth: Evolution is a consequence of natural selection.
  • The distinction between evolution and natural selection as processes or end results remains unclear.
• Influences on Darwin: Thomas Malthus’s work on populations.
• Factual Observations Underlying Natural Selection:
  • Natural resources are limited.
  • Populations are generally stable in size (except seasonal fluctuations).
  • Members of a population vary in characteristics (no two individuals are alike), and most variations are inherited.
  • Theoretical exponential population growth versus real-world limited sizes implies competition for resources, meaning only some survive and flourish.
• Darwin’s Insight: Heritable variations that improve resource utilization enable individuals to reproduce and leave more progeny. Over many generations, this leads to changes in population characteristics and the appearance of new forms.

Mechanism of Evolution

• Origin of Variation:
  • Darwin observed inheritable ‘factors’ influencing phenotype but remained silent on their origin.
  • Hugo deVries (early 20th century), based on his work on evening primrose, introduced the idea of mutations – large differences arising suddenly in a population.
  • DeVries believed mutations cause evolution, not minor variations. He described mutations as random and directionless.
  • He called mutation-caused speciation saltation (single step large mutation).
  • In contrast, Darwinian variations were considered small and directional, leading to gradual evolution.
• Studies in population genetics later clarified these concepts.

Hardy-Weinberg Principle

• Core Principle: In a given population, the frequency of alleles of a gene or locus remains fixed and constant from generation to generation.
• Genetic Equilibrium: The gene pool (total genes and their alleles in a population) remains constant. The sum total of all allelic frequencies is 1.
• Algebraic Equation: For a gene with two alleles, A and a, with frequencies p and q respectively in a diploid population:
  • Frequency of AA individuals = p².
  • Frequency of aa individuals = q².
  • Frequency of Aa individuals = 2pq.
  • Therefore, p² + 2pq + q² = 1 (This is the binomial expansion of (p+q)²).
• Evolutionary Change: When measured frequencies differ from expected values, it indicates the extent and direction of evolutionary change. A disturbance in genetic equilibrium (change in allele frequency) is interpreted as evolution.

Factors Affecting Hardy-Weinberg Equilibrium (and causing Evolution)

1. Gene Migration or Gene Flow: Movement of a section of a population to another place changes gene frequencies in both the original and new populations (new genes/alleles are added to the new population and lost from the old).
2. Genetic Drift: A chance change in allele frequency.
   • Founder Effect: When the change in allele frequency is so significant in a new population sample that it becomes a different species; the original drifted population becomes the “founders”.
3. Mutation: Leads to new phenotypes when pre-existing advantageous mutations are selected, resulting in speciation over generations.
4. Genetic Recombination: During gametogenesis, leads to variation.
5. Natural Selection: A process where heritable variations enabling better survival lead to increased reproductive success and more progeny.
   • Variations (from mutation, recombination, gene flow, or genetic drift) change gene/allele frequencies in future generations.
   • Natural selection can lead to different outcomes in population characteristics (Figure 6.8):
     • Stabilisation: More individuals acquire the mean character value.
     • Directional Change: More individuals acquire a value other than the mean character value.
     • Disruption: More individuals acquire peripheral character values at both ends of the distribution curve.

A Brief Account of Evolution

• 2000 mya: First cellular forms of life appeared. The mechanism of evolution from non-cellular aggregates to cells with membranes is unknown.
• Some early cells developed the ability to release O2, possibly through reactions similar to photosynthesis.
• 500 mya: Single-celled organisms evolved into multicellular life forms. Invertebrates were formed and active.
• 350 mya: Jawless fish probably evolved. Fish with stout fins could move on land and return to water.
• 320 mya: Sea weeds and a few plants existed. Plants were the first organisms to invade land and were widespread when animals moved onto land.
• Fish to Amphibians:
  • In 1938, a Coelacanth fish (thought extinct) was caught in South Africa.
  • Lobefins evolved into the first amphibians (lived on both land and water). These were ancestors of modern frogs and salamanders.
• Amphibians to Reptiles:
  • Amphibians evolved into reptiles, which lay thick-shelled eggs that don’t dry up (unlike amphibian eggs).
  • Modern descendants include turtles, tortoises, and crocodiles.
• Dominance of Reptiles: For about 200 million years, reptiles of various shapes and sizes dominated Earth.
  • Giant ferns (pteridophytes) formed coal deposits.
  • Some land reptiles returned to water and evolved into fish-like reptiles (e.g., Ichthyosaurs) around 200 mya.
  • Land reptiles included dinosaurs, such as the Tyrannosaurus rex (20 feet tall, fearsome teeth).
  • 65 mya: Dinosaurs suddenly disappeared from Earth. Possible reasons include climatic changes or evolution into birds. Small reptiles from that era still exist.
• Mammals:
  • The first mammals were small, like shrews.
  • They were viviparous (protected unborn young inside the mother’s body) and more intelligent in sensing and avoiding danger.
  • Mammals took over after reptiles declined.
  • Continental Drift’s Impact:
    • When South America joined North America, native South American mammals (resembling horse, hippopotamus, bear, rabbit) were overridden by North American fauna.
    • Pouched mammals of Australia survived due to isolation and lack of competition.
  • Some mammals adapted to living wholly in water (e.g., whales, dolphins, seals, sea cows).
  • The evolution of man, with language skills and self-consciousness, is considered the most successful story.

Origin and Evolution of Man

• 15 mya: Primates like Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed. They were hairy and walked like gorillas and chimpanzees.
• 3-4 mya: Man-like primates with hominid features walked upright in eastern Africa (fossils found in Ethiopia and Tanzania), probably not taller than 4 feet.
• 2 mya: Australopithecines lived in East African grasslands. They hunted with stone weapons but primarily ate fruit.
• Homo habilis: The first human-like being, with brain capacities between 650-800 cc. They probably did not eat meat.
• Homo erectus (~1.5 mya): Fossils found in Java (1891). Had a larger brain (around 900 cc) and probably ate meat.
• Neanderthal man (~100,000-40,000 years back): Lived in the Near East and Central Asia. Brain size was 1400 cc. They used hides for protection and buried their dead.
• Homo sapiens:
  • Arose in Africa and then migrated across continents, developing into distinct races.
  • Modern Homo sapiens appeared during the ice age (between 75,000-10,000 years ago).
  • Pre-historic cave art developed around 18,000 years ago (e.g., Bhimbetka rock shelter in Madhya Pradesh).
  • Agriculture began around 10,000 years ago, leading to human settlements.