Unit 2: Genetics and Evolution - Chapter 3: Evolution

2.3 Evolution

Evolution is the process of gradual change in the characteristics of a population of organisms over successive generations, leading to the development of new species from pre-existing ones. It is a slow, continuous, and irreversible process.

Origin of Life

• Abiogenesis (Spontaneous Generation): The discredited theory that life arose spontaneously from non-living matter (e.g., maggots from decaying meat).

• Biogenesis: The theory that life arises only from pre-existing life (Louis Pasteur's experiment disproved spontaneous generation).

• Oparin-Haldane Theory (Chemical Evolution):

  • Proposed independently by A.I. Oparin (Russia) and J.B.S. Haldane (England/India).
  • States that life originated from non-living organic molecules (e.g., amino acids, sugars, nitrogenous bases) that formed spontaneously under the primitive Earth conditions.
  • Primitive Earth had a reducing atmosphere (no free oxygen), high temperature, volcanic storms, and UV radiation.
  • Simple inorganic molecules combined to form complex organic molecules, which then aggregated to form protobionts (pre-cells) and eventually living cells.

• Miller-Urey Experiment (1953):

  • Stanley Miller and Harold Urey experimentally demonstrated the Oparin-Haldane theory.
  • They created an apparatus simulating primitive Earth conditions (reducing atmosphere of CH₄, H₂, NH₃, and water vapor, high temperature, electrical discharges for lightning).
  • After a week, they observed the formation of simple organic molecules like amino acids, sugars, and nitrogenous bases.
  • This experiment provided strong evidence for chemical evolution.

Evidences of Evolution

1. Morphological and Anatomical Evidences:

   • Homologous Organs: Organs that have the same basic structural plan and origin but perform different functions.
     • Indicates: Divergent evolution (common ancestry).
     • Example: Forelimbs of whales, bats, cheetahs, and humans (all have humerus, radius, ulna, carpals, metacarpals, phalanges, but used for swimming, flying, running, grasping respectively).
   • Analogous Organs: Organs that have different basic structural plans and origins but perform similar functions.
     • Indicates: Convergent evolution (different ancestors evolving similar adaptations due to similar environmental pressures).
     • Example: Wings of insects and birds (both used for flight, but structurally different).

2. Embryological Evidences:

   • Recapitulation Theory (Biogenetic Law): Proposed by Ernst Haeckel.
     • States that "Ontogeny recapitulates Phylogeny," meaning the developmental history of an organism (ontogeny) briefly repeats its evolutionary history (phylogeny).
     • Example: Vertebrate embryos (fish, salamander, chick, human) all possess gill slits and a tail at some stage of their development, even though these features are not present in the adult forms of many of them.
     • Criticism: Karl Ernst von Baer disproved the extreme interpretation of this law, stating that embryos never pass through the adult stages of other animals.

3. Palaeontological Evidences:

   • Fossils: Preserved remains or traces of organisms from the past.
   • Significance: Provide direct evidence of past life forms and their evolutionary changes over geological time.
   • Example: Fossil records of horses show a gradual increase in size, reduction in number of toes, and modification of teeth for grazing.

4. Biogeographical Evidences:

   • The study of the distribution of species across geographical regions.
   • Example: Darwin's finches on the Galapagos Islands. Different islands had finches with different beak shapes adapted to different food sources, suggesting they evolved from a common ancestor that colonized the islands.
   • Adaptive Radiation: 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).
     • Example: Darwin's finches, Australian marsupials (evolved from a common marsupial ancestor, radiating to different habitats and developing different forms).

5. Molecular Evidences:

   • Similarities in DNA, RNA, and protein sequences among different organisms.
   • Example: The closer the evolutionary relationship between two species, the more similarities there are in their DNA sequences and protein structures (e.g., human and chimpanzee DNA are 98% similar).

Darwin’s Theory of Natural Selection

• Proposed by Charles Darwin in his book "On the Origin of Species by Means of Natural Selection" (1859).
• Key Concepts:
  1. Overproduction/Prodigality of Production: Organisms produce more offspring than can survive.
  2. Struggle for Existence: Due to overproduction, individuals compete for limited resources (food, space, mates) and against environmental challenges (predators, diseases).
  3. Variation: Individuals within a population show variations in their traits. These variations are heritable.
  4. Survival of the Fittest (Natural Selection): Individuals with variations that are advantageous in a particular environment are better adapted, survive, and reproduce more successfully than others. Nature selects these favorable variations.
  5. Inheritance of Favorable Variations: The advantageous variations are passed on to the next generation.
  6. Speciation: Over many generations, the accumulation of favorable variations leads to the formation of new species.
• Role of Malthus: Darwin was influenced by Thomas Malthus's essay on population, which highlighted the potential for populations to grow exponentially while resources grow arithmetically, leading to struggle.

Neo-Darwinism (Modern Synthesis of Evolution)

• A synthesis of Darwinian natural selection with Mendelian genetics.
• Explains the sources of variation and how they are acted upon by natural selection.
• Factors contributing to Neo-Darwinism:
  • Mutation: Sudden heritable changes in DNA sequence, providing new variations.
  • Recombination: Reshuffling of genes during sexual reproduction (crossing over, independent assortment), creating new combinations of existing variations.
  • Natural Selection: Differential survival and reproduction of individuals based on their traits.
  • Gene Flow (Gene Migration): Movement of genes between populations due to migration of individuals.
  • Genetic Drift: Random changes in allele frequencies in a population, especially significant in small populations.
    • Founder Effect: When a small group of individuals establishes a new population, the allele frequencies in the new population may differ significantly from the original population due to chance.
    • Bottleneck Effect: A sharp reduction in the size of a population due to environmental events (e.g., natural disaster, disease), leading to a reduction in genetic diversity.

Examples of Evolution

• Giraffe: Lamarck proposed that giraffes stretched their necks to reach leaves, and this acquired trait was inherited. Darwin explained that giraffes with naturally longer necks had an advantage in reaching food, survived better, and passed on their genes, leading to a gradual increase in neck length over generations.
• Industrial Melanism:
  • Example: Peppered moth (Biston betularia) in England.
  • Before industrialization: White-winged moths were more common (camouflaged against lichen-covered trees).
  • After industrialization: Soot and smoke darkened tree trunks, and lichens disappeared. Dark-winged moths became more common (camouflaged against dark trees), while white-winged moths were easily preyed upon.
  • Explanation: Natural selection favored the dark-winged moths in the polluted environment.
• Antibiotic Resistance:
  • When bacteria are exposed to antibiotics, susceptible bacteria are killed, but resistant variants survive and reproduce.
  • Over time, the population becomes dominated by antibiotic-resistant bacteria.
  • Explanation: Natural selection favors resistant bacteria in the presence of antibiotics.

Mutation Theory: Hugo de Vries

• Proposed by Hugo de Vries based on his work on evening primrose (Oenothera lamarckiana).
• Key Idea: Evolution occurs through sudden, large, and discontinuous changes called mutations (saltation).
• Contrast with Darwin: Darwin believed in gradual, continuous variations.
• De Vries's theory emphasized the importance of mutations as the raw material for evolution, which is now incorporated into Neo-Darwinism.

Hardy–Weinberg Principle

• States that in a large, randomly mating population, in the absence of evolutionary influences (mutation, gene flow, genetic drift, natural selection, non-random mating), the allele and genotype frequencies will remain constant from generation to generation.
• Equation: p² + 2pq + q² = 1
  • p = frequency of the dominant allele
  • q = frequency of the recessive allele
  • p² = frequency of homozygous dominant genotype
  • q² = frequency of homozygous recessive genotype
  • 2pq = frequency of heterozygous genotype
• Factors affecting Hardy-Weinberg Equilibrium (causing evolution):
  • Gene migration (gene flow)
  • Genetic drift
  • Mutation
  • Genetic recombination
  • Natural selection
• Numericals: Used to calculate allele and genotype frequencies in a population.

Types of Natural Selection

Natural selection can lead to different patterns of phenotypic change in a population:

1. Directional Selection:

   • Favors one extreme phenotype, shifting the population mean towards that extreme.
   • Example: Industrial melanism (selection for dark moths).

2. Disruptive Selection:

   • Favors individuals at both extremes of the phenotypic range over intermediate phenotypes.
   • Can lead to speciation.
   • Example: Beak size in finches, where small beaks are good for small seeds and large beaks for large seeds, but intermediate beaks are inefficient.

3. Stabilizing Selection:

   • Favors intermediate phenotypes and acts against extreme variations.
   • Reduces phenotypic variation and maintains the status quo.
   • Example: Human birth weight (intermediate weight babies have higher survival rates).

Adaptive Radiation

• 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).
• Darwin’s Finches: A classic example. A single ancestral species of finch colonized the Galapagos Islands and diversified into many new species, each adapted to a different ecological niche (e.g., different food sources, leading to different beak shapes).
• Australian Marsupials: A variety of marsupials (e.g., koala, kangaroo, wombat, Tasmanian wolf) evolved from a common ancestral marsupial within the Australian continent, adapting to different habitats and lifestyles.

Human Evolution

• The evolutionary history of humans, tracing the lineage from early primates to modern humans (Homo sapiens sapiens).
• Key Stages and Traits:
  • Dryopithecus: Ape-like, hairy, walked like gorillas and chimpanzees. (Approx. 15 mya)
  • Ramapithecus: More man-like, walked upright. (Approx. 15 mya)
  • Australopithecus: Lived in East African grasslands, ate fruit, hunted with stone weapons, walked upright. (Approx. 2 mya)
  • Homo habilis: First human-like hominid, brain capacity 650-800 cc, probably did not eat meat. (Approx. 2 mya)
  • Homo erectus: Brain capacity 900 cc, ate meat, probably lived in groups, used fire. (Approx. 1.5 mya)
  • Neanderthal Man (Homo neanderthalensis): Brain capacity 1400 cc, lived in East and Central Asia, used hides to protect their bodies, buried their dead. (100,000-40,000 years ago)
  • Homo sapiens (Cro-Magnon Man): Arose in Africa, brain capacity 1450 cc, developed cave art. (75,000-10,000 years ago)
  • Homo sapiens sapiens (Modern Man): Arose during ice age, developed agriculture, settled human habitations. (10,000 years ago to present)

Human evolution is a complex process influenced by genetic changes, environmental pressures, and cultural developments. The fossil record provides crucial insights into this journey.