Chapter 24: Evolution

Evolution: Process by which genetic changes take place in populations of organisms over millions of years. Contrast with Theory of Special Creation (divine creation of fixed species).

Evolution of Eukaryotes

Hypotheses include:

• Hydrothermal Vent Hypothesis: Life originated in deep ocean hot springs (Archaebacteria).
• Endosymbiont Hypothesis: Eukaryotes evolved from large anaerobic prokaryotes ingesting small aerobic bacteria (becoming mitochondria) and photosynthetic bacteria (becoming chloroplasts). Lynn Margulis proposed this.
• Membrane Invagination Hypothesis: Inward folding of plasma membrane formed organelles.

Lamarckism (Jean Baptiste Lamarck)

Proposed in 1809. Mechanisms: 1. Use and Disuse: Organs used become stronger (e.g., Blacksmith's arm), unused degenerate. 2. Inheritance of Acquired Characters: Modifications acquired during life are passed to offspring (e.g., Giraffe's neck stretching).

Darwinism (Charles Darwin)

Proposed Natural Selection in 'The Origin of Species' (1859). Observations from Voyage of Beagle (Galapagos Islands).

• Descent with Modification: All life related through common ancestor.
• Natural Selection: Driving force. Based on Overproduction, Struggle for Existence, Survival of the Fittest (Variations), and Evolution of new species.

Neo-Darwinism (Modern Synthesis)

Reconciliation of Darwin's theory with Genetics (Mendel's work) and Population Genetics.

• Biogeography: Distribution of species (e.g., Galapagos fauna similar to South America).
• Fossil Record: Chronological appearance of vertebrates (Fish -> Amphibians -> Reptiles -> Birds/Mammals). Oldest fossils are prokaryotes.
• Comparative Anatomy:
  • Homologous Structures: Same structure, different function (e.g., Forelimbs of man, bat, whale). Divergent Evolution.
  • Analogous Structures: Different structure, same function (e.g., Wings of bat and insect). Convergent Evolution.
  • Vestigial Organs: Reduced/useless organs (e.g., Appendix, Pelvis in whales).
• Comparative Embryology: Similar embryonic stages (e.g., Gill pouches in vertebrates).
• Molecular Biology: Universal genetic code, similarity in DNA/Protein sequences (Cytochrome c).

Population: Group of interbreeding individuals of same species in same area.

Gene Pool: Total aggregate of genes/alleles in a population.

Hardy-Weinberg Theorem

Allele and genotype frequencies in a non-evolving population remain constant unless agents of change act. Equation: p² + 2pq + q² = 1 (Genotype freq) and p + q = 1 (Allele freq).

Factors Affecting Gene Frequency

• Mutation: Ultimate source of change.
• Migration: Gene flow between populations.
• Genetic Drift: Random change in allele frequency (significant in small populations).
• Non-Random Mating: Inbreeding changes genotype frequency (increases homozygosity).
• Selection: Artificial or Natural favorable traits.

Species in imminent danger of extinction. Threatened Species: Likely to become endangered.

Causes: Habitat destruction (Deforestation), Pollution, Hunting, Invasive species.

Status in Pakistan:

• Extinct: Cheetah, Tiger, Asian Lion, Indian Rhino.
• Endangered: Indus Dolphin, Markhor, Houbara Bustard, Marco Polo Sheep.