Biology and its scope
Biology (Greek bios = life, logos = study) is the science of living organisms. The term was coined independently by Lamarck and Treviranus (1802). It is broadly divided into Botany (plants), Zoology (animals) and Microbiology (micro-organisms).
Key branches worth remembering for MCQs:
| Branch | Study of |
|---|---|
| Cytology | Cell structure and function |
| Histology | Tissues |
| Morphology | External form and structure |
| Anatomy | Internal structure |
| Physiology | Functions of body parts |
| Taxonomy | Classification and nomenclature |
| Ecology | Organism-environment relationships |
| Genetics | Heredity and variation |
| Embryology | Development from zygote |
| Ethology | Animal behaviour |
Origin of life
The Earth is about 4.6 billion years old and life appeared roughly 3.5-3.8 billion years ago. The main ideas about how life began:
- Special creation — life created by a supernatural power (non-scientific).
- Spontaneous generation (Abiogenesis) — life arose from non-living matter; supported by Aristotle. Disproved by Francesco Redi, Spallanzani and finally Louis Pasteur (swan-necked flask experiment, showing "life comes only from pre-existing life", i.e. Biogenesis).
- Cosmozoic / Panspermia theory — life came from space as spores (Richter, Arrhenius).
- Chemical / Oparin-Haldane theory — the modern, widely accepted view.
The Oparin-Haldane hypothesis
A. I. Oparin (1924) and J. B. S. Haldane (1929) independently proposed that life arose through gradual chemical evolution in the primitive ocean (the "hot dilute soup" / primordial soup). The early reducing atmosphere contained methane (CH4), ammonia (NH3), water vapour (H2O) and hydrogen (H2), but no free oxygen. Energy from UV rays, lightning and volcanic heat drove the formation of simple organic molecules, which accumulated into coacervates (Oparin) or microspheres, the precursors of the first cell.
Miller-Urey experiment (1953)
Stanley Miller and Harold Urey tested Oparin's idea. They circulated CH4, NH3, H2 and water vapour in a closed apparatus and passed electric sparks (simulating lightning) for a week. They obtained amino acids (glycine, alanine, aspartic acid), sugars and other organics, proving that biomolecules can form abiotically under primitive-Earth conditions.
High-yield: Miller-Urey used CH4, NH3, H2 and H2O with electric discharge and produced amino acids. The primitive atmosphere was reducing with no free oxygen — a favourite one-liner.
Organic evolution
Evolution is the slow, gradual, continuous and irreversible change in organisms over generations producing new species from pre-existing ones.
Lamarckism (Theory of inheritance of acquired characters)
Jean Baptiste Lamarck proposed that use and disuse of organs and inheritance of acquired characters drive evolution (classic example: long neck of the giraffe). Weismann's germplasm theory (cutting tails of mice for generations) disproved inheritance of acquired somatic characters.
Darwinism (Theory of Natural Selection)
Charles Darwin (On the Origin of Species, 1859), with independent support from A. R. Wallace, proposed natural selection based on:
- Overproduction — organisms produce more offspring than can survive.
- Struggle for existence — intraspecific, interspecific and environmental.
- Variation — individuals differ.
- Survival of the fittest / natural selection — favourable variations are selected.
- Origin of species — accumulation of variations over time.
Neo-Darwinism and Mutation theory
Modern synthetic theory combines Darwinian natural selection with Mendelian genetics. Hugo de Vries proposed the Mutation theory (sudden, discontinuous, heritable changes; based on Oenothera lamarckiana, evening primrose). Sources of variation include mutation, recombination, gene flow, genetic drift and natural selection acting on them.
Evidences of evolution
| Evidence | Example |
|---|---|
| Morphological / Anatomical | Homologous organs (same origin, different function: forelimbs of man, whale, bat) show divergent evolution; Analogous organs (different origin, same function: wings of insect and bird) show convergent evolution |
| Vestigial organs | Vermiform appendix, nictitating membrane, coccyx, wisdom teeth in humans |
| Palaeontological (fossils) | Archaeopteryx — connecting link between reptiles and birds; horse evolution series |
| Embryological | Recapitulation theory (Haeckel) — "ontogeny repeats phylogeny" |
| Connecting links | Peripatus (Annelida-Arthropoda), Archaeopteryx (Reptile-Bird), Duck-billed platypus (Reptile-Mammal) |
| Biochemical / Molecular | Universality of DNA, ATP and the genetic code |
Hardy-Weinberg principle states that in a large, randomly mating population with no mutation, migration or selection, allele and genotype frequencies remain constant (genetic equilibrium): p squared + 2pq + q squared = 1.
Quick revision
- Term biology coined by Lamarck and Treviranus (1802).
- Life arose ~3.5 billion years ago; Earth ~4.6 billion years old.
- Pasteur established biogenesis; Oparin-Haldane proposed chemical evolution.
- Primitive atmosphere was reducing, with no free oxygen.
- Miller-Urey produced amino acids from CH4, NH3, H2, H2O + spark.
- Homologous = divergent; Analogous = convergent evolution.
- Archaeopteryx = reptile-bird connecting link; appendix = vestigial organ.
- De Vries gave the Mutation theory using Oenothera.