Biology IV: Evolution and the Diversity of Organisms
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Origin of Life and Evolution
- Origin of life: the Oparin-Haldane hypothesis and the Miller-Urey experiment (1953) showed that organic molecules can form under simulated early-Earth conditions. The RNA world hypothesis proposes that RNA preceded DNA and proteins.
- Evolution by natural selection (Darwin, 1859; also Wallace): organisms vary; some variation is heritable; more offspring are produced than survive; individuals with advantageous traits leave more offspring. Over generations, populations change.
- Evidence: the fossil record and transitional forms; comparative anatomy (homologous and analogous structures; vestigial organs); embryology; biogeography; molecular evidence (DNA and protein similarity); direct observation (antibiotic resistance, pesticide resistance).
- Hardy-Weinberg principle (Hardy, 1908): in a non-evolving population, p + q = 1 and p² + 2pq + q² = 1. It requires no mutation, no selection, random mating, a large population, and no gene flow. Use it to estimate carrier frequencies: if q² = 1 in 10 000, then q = 0.01 and the carrier frequency 2pq ≈ 0.02.
- Speciation: allopatric (geographic isolation) and sympatric (for example, polyploidy in plants); reproductive barriers; adaptive radiation.
- Evolution of humans: primates, hominids, key fossils; evolution is a branching bush.
Classification
Organisms are classified by the Linnaean hierarchy (domain, kingdom, phylum, class, order, family, genus, species) with binomial nomenclature (Genus species). The three-domain system recognizes Bacteria, Archaea, and Eukarya. Phylogenetic trees show evolutionary relationships. Viruses are acellular and are not placed in the domains.
The Major Groups
| Group | Key features | Examples |
|---|---|---|
| Bacteria and Archaea | Prokaryotic; many shapes (cocci, bacilli, spirilla) | Escherichia coli; methanogens |
| Protists | Mostly unicellular eukaryotes | Amoeba, Paramecium, Plasmodium, algae |
| Fungi | Heterotrophs with chitin walls; hyphae and mycelium; absorptive nutrition | Yeast, mushrooms, moulds |
| Plants | Bryophytes (no vascular tissue); pteridophytes (ferns; vascular, spores); gymnosperms (naked seeds); angiosperms (flowers and fruits; monocots and dicots) | Mosses, ferns, pines, rice |
| Animals | Phyla: Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, Chordata | Sponges, jellyfish, tapeworms, earthworms, snails, insects, starfish, vertebrates |
Chordates include fish, amphibians, reptiles, birds, and mammals; know the diagnostic features of each class and the trends in vertebrate evolution.
Sri Lanka's Biodiversity
Sri Lanka, together with the Western Ghats of India, is recognized as one of the world's biodiversity hotspots (Myers et al., 2000), with high levels of endemism among its plants, amphibians, reptiles, and freshwater fish. The Sinharaja Forest Reserve is a UNESCO World Heritage Site and a centre of endemic biodiversity (UNESCO, n.d.). Learn examples of endemic species, the major ecosystems (rainforest, dry zone forest, mangroves, coral reefs, wetlands), and the threats to them (see Chapter 13).
Common Mistakes
- Treating evolution as "progress" toward a goal.
- Saying that individuals evolve (populations evolve).
- Misapplying the Hardy-Weinberg equation (check that the population is in equilibrium).
CHAPTER 7