Topic 5 covers evolutionary biology: the evidence for evolution, the mechanism of natural selection, how new species form, and how organisms are classified. It also discusses biodiversity and its conservation.
Fossil record: shows gradual change over time and transitional forms (e.g., Archaeopteryx). Homologous structures: similar anatomy despite different functions (e.g., pentadactyl limb in mammals, birds, whales). Comparative DNA/protein sequences: closely related species have more similar sequences. Selective breeding demonstrates that populations can change. Observed evolution: antibiotic resistance in bacteria, peppered moth.
Mechanism: (1) Variation exists within a population (due to mutation and sexual reproduction). (2) There is a struggle for survival (overproduction of offspring). (3) Individuals with advantageous traits are more likely to survive and reproduce. (4) These traits are passed to offspring. Over many generations, allele frequencies change → population adaptation. Natural selection acts on phenotypes but changes genotype frequencies.
Speciation: formation of new species. Allopatric: geographic isolation separates populations → different selection pressures → reproductive isolation. Sympatric: speciation without geographic isolation (e.g., polyploidy in plants). Reproductive isolation: pre-zygotic (temporal, behavioural, mechanical) and post-zygotic (hybrid inviability, sterility) barriers prevent gene flow between populations.
Binomial nomenclature: Genus species (italicised, genus capitalised). Hierarchical classification: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Three domains: Bacteria, Archaea, Eukarya. Cladistics: classification based on shared derived characteristics and molecular evidence. Cladograms show evolutionary relationships. Analogous vs homologous structures.
In science, a theory is not a guess — it is a well-supported explanation backed by extensive evidence. The theory of evolution by natural selection is supported by fossils, DNA evidence, comparative anatomy, observed speciation, and laboratory experiments. It is the unifying framework of all biology. "Theory" in science means it has been repeatedly tested and confirmed, not that it is uncertain.
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