Variation, natural selection and classification

Natural selection and evolution

Darwin's theory of natural selection (published 1859, On the Origin of Species) explains how species change over time:

1. Overproduction: organisms produce more offspring than can survive.
2. Variation: individuals within a population show variation (due to mutations and sexual reproduction).
3. Competition: individuals compete for limited resources (food, mates, space).
4. Survival of the fittest: individuals best adapted to their environment survive and reproduce.
5. Inheritance: favourable alleles are passed to offspring.
6. Gradual change: over many generations, the frequency of favourable alleles increases in the population — the species changes (evolves).

Key example — antibiotic resistance in bacteria:

• Random mutation in one bacterium gives resistance to antibiotic.
• Antibiotic applied → non-resistant bacteria killed; resistant bacterium survives.
• Resistant bacterium reproduces rapidly (no competition).
• Resistance allele spreads through population → resistant strain.

Speciation: when populations of the same species become isolated (geographically or reproductively), natural selection acts differently on each, eventually producing new species that cannot interbreed.

Evidence for evolution

1. Fossil record: shows gradual change in species over time; oldest rocks → oldest (simplest) fossils.
2. Comparative anatomy: homologous structures (e.g. pentadactyl limb) shared by related species indicate common ancestry.
3. DNA evidence: similarity in DNA sequences — more closely related species share more DNA.
4. Antibiotic/pesticide resistance (observable natural selection in real time).

Classification

The taxonomic hierarchy (largest → smallest): Kingdom → Phylum → Class → Order → Family → Genus → Species

Mnemonic: King Philip Came Over From Germany Saturday.

Species definition: organisms that can interbreed to produce fertile offspring.

Binomial nomenclature: each species has a two-part Latin name — Genus species (e.g. Homo sapiens). Genus is capitalised; species is lower case. Italic or underlined.

Five kingdoms (traditional CCEA classification)

1. Animals (Animalia) — multicellular, eukaryotic, no cell wall, heterotrophic
2. Plants (Plantae) — multicellular, eukaryotic, cell wall (cellulose), autotrophic
3. Fungi — eukaryotic, cell wall (chitin), saprophytic (absorb nutrients from dead matter)
4. Protists — mostly unicellular eukaryotes (e.g. Amoeba, Plasmodium)
5. Prokaryotes (Monera) — unicellular, no membrane-bound nucleus (bacteria)

Biodiversity

Biodiversity refers to the variety of living organisms in an area — including species diversity, genetic diversity, and ecosystem diversity.

Threats to biodiversity: habitat destruction, pollution, climate change, invasive species, overexploitation.

Conservation: protected areas (nature reserves), captive breeding, seed banks, international agreements (e.g. CITES).