CB4 — Natural Selection and Genetic Modification

Darwin's theory of evolution by natural selection

Charles Darwin published On the Origin of Species in 1859 after his voyage on HMS Beagle. His theory has four key components:

1. Variation: individuals within a population show heritable variation (due to mutations and sexual reproduction).
2. Overproduction: organisms produce more offspring than can survive (Malthus's influence).
3. Competition/selection pressure: resources are limited → competition → struggle for survival.
4. Survival of the fittest (natural selection): individuals with advantageous alleles are more likely to survive, reproduce, and pass on those alleles. Over many generations, advantageous alleles become more common → the population changes → eventually a new species may form (speciation).

Evidence for evolution:

• Fossil record: transitional fossils show gradual change over time. Gaps exist because fossilisation is rare.
• Antibiotic resistance: bacteria evolve resistance rapidly (observable natural selection in real time).
• Comparative anatomy: homologous structures (same basic structure, different function, e.g., pentadactyl limb in mammals) suggest common ancestry.
• DNA/protein evidence: closely related species have more similar DNA sequences.

Speciation occurs when populations of the same species become isolated (geographically or reproductively) → different mutations accumulate + different natural selection pressures → populations diverge until they can no longer interbreed.

Classification

Taxonomy is the science of naming and classifying organisms. The hierarchy (from broadest to most specific): Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species (Mnemonic: Dear King Philip Came Over For Good Soup)

Binomial nomenclature: two-part Latin name = Genus + species. Written in italics; Genus capitalised, species lower case. E.g., Homo sapiens.

The five kingdoms: Animals, Plants, Fungi, Protists, Prokaryotes (Monera). Modern classification uses three domains: Bacteria, Archaea, Eukarya (based on molecular/genetic evidence).

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

Fossil record and evidence of common ancestors

Fossils form when organisms are buried and mineralised. They can preserve: hard parts (bones, shells), impressions (footprints), whole organisms in amber or ice.

Limitations of the fossil record: soft-bodied organisms rarely fossilise; many fossils destroyed by geological activity; we have only found a tiny fraction.

Antibiotic resistance — natural selection in action

1. Random mutation produces an allele conferring antibiotic resistance in one bacterium.
2. Antibiotic is introduced → selection pressure.
3. Non-resistant bacteria die; resistant bacterium survives and reproduces rapidly (binary fission).
4. Resistant allele spreads through population.
5. Eventually entire population is resistant.

To slow resistance: complete full antibiotic courses (don't stop early); don't use antibiotics for viral infections; rotate antibiotic types.

Genetic modification (GM)

Genetic modification (genetic engineering) involves transferring a gene from one organism to another (often different species) to give a desired characteristic.

Method using bacteria:

1. Isolate the desired gene using restriction enzymes (cut DNA at specific sequences, producing "sticky ends").
2. Cut plasmid (small circular bacterial DNA) with the same restriction enzyme.
3. Join gene into plasmid using DNA ligase (creates recombinant plasmid).
4. Insert plasmid into bacterium (transformation).
5. Bacteria multiply → all produce the protein encoded by the inserted gene.

Examples:

• Insulin production: human insulin gene inserted into E. coli → bacteria produce human insulin at scale → used by diabetics.
• GM crops: herbicide-resistant or pest-resistant crops (e.g., Bt cotton with insecticidal protein gene).
• Golden rice: beta-carotene gene inserted → addresses vitamin A deficiency.

Ethical issues: unintended effects on ecosystems; "playing God"; gene flow to wild populations; corporate control of food supply; allergenicity concerns.

Selective breeding (artificial selection)

Humans have selectively bred organisms for thousands of years by choosing parents with desired traits:

• Crops: higher yield, disease resistance, nutritional value (e.g., wheat, maize).
• Animals: higher milk yield in cattle; docile temperament in dogs.
• Disadvantage: reduces genetic diversity → populations more vulnerable to new diseases.