Evolution and Natural Selection

🦕 Natural Selection

Natural selection is the mechanism by which evolution occurs. Better-adapted individuals are more likely to survive and reproduce, passing on their advantageous alleles.

• 1. Variation — individuals in a population show inherited variation (from mutations and sexual reproduction)
• 2. Overproduction — more offspring are produced than can survive
• 3. Selection pressure — competition, predation, disease → not all survive
• 4. Differential survival — individuals with favourable adaptations more likely to survive ("survival of the fittest")
• 5. Inheritance — survivors reproduce and pass on their advantageous alleles
• 6. Gradual change — over many generations, allele frequencies shift → evolution

🦕 Evidence for Evolution

• Fossil record — sequence of fossils shows gradual changes in organisms over time
• Comparative anatomy — homologous structures (e.g. pentadactyl limb in mammals) suggest common ancestry despite different functions
• DNA evidence — closely related species share more DNA sequences
• Observable examples — antibiotic resistance in bacteria; industrial melanism in peppered moths

🦕 Speciation

Speciation is the formation of new species from existing ones.

• Allopatric speciation: geographic isolation → populations cannot interbreed → different selection pressures → divergence → eventually cannot interbreed even if reunited
• A species is defined as a group of organisms that can interbreed and produce fertile offspring
• Genetic drift — random changes in allele frequency; more significant in small populations