# Evolution and Natural Selection
Evolution explains how species change over time through natural selection. It is one of the most important concepts in biology, linking genetics, ecology, and biodiversity.
1. Variation
| Type | Cause | Examples |
|---|---|---|
| Genetic | Mutations, meiosis, sexual reproduction | Eye colour, blood type |
| Environmental | Conditions organism lives in | Scars, language |
| Both | Combination of genes and environment | Height, weight |
Mutations: random changes in DNA sequence. Most have no effect; some are harmful; rarely, some are beneficial.
2. Natural Selection (Darwin)
- Variation exists within a population
- Individuals with beneficial characteristics are better adapted to the environment
- These individuals are more likely to survive and reproduce
- They pass on their advantageous alleles to offspring
- Over many generations, the proportion of the population with the beneficial trait increases
This is "survival of the fittest."
3. Evidence for Evolution
- Fossil record: shows gradual changes in organisms over time
- Antibiotic-resistant bacteria: natural selection observed in real time
- Homologous structures: similar bone structures in different species
- DNA analysis: more similar DNA = more closely related
4. Selective Breeding (Artificial Selection)
Humans choose organisms with desired characteristics to breed.
- Choose parents with desired trait
- Breed them together
- Select best offspring
- Repeat over many generations
Examples: crops with higher yield, dogs with specific temperaments, cattle with more milk.
Risk: reduces genetic variation → susceptibility to disease.
5. Genetic Engineering
Transferring a gene from one organism to another.
- Identify desired gene
- Cut gene out using restriction enzymes
- Insert into vector (plasmid)
- Transfer to target organism
- Gene expressed → desired protein produced
Examples: insulin production in bacteria, Bt crops (insect-resistant), golden rice.
GM Crops: Pros and Cons
- Pros: higher yield, pest resistance, vitamin-enriched
- Cons: unknown long-term effects, cross-pollination with wild plants, ethical concerns
6. Classification
Linnaean system: Kingdom → Phylum → Class → Order → Family → Genus → Species
Modern classification also uses DNA analysis and the three-domain system (Bacteria, Archaea, Eukaryota).
7. Practice Questions
- Describe natural selection in your own words.
- Explain how antibiotic-resistant bacteria evolve.
- Give two advantages and two disadvantages of selective breeding.
- Describe the steps of genetic engineering.
- What evidence supports the theory of evolution?
Want to check your answers and get step-by-step solutions?
Summary
- Natural selection: variation → advantageous traits → survival → reproduction → alleles passed on
- Evidence: fossils, DNA, resistant bacteria, homologous structures
- Selective breeding: humans select desired traits; reduces genetic variation
- Genetic engineering: gene transfer; e.g. insulin production, GM crops
- Classification: Linnaean system and three-domain system