4.4.3 Natural Selection

Key Stages of Evolution via Natural Selection

1. Variation

• There is a variety of phenotypes within a population, caused by genetic mutations, meiosis, and sexual reproduction.

2. Environmental Change

• A change in the environment introduces new selection pressures, such as changes in food availability, predators, or climate.

3. Selective Advantage

• Some individuals possess advantageous alleles that provide a selective advantage.
• These individuals are more likely to survive and reproduce, passing their advantageous alleles to their offspring.

4. Inheritance of Alleles

• The advantageous alleles are inherited by the next generation, increasing their frequency in the population over time.

5. Change in Allele Frequency

• Over generations, the frequency of advantageous alleles increases, leading to changes in the population's genetic makeup and eventual evolution.

Example: Peppered Moths

• In pre-industrial Britain, light-coloured moths were more common as they were camouflaged against tree bark.
• Following industrialisation, pollution darkened the bark, favouring dark-coloured moths.
• Selective pressure shifted due to predation by birds, increasing the frequency of alleles for darker colouring.

Key Terms to Remember

• Phenotype: The observable characteristics of an organism.
• Selective Pressure: An environmental factor that affects an organism's survival or reproduction.
• Evolution: A change in allele frequencies in a population over time.