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NATURAL SELECTION
IGCSE Biology Revision Notes
Darwin’s Original Idea
1. Individuals in a species show a wide range of variation
2. Variation is caused by differences in their genes
3. Individuals with characteristics best adapted to the
environment are more likely to survive and reproduce
4. These individuals’ genes are passed to their offspring.
5. Individuals that are poorly adapted to their environment
are less likely to survive and reproduce.
6. Their genes will be less common in the next generation.
7. Given enough time, a species will gradually evolve.
Standard modern version
1.
The environment creates selection pressure by limiting the
resources available to individuals (e.g. food, shelter, protection from
predators)
2.
Mutations and meiosis create genetic variation
3.
Organisms in a species are in competition – characteristics that are
well adapted create a selective advantage
4.
Individuals with the characteristics best adapted to the environment
are more likely to survive and reproduce: they are selected for
5.
These individuals’ genes are passed to their offspring.
6.
Individuals that are poorly adapted to their environment are less
likely to survive and reproduce: they are selected against.
7.
Their genes will be less common in the next generation.
Examples: Peppered Moth
• Industrial pollution in the 19th Century killed light-coloured
lichens and made trees dark. That created a selection
pressure as the kind camouflage needed to avoid
detection by birds changed
• Dark moths had a selective advantage and so were
selected for, and more of the alleles responsible for the
dark colour were present in the next generation
• Light moths had a selective disadvantage and so were
selected against, so fewer of the alleles responsible for
the light colour were present in the next generation
• In industrial areas, dark moths became common and light
moths became rare
Example: Antibiotic resistance
• Selection pressure: Antibiotic that kill bacteria or prevent
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them reproducing
Selective advantage: Resistance to antibiotic
Selected for: Bacteria that are resistant
Selective disadvantage: Susceptibility to antibiotic
Selected against: Bacteria that are susceptible
Result: When a new antibiotic is introduced to the
environment, after some time there are more resistant
bacteria and fewer susceptible bacteria in the population
Problem: There are now many, many antibiotic-resistant
bacteria, and fewer effective antibiotics available!