Natural selection
Natural selection
- Natural selection explains how a species becomes better suited to its environment:
- there is genetic variation (from mutation),
- organisms produce many offspring — more than can survive,
- there is a struggle to survive, with competition for food and space,
- the best-adapted are more likely to survive and reproduce,
- they pass on their alleles, so helpful alleles become more common.
Practice
Put the steps of natural selection in order.
Variation → too many offspring → best-adapted survive and reproduce → their alleles spread.
Practice
The original source of the variation that natural selection acts on is:
Mutation creates the new alleles (variation); natural selection then favours the helpful ones.
A real example (Supplement)
- Over many generations this makes the population more suited to its environment (adaptation).
- Antibiotic-resistant bacteria: a few carry a resistant allele, survive the antibiotic, then multiply — until the whole population is resistant.
Practice
The rise of antibiotic-resistant bacteria is an example of natural selection.
Resistant bacteria survive the antibiotic and multiply, so the resistant allele spreads — natural selection in action.
You've got it
Key idea
- natural selection: variation → too many offspring → struggle/competition → best-adapted survive → pass on alleles
- helpful alleles become more common each generation
- antibiotic resistance is natural selection in action (Supplement)