Natural selection Cambridge IGCSE Co-ordinated Sciences (Double Award) revision
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In plain words
In any group of animals or plants, some are a bit better suited to their surroundings than others. They're more likely to survive and have young, and the young inherit whatever gave their parents the edge. Repeat that for thousands of generations and the whole population changes. Nobody chooses: it just happens.
Three things to know
- Natural selection: there is variation within a population; more offspring are born than can survive; there is a struggle for survival; the best adapted survive and reproduce; they pass their alleles on.
- Over many generations the helpful alleles become more common. This is how populations become adapted to their environment, and how species evolve.
- Antibiotic resistance is natural selection you can watch: a few bacteria happen to be resistant, the antibiotic kills the rest, the survivors multiply, and soon the whole population is resistant.
Tips and tricks
- Bacteria don't "become" resistant because of the antibiotic. Resistant ones were already there by mutation; the antibiotic only removes their competition.
- A full answer runs: variation → competition → best adapted survive → reproduce → pass on alleles → more common over generations.
It lands in your notebook with its questions as flashcards.
Natural selection: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What does natural selection act on?
Without differences there is nothing to select.
Which individuals are most likely to survive and reproduce?
They are best suited to their environment.
What is passed on to the offspring of survivors?
So the useful features become more common.
How do antibiotic-resistant bacteria first arise?
The antibiotic then selects for them.
What happens to a helpful allele in a population over many generations?
Individuals with it leave more offspring.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 9 marks. Write your answers on paper, then check them.
Natural selection
Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Describe the process of natural selection.[4]
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Individuals in a population vary. More offspring are produced than can survive, so there is competition (a struggle for survival). Those with features best adapted to the environment are more likely to survive and reproduce. They pass the alleles for those features to their offspring.
Explain how a population of bacteria becomes resistant to an antibiotic.[3]
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By chance (mutation) a few bacteria are resistant. When the antibiotic is used, the non-resistant bacteria are killed and the resistant ones survive. They reproduce, passing on the resistance, until most of the population is resistant.
Give one difference between natural selection and selective breeding.[2]
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In natural selection the environment determines which individuals survive and breed; in selective breeding humans choose. (Natural selection is also usually much slower.)
Answers: Natural selection
- 1. Individuals in a population vary. More offspring are produced than can survive, so there is competition (a struggle for survival). Those with features best adapted to the environment are more likely to survive and reproduce. They pass the alleles for those features to their offspring.
- 2. By chance (mutation) a few bacteria are resistant. When the antibiotic is used, the non-resistant bacteria are killed and the resistant ones survive. They reproduce, passing on the resistance, until most of the population is resistant.
- 3. In natural selection the environment determines which individuals survive and breed; in selective breeding humans choose. (Natural selection is also usually much slower.)