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Darwin's theory of evolution by natural selection

Reproduction and inheritance · Cell division, variation and evolution · note 7 of 8

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Darwin's theory of evolution by natural selectionSpec 3.38

In short

Natural selection is the process by which evolution happens. Individuals in a population show genetic variation, more offspring are born than can survive, and they compete for resources. Those best suited to their environment are more likely to survive, reproduce and pass on their alleles, so the proportion with advantageous alleles increases over many generations.

Evolution is a change in the inherited characteristics of a population over many generations. Darwin's theory explains how it happens by natural selection.

  1. Individuals in a population show variation. Some of this variation is genetic, and arises from mutation and sexual reproduction.
  2. More offspring are born than can survive, so individuals compete for resources such as food, space and mates. Some are eaten or die of disease.
  3. Individuals with characteristics that suit their environment are more likely to survive and reproduce.
  4. They pass on the alleles for those characteristics to their offspring.
  5. Over many generations, the proportion of the population with the advantageous alleles increases.

This is sometimes summed up as 'survival of the fittest'. 'Fittest' means best suited to the environment, not strongest.

Common mistake:

Individuals do not change during their lives to cope with their environment. The advantageous allele has to be there already, and the proportion of individuals with it increases over generations.

Exam tip:

Write the full chain in your answer: variation, competition, survival and reproduction of the better adapted, alleles passed on, proportion increases.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

What is the difference between mitosis and meiosis?

Mitosis produces two diploid cells that are genetically identical to the parent cell and is used for growth, repair, cloning and asexual reproduction. Meiosis produces four haploid gametes, each with half the number of chromosomes and genetically different from each other. In humans, mitosis gives cells with 46 chromosomes and meiosis gives gametes with 23.

How does meiosis create genetic variation?

Meiosis creates genetic variation because the four gametes it makes are genetically different from each other and from the parent cell. Fertilisation is also random, so any sperm can fuse with any egg. This is why brothers and sisters are not identical to each other or to either parent.

How does natural selection lead to evolution?

Natural selection leads to evolution because individuals in a population show genetic variation and compete for resources. Those with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations, the proportion of the population with the advantageous alleles increases, so inherited characteristics change.

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