IGCSE Biology / Selective breeding
Comparing artificial and natural selection revision notes
Edexcel International GCSE Biology (4BI1), chapter 20: Selective breeding. Free lesson.
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Natural and artificial selection both act on variation that already exists, but they differ in who selects which individuals reproduce. Artificial selection can change a population quickly, but it reduces genetic variation and can increase vulnerability to disease.
What natural and artificial selection have in common
Both processes act on existing variation in a population. Neither creates new characteristics. In each, some individuals reproduce more than others, so the alleles they carry become more common over generations. The environment selects in natural selection. A human being selects in artificial selection.
How the two types of selection differ
Natural selection favours characteristics that help an organism survive and reproduce in its environment. Artificial selection favours whatever characteristic a human wants, even if it makes survival harder. Natural selection usually takes many thousands of generations. Artificial selection can produce a marked change in tens of generations.
Why artificial selection is faster
Artificial selection creates a stronger selection pressure, meaning a stronger difference in which individuals are allowed to reproduce. A breeder may choose only a handful from hundreds. Their alleles then make up a much larger share of the next generation, so allele frequencies shift more each generation. In nature, a wider range of individuals often reproduces, so the shift is usually smaller.
Why artificial selection reduces variation
When only a few individuals with a desired characteristic reproduce, only their alleles are passed on. Alleles carried by the other individuals may be lost. Repeating this over generations makes the population genetically similar, reducing genetic variation. Closely related individuals may then be bred together. This is inbreeding. Inbreeding makes harmful recessive alleles more likely to occur in the homozygous state, where both copies of a gene are recessive. A population with little variation may have no individual carrying an allele that gives resistance to a new disease.
Why conserving wild relatives matters
Wild relatives of crops are worth conserving because they may carry alleles that domesticated varieties have lost. These can include alleles for resistance to diseases that have not appeared yet.