Edexcel IGCSE Biology · Spec 5.10-5.11
Selective Breeding
How selective breeding produces plants and animals with desired characteristics over generations.
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Selective Breeding
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Explained
Selective breeding, and what it costs
Selective breeding means choosing which organisms reproduce, so that a characteristic humans want becomes more common over generations. Humans have been doing it for thousands of years, which is why every breed of dog descends from the wolf and why modern wheat looks nothing like the wild grass it came from.
The process
- Choose parents that show the desired characteristic most strongly.
- Breed them together.
- From the offspring, choose those that show the characteristic best.
- Breed those together, and repeat over many generations.
Two words carry the marks. The characteristic must be inherited, because breeding cannot pass on something caused by the environment. And it takes many generations, because each round only shifts the population a little.
What it is used for
In plants: higher crop yield, resistance to disease, larger or sweeter fruit, and tolerance of drought or salty soil.
In animals: cows that produce more milk, sheep with thicker wool, chickens that lay more eggs, and cattle with more meat. Also temperament, which is why domestic animals are docile.
The risks
Every risk here comes from the same source. Breeding from a small number of chosen individuals narrows the gene pool, so the population loses genetic variation.
Less variation means less chance that any individual happens to be resistant when a new disease appears, so a whole crop or herd can be wiped out. It also means harmful recessive alleles are more likely to be inherited from both parents and so to show, which is why some pedigree dog breeds have characteristic inherited health problems.
And if conditions change, the population has less to select from, so it is less able to adapt. That is the same argument as the one for why variation matters in natural selection, turned round.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for differences between genetic modification and selective breeding. It credits four contrasts: genetic modification uses foreign DNA from another species and produces transgenic organisms, it is quicker and needs fewer generations, it involves asexual reproduction or cloning, and it uses enzymes, plasmids and vectors.
It then says to allow the converse, and spells out what that converse is: selective breeding uses the same species, is slower, and uses sexual reproduction, crossing, pollination or fertilisation.
That list is worth reading as a definition of selective breeding by contrast. It works only within a species, it works through ordinary sexual reproduction, and it is slow. Those three facts answer most comparison questions on this topic.
Notice that the mark scheme accepts the converse but does not award it automatically. You still have to write it. Describing only genetic modification, in a question asking for a difference, leaves the examiner with one side of a comparison.
Selective breeding and natural selection
The mechanism is identical. In both cases, individuals with certain alleles reproduce more than others, so those alleles become more common over generations.
The difference is who does the selecting. In natural selection the environment does it, and the characteristics that spread are the ones that help survival. In selective breeding humans do it, and the characteristics that spread are the ones humans find useful, which are sometimes actively bad for the animal.
If a question asks you to compare them, that is the contrast to draw: same process, different selector, different outcome.
Spec 5.10-5.11
What you need to know
- Describe how selective breeding works
- Give plant and animal examples
- Explain its risks
Active recall
Quick check
Answer each question before opening the answer.
Outline the steps of selective breeding.
Choose parents with the desired characteristic, breed them, select the best offspring, and repeat over many generations.
State one problem with selective breeding.
It reduces genetic variation, which can make a population more vulnerable to disease.
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