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The key difference
Natural selection is driven by the environment: organisms with advantageous characteristics survive and reproduce. Selective breeding is driven by humans: organisms with desired traits are chosen as parents. Natural selection increases diversity; selective breeding often reduces it.
Both natural selection and selective breeding result in populations changing over time, but the driving force and outcomes are very different. Understanding this distinction is critical for Topics 18 and 21 of the IGCSE Biology syllabus.
Feature
Natural Selection
Selective Breeding
Who selects?
The environment / nature
Humans
What is selected for?
Characteristics that increase survival and reproduction in a given environment
Characteristics that are useful or desirable to humans (e.g. High yield, disease resistance)
Timescale
Slow: occurs over many generations (thousands to millions of years)
Relatively fast: visible changes within a few generations
Effect on genetic diversity
Maintains or increases diversity (varied alleles in the population)
Reduces genetic diversity (inbreeding narrows the gene pool)
Outcome
Populations become better adapted to their environment: can lead to new species
Breeds or varieties with exaggerated desired traits: does not create new species
Process
Variation → competition → survival of the fittest → reproduction → advantageous alleles passed on
Humans choose parents with desired traits → cross them → select best offspring → repeat over generations
Example
Peppered moths: dark moths survived better on soot-covered trees during industrial revolution
High-yield wheat varieties bred by crossing plants with the largest grain heads over many generations
Why Selective Breeding Can Be Risky
Selective breeding reduces genetic diversity because only individuals with the desired traits are bred. Over time, the gene pool narrows. This creates problems:
Inbred populations are more susceptible to disease: if one individual is vulnerable, most others will be too
Reduced ability to adapt to environmental changes
Increased likelihood of genetic defects due to homozygosity of harmful recessive alleles
For example, many pedigree dog breeds suffer from health problems (hip dysplasia, breathing difficulties) as a direct result of selective breeding for extreme physical traits.
The Steps of Natural Selection
The four steps of natural selection, in order, are:
Variation: individuals in a population show variation in their characteristics (due to genetic differences)
Competition: organisms compete for limited resources (food, territory, mates)
Survival: individuals with characteristics best suited to the environment are more likely to survive ("survival of the fittest")
Reproduction: survivors pass their advantageous alleles to their offspring, so the proportion of the advantageous allele increases in the next generation
Try a comparison question (4 marks)
Explain one advantage and one disadvantage of selective breeding in food production. [4]
Show the answer and marking guidance
Advantage: selective breeding can increase crop yield / improve disease resistance / produce desired characteristics quickly over a few generations
This benefits food production by increasing food supply / reducing losses to disease / meeting consumer demand
Disadvantage: selective breeding reduces genetic diversity / narrows the gene pool
This means the crop is more vulnerable to new diseases or environmental changes: if one plant is susceptible, most will be
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Saying natural selection is "random": the variation is random, but the selection is not (it favours better-adapted organisms)
Confusing selective breeding with genetic engineering: selective breeding uses traditional crossing, not direct DNA manipulation
Forgetting that natural selection requires pre-existing variation: it does not create new characteristics, it selects from existing ones
Claiming selective breeding creates new species: it creates breeds or varieties within a species