Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Artificial selection: humans decide which individuals reproduce, choosing traits they want. Over generations those traits become more common.
- It needs variation, heritability and differential reproduction, just like natural selection. Only the selecting agent differs.
- Selection uses existing heritable variation. New alleles come from mutation or from crossing with another population.
- Effects: the mean shifts in the chosen direction; variation within the population falls; differences between breeds or varieties grow.
- Examples: kale, cabbage, Brussels sprouts, kohlrabi, broccoli and cauliflower from wild cabbage; maize from teosinte; dog breeds from wolves.
- Humans also select by accident (pesticide and antibiotic resistance, fishing that removes large fish).
Key relationships
| Cause | Effect on the selected population |
|---|---|
| only parents with the wanted phenotype breed | mean of the trait moves toward that phenotype |
| carriers of unwanted alleles do not breed | those alleles are lost; variation falls |
| few parents used | more alleles lost by chance; population becomes uniform |
| different breeders choose different traits | breeds or varieties diverge from each other |
| little variation left | response to selection slows and plateaus |
| uniform crop or herd | one pathogen can spread through nearly all individuals |
| Calculation | How |
|---|---|
| change in mean | final mean − starting mean |
| percentage change | (final − start) ÷ start × 100 |
| change per generation | total change ÷ number of generations |
Assumptions behind the data
- A selected line only shows a genetic response if it is compared with an unselected control kept in the same conditions.
- The trait must be at least partly heritable; changes caused by diet or training are not passed on.
- A few fictional generations support a conclusion; they do not prove it.
Mistakes to avoid
- Saying breeders “create” new traits or alleles.
- Confusing artificial selection with genetic engineering.
- Claiming selection increases diversity within a breed.
- Assuming a trait humans select must help the organism survive in the wild.
- Describing a graph without quoting values or comparing with the control.
- Calling a fall in variation “evolution stopping”: the population has still evolved.
Quick self-check
- Why can selection for larger fruit stop working after many generations? (Alleles for larger fruit become fixed and others are lost, so little heritable variation remains.)
- Mean seed mass rises from 30 mg to 39 mg. Percentage change? ((39 − 30) ÷ 30 × 100 = 30%)
- Why is a field of one clone at risk? (Every plant has the same genotype; a pathogen that infects one can infect all.)
Next: practice questions.