AP Biology · Unit 7 · Topic 7.3
Artificial selection is the same arithmetic with a human doing the choosing. Pick your breeders, watch the mean move, and watch what it costs you.
Learning objective 7.3.A Explain how humans can affect diversity within a population.
Four thousand individuals, a trait that varies continuously, and a breeder who keeps the top fifth. The bars are the whole population; the highlighted part is what gets bred from.
| Quantity | Heritable trait | Trait with no heritable component |
|---|
The second column of the table is Topic 7.1's heritability requirement, run as an experiment. The breeder did everything right — the selection differential is just as large — and the population did not move, because nothing that made those parents exceptional was passed on. A breeder can choose the parents. They cannot choose what gets inherited.
The same program, run for twelve generations. Watch the two lines separately: one is what the breeder wanted, the other is what it took to get it.
| Top fraction kept as parents | Gain in the mean | Heritable variance remaining |
|---|
Natural fitness here is highest near the population's original mean — which is what being adapted to an environment means. The breeding program has been dragging the trait away from it the whole time.
| Artificial selection | Natural selection |
|---|
Read the last column. In every case the trait humans selected for came with something the organism lost, and in most cases what it lost was the ability to get by without us. That is not a failure of the breeding — it is what selecting for one thing rather than survival produces.
Answer these in your own words before you open the answers.
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