1The same chain, a different chooser

Compare the two processes link by link.

The last row matters. A dairy cow selected for enormous milk yield, or a dog breed selected for a very short face, may be less able to survive without human care. Artificial selection does not have to improve fitness in the wild; it serves the breeder.

Artificial selection works only on heritable variation that is already there: once the alleles for a trait are fixed in a line, choosing harder does nothing more.

2What humans have done with it

Real examples show how far it can go.

• Wild cabbage (Brassica oleracea), a single wild species, has been bred into cabbage (large leaf bud at the tip), kale (large leaves), broccoli and cauliflower (flower clusters), Brussels sprouts (side buds) and kohlrabi (swollen stem). Breeders selected different parts of the plant in different lines, and every one of these crops is still the same species.
• Dogs descend from wolves. Selection for herding, guarding, hunting, size and appearance produced breeds that differ in body mass by more than a factor of fifty.
• Maize descends from a wild grass, teosinte, whose small ears carry a few hard-cased kernels. Selection over thousands of years produced large cobs with hundreds of exposed kernels.

People also select without meaning to. Spraying a field with a herbicide, treating a patient with an antibiotic, or catching only the largest fish all decide which individuals reproduce. That is artificial selection too, and topic 7.8 returns to it.

3Reading a selection experiment

An invented experiment. A plant breeder wants a fast-cycling plant that flowers sooner. She grows 200 plants, records the number of days from sowing to the first open flower, and lets only the 20 earliest-flowering plants cross with each other. She repeats this for five generations. A control line is grown alongside, with 20 parents chosen at random each generation.

Describe it the way the exam wants, with specific values.

• A specific data point. In generation 3 the selected line flowered after a mean of 25.3 days.
• The trend. The mean in the selected line fell every generation, from 30.0 days to 24.3 days. The control stayed between 29.8 and 30.2 days.
• The pattern in the rate. The fall was largest at first (1.9 days between generations 0 and 1) and smallest at the end (0.3 days between generations 4 and 5).
• The variation. The range in the selected line narrowed from 17 days (22–39) to 8 days (20–28). Late-flowering plants vanished; the earliest value hardly moved.

Then the percentage change, a calculation the formula sheet expects you to handle.

Now explain each pattern.

• Why the mean fell: early flowering was partly inherited, so offspring of early-flowering parents also tended to flower early.
• Why the control matters: the control line, grown in the same conditions, did not change. So the change in the selected line was caused by the selection, not by the greenhouse, the weather or the seed batch.
• Why the response slowed: each generation, alleles for late flowering were lost from the selected line, and the alleles for early flowering became common. With less heritable variation left, choosing the earliest plants shifted the next generation less. The lower limit near 20 days may also be a physiological limit: a plant needs some time to grow before it can flower.
• Why the range narrowed: the selected line now descends from a small set of similar parents, so it carries less genetic variation.

4What artificial selection does to diversity

The CED's learning objective is about diversity within a population, so be precise about which level you mean.

Within a selected line or breed, diversity falls. Each generation only a few chosen individuals breed, and they are chosen to be alike. Alleles carried by the rejected individuals are lost. Breeders often cross close relatives to fix a trait, which increases the share of homozygous individuals, so recessive alleles for genetic disorders show up more often. Many pedigree dog breeds have high rates of particular inherited diseases for this reason.

Across a whole species, diversity of form can rise. Selecting different lines in different directions creates many distinct varieties, like the cabbage crops or the dog breeds. The species as a whole shows a wider range of phenotypes than its wild ancestor did.

Crops grown as one variety are vulnerable. When a field, or a whole region, is planted with a single highly selected variety, every plant shares the same alleles for disease resistance. A pathogen that can attack one plant can attack them all. Topic 7.11 develops this: genetically uniform populations are at greater risk when the environment changes.