Artificial Selection: Practice Questions (Biology 7.3)

Answer: (A). The farmer chooses which plants reproduce, based on a heritable trait (ripening time). Repeated each year, early ripening becomes more common.

• (B) is caused by human activity, but the antibiotic, not a person, decided which bacteria reproduced. It is natural selection in a human-made environment.
• (C) is genetic engineering: DNA is changed directly instead of choosing parents from existing variation.
• (D) is a change caused by the environment. Taller growth from fertiliser is not inherited, and no plants were chosen as parents.

Answer: (A). Selection needs heritable variation. After many generations, alleles for finer wool are fixed and others are lost, so the parents chosen are genetically almost the same as the rest. Offspring then resemble the flock average, and the mean stops changing: a selection plateau.

• (B) is goal-directed thinking. Populations do not “need” or aim for anything; selection only sorts existing variation.
• (C) is false. Mutations keep arising at a low rate whether or not a population is selected. They are simply too rare to restart a fast response on their own.
• (D) overstates the case. Genes still affect the trait; what has run out is genetic variation among the sheep.

Answer: (A). Egg number: (296 − 180) ÷ 180 × 100 = 64.4%. Alleles: (2.6 − 4.8) ÷ 4.8 × 100 = −45.8%. The rise per generation was 5.2 eggs (generations 0–10), then 3.9 (10–20), then 2.5 (20–30), so it slowed.

• (B) 3.9 is the overall average (116 ÷ 30) and the middle interval’s rate. The rate fell from 5.2 to 2.5, so it was not constant.
• (C) mistakes a correlation for a cause. Both changes are effects of the same selection; losing alleles does not by itself raise egg number.
• (D) divides final by start (296 ÷ 180 = 1.64) and reads the ratio as a percentage increase. The increase is 64%, not 164%.

(a) Selected line: mean root length rises every generation, from 42 mm to 60 mm, by 4–5 mm each time. Unselected line: stays between 41 and 43 mm with no trend. By generation 4 the selected line is 17 mm longer than the unselected line.

(b) (60 − 42) ÷ 42 × 100 = 18 ÷ 42 × 100 = +42.9%. Per generation: 18 ÷ 4 = 4.5 mm.

(c) The unselected line shows what happens in the same conditions without selection. Because it did not change, the rise in the selected line can be attributed to selection (a genetic change), not to the room, the compost or the measuring method.

(d) The mean should stay about 60 mm, perhaps fluctuating a little. Selection changed allele frequencies; random breeding does not undo that, so the line does not “spring back” to 42 mm. (Accept a slight fall if justified, for example by natural selection against long roots in the lab.)

(a) Cuttings are produced by mitosis, so every plant is a clone of the original selected plant, with the same genotype. Earlier selection had already kept only plants with the wanted fruit, removing other alleles.

(b) Because all plants share one genotype, they share the same susceptibility. If the fungus can infect one plant, it can infect all, and there are no resistant individuals to survive and slow the spread. In a variable population some plants would carry alleles for resistance.

(c) Any two: cross the variety with wild relatives or other varieties and select resistant offspring that still taste good; use seeds from a gene bank of old varieties; grow several varieties instead of one. Disadvantage (one): offspring of crosses vary, so fruit is less uniform; restoring the wanted traits takes many generations.

(a) Mean age at maturity fell every decade, from 6.1 to 4.3 years, a change of −1.8 years or (4.3 − 6.1) ÷ 6.1 × 100 = −29.5%. Mean length at maturity also fell, from 62 to 49 cm (−21.0%). The fall in age was largest from 1990 to 2000 (0.6 years) and smallest from 2010 to 2020 (0.3 years).

(b) Nets remove large fish, which are usually older. Fish that inherit a tendency to mature young and small can spawn before they reach catchable size, so they leave more offspring. Alleles for early maturity therefore become more common in each generation.

(c) The data fit the claim but do not prove a genetic change. Alternative: with fewer fish there is more food per fish, or the water has warmed, so fish grow faster and mature earlier without any change in their genes (a non-heritable response to the environment). Test: rear offspring from this stock and from a lightly fished stock in identical tanks with the same food and temperature. If the heavily fished stock’s offspring still mature earlier, the difference is genetic.

Note: no one chose the winners on purpose, so strictly this is natural selection in a human-made environment. Accept either label if the mechanism is explained.

(a) Oil content rose from 4.0% to 6.7%, an overall rise of 67.5%. The gain was 2.3 percentage points in generations 0–6 (4.0 to 6.3) but only 0.4 in generations 6–12 (6.3 to 6.7). The gain per two generations fell each time: 0.9, 0.8, 0.6, 0.3, 0.1, then 0. Oil content reached a plateau of 6.7% from generation 10.

(b) At first the population held many alleles affecting oil content, so the chosen parents differed genetically from the average and the response was fast. Each round fixed favourable alleles and removed others, so heritable variation shrank. When almost none remained, choosing parents no longer changed the offspring, and the mean stopped rising.

(c) The cross brought in new alleles from the wild population (gene flow into the line), including some that raise oil content and were not present before. This restored heritable variation, so selection could act again: the mean rose 0.7 percentage points, from 6.7% to 7.4%.