Artificial Selection

High: 4.7% to 19.8% with nearly equal steps each 10 generations. Low: 4.7% to 1.8% in the first 30 generations, then only to 0.8% over the next 70.

• Correct: The high line rose steadily, by about 15 percentage points; the low line fell fast, then leveled off.: Correct: a steady rise in one line, a fast fall that flattens in the other.
• Both lines changed at a steady rate, so the low line will reach 0% oil within a few more generations.: The low line slowed sharply after 30 generations, so it is not falling at a steady rate toward 0%.
• Both lines changed quickly at first and then stopped changing after about 30 generations.: The high line kept rising by more than 1 percentage point every 10 generations through generation 100.
• The high line leveled off after 50 generations, while the low line kept falling at an even rate to the end.: This reverses the two lines: the high line kept rising, and the low line leveled off.

A polygenic trait has many alleles that each add a little. Each round of selection makes some of them more common, but plenty of heritable variation is left (and mutation adds a little), so the next round can still move the mean.

• Correct: Oil content depends on many genes, so heritable variation remained for each round of selection.: Correct: continued response needs continued heritable variation.
• Each generation the plants made more oil because they were grown in fields that suited high-oil corn.: Both lines grew in the same fields, so the fields cannot explain why only one line rose.
• The plants in the high line learned to store more oil, and passed what they learned to their kernels.: Plants do not learn, and changes in an individual are not inherited; the line changed because of which plants bred.
• Choosing the 12 best ears each generation caused fresh mutations for high oil content in their kernels.: Choosing parents does not cause mutations; it changes which existing alleles are passed on.

Kernels with almost no oil grow poorly, which limits how far the line can go, and once nearly every plant carries the low-oil alleles, there is little heritable variation for further selection to act on.

• Correct: Kernels need some oil to grow, and most low-line plants already carried low-oil alleles, leaving little variation.: Correct: a physical limit plus used-up variation slows the response.
• The breeders lost interest in the low line and stopped choosing the ears with the least oil after generation 30.: The method was the same for both lines every generation, as the stimulus states.
• Corn has no alleles for less oil than the starting variety held, so the low line could not drop below 4.7%.: The low line did fall far below 4.7%, to 0.8%, so the starting variety did hold alleles for less oil.
• The low line was too small a population for selection to work, while the high line was a large one.: Both lines were run the same way with the same number of selected ears.

Resistance mutations arise at random in a huge population. Each dose kills the less resistant cells, so the more resistant ones take over; later random mutations in those survivors can raise resistance again, giving a rise in steps.

• Correct: Cells whose random mutations raised resistance survived each higher dose and multiplied, again and again.: Correct: random variation plus repeated selection by the drug.
• The antibiotic damaged DNA in a way that changed the drug's target, so the cells that it reached became resistant to it.: Mutations are not directed at the drug's target by the drug; resistant variants are random, and the drug removes the others.
• Each cell gradually got used to the antibiotic, and its tolerance was passed on when it divided.: Tolerance gained by one cell during its life is not what changes the MIC; resistant mutants replace susceptible cells.
• The rising dose broke the antibiotic down faster, so later doses had less effect on the cells.: The MIC is measured on fresh samples with a set concentration, so it reflects the cells, not breakdown of the dose.

Which resistance mutations appear, and when, is a matter of chance. Flasks started alike can follow different paths, which is evidence that the mutations were not directed by the drug.

• Correct: Resistance mutations arise at random, so each flask gained different mutations at different times.: Correct: random mutation makes replicate populations differ.
• The flasks probably received different doses, since identical handling should give identical results.: The doses were the same; differences between replicates are expected when mutations arise by chance.
• Flask 2 started with more resistant cells, because it was taken from the top of the starting culture.: All flasks had the same MIC on day 0, so there is no evidence that flask 2 started different.
• Bacteria in flask 3 were less able to sense the antibiotic, so fewer of them changed.: Bacteria do not change in response to sensing a drug; random mutants are selected.