Unit 7 practice test: Natural Selection

Fitness gained 0.32 in the first 2,000 generations but only 0.08 in the last 20,000, and it was still rising at the end.

• Correct: It rose quickly at first, then kept rising more slowly through generation 40,000: Correct: fast early gains, then slower but continuing gains.
• It rose at a steady rate, gaining the same amount in each block of 1,000 generations: The gain per 1,000 generations fell from 0.16 early to 0.004 late, so the rate was not steady.
• It rose until generation 10,000 and stayed exactly the same after that: The mean kept rising after generation 10,000, from 1.50 to 1.60 to 1.68.
• It rose slowly at first and then faster and faster as mutations piled up: The pattern is the reverse: the fastest gains came first.

Because the ancestor is kept alive in the freezer, any later sample can compete against it head to head at the same time, in the same medium and temperature, so differences in growth reflect genetic change rather than different conditions.

• Correct: Ancestral and evolved cells can be revived and grown together in one flask under the same conditions: Correct: a living ancestor allows a direct, side-by-side comparison.
• Freezing stops new mutations, so the evolved populations have the same genes as the ancestor when tested: The evolved populations carry new mutations that arose before freezing; freezing does not undo them.
• Freezing kills the least fit cells, so the cells measured are mostly the ones with beneficial mutations: Freezing is used to store whole samples, not to pick out fitter cells.
• Frozen samples let researchers skip the daily transfers and still reach 40,000 generations sooner: Generations happen only while cells grow; frozen cells do not reproduce.

Intervals: 5,000 gives 1.39-1.49 and 40,000 gives 1.61-1.75, which do not overlap. 10,000 gives 1.45-1.55 and 20,000 gives 1.54-1.66, which overlap, so those two means are not shown to differ.

• Correct: Fitness at 40,000 generations differs from fitness at 5,000, but 10,000 and 20,000 are not shown to differ: Correct: non-overlapping intervals at 5,000 and 40,000; overlapping ones at 10,000 and 20,000.
• Each mean in the table differs from the mean before it, because each one is higher than the last: A higher mean is not enough; 10,000 (1.45-1.55) and 20,000 (1.54-1.66) overlap.
• The means at generations 20,000 and 40,000 differ from each other, because their two intervals do not overlap: 20,000 (1.54-1.66) and 40,000 (1.61-1.75) do overlap.
• The means are not shown to differ, because each interval overlaps with the one after it: 0 and 2,000 (1.00 and 1.28-1.36) do not overlap, and neither do 5,000 and 40,000.

Twelve independent populations all improving in the same conditions points to natural selection by those conditions. Which beneficial mutations arose first was a matter of chance, so the genetic routes differed.

• Correct: The shared conditions selected for faster growth, while which mutations arose differed by chance: Correct: the same pressure, different chance mutations, the same outcome.
• The ancestor already carried the beneficial mutations, so the 12 flasks needed no new mutations: All 12 began from one cell, but they gained fitness through different new mutations that arose later.
• Genetic drift by itself raised fitness, since drift tends to move separate populations in one direction: Drift is random in direction; it would not raise fitness in all 12 populations.
• The populations exchanged genes during the daily transfers, so the 12 ended with matching mutations: Each population is transferred into its own flask, and the stem says the mutations mostly differed.

Layer 3 lies above the ash dated to 23 million years (layer 4) and below the ash dated to 5.3 million years (layer 2), so it formed between those times.

• Correct: Between 5.3 and 23 million years old: Correct: the ash layers bracket the fossil.
• Exactly 14 million years, halfway between the ash layers: Sediment does not build up at a steady rate, so the midpoint is not a reliable estimate.
• Older than 23 million years: Layer 3 lies above the 23-million-year ash, so it is younger.
• Younger than 5.3 million years: Layer 3 lies below the 5.3-million-year ash, so it is older.

The radiometric clock measures time since a mineral crystallized. In ash, that is the eruption, so the date is the age of the layer. Sandstone grains were worn from older rocks, so their isotopes record the age of those older rocks.

• Correct: Ash crystals form as molten rock cools, starting the clock; sedimentary grains are older than the layer itself.: Correct: the clock must start when the layer forms.
• Sedimentary rocks contain no radioactive isotopes, so no radiometric method works on them.: Sedimentary grains do contain radioactive isotopes; they give the age of the grains' source rocks, not of the layer.
• Volcanic ash decays faster than sedimentary rock, so it gives a more precise age in fewer years.: Decay rates depend on the isotope, not on the rock type.
• Fossils are destroyed when sedimentary rock is heated, so the rock around them is not tested.: Dating does not require heating the rock; the problem is when its minerals formed.

Ordered by age, the fossils show bodies getting larger and side toes becoming smaller and fewer: change over time in one group, as descent with modification predicts. The record cannot show which fossil, if any, was a direct ancestor of another.

• Correct: Horse-like mammals changed over time toward larger bodies and fewer toes: descent with modification.: Correct: a trend over time, without overclaiming direct parentage.
• Each fossil is the direct parent species of the fossil in the layer above it.: Fossils show related forms in order, but not that one was the direct ancestor of the next; they may be side branches.
• The one-toed mammal lived at the same time as the dog-sized mammal, but its fossils were buried higher.: Superposition places layer 1 above the 5.3-million-year ash and layer 5 below the 23-million-year ash, so they did not live together.
• The animals lost toes during their lives as they ran on harder ground, and passed the change on.: Changes during life are not inherited; the trend comes from selection on heritable variation.