7.4 Population Genetics

1. AOnly natural selection can change allele frequencies; random events affect individuals but leave the population unchanged.
   A student who thinks all evolution is caused by natural selection picks this. Chance events can change which alleles are passed on, so they can change a population's allele frequencies.
2. BChance events, such as which individuals happen to reproduce, can change allele frequencies whatever the alleles' effects on fitness. Correct
   Random events also drive evolution. Mutation adds new alleles at random, and chance events decide which individuals survive and reproduce and which alleles are passed on, so allele frequencies can change without regard to fitness.
3. CRandom events do change allele frequencies, but over many generations their effects cancel out and so cause no evolution.
   A student who thinks random events cancel out picks this. Chance changes in one generation become the starting point for the next, so they build up rather than cancel, especially in small populations.
4. DRandom changes in allele frequencies tend to make populations better adapted, as useful changes are the ones that last.
   A student who thinks drift improves adaptation picks this. Random changes are unrelated to fitness; a harmful allele can become common by chance.

CED 7.4.A.1 · Read this in Fix

1. AThe higher temperature caused mutation H to occur, so it appeared only in the population at 42 °C.
   A student who thinks the environment causes the mutations that help organisms survive it picks this. Mutation H also appeared at 37 °C (1% at generation 150), where it did not spread.
2. BCells at 42 °C acquired mutation H because they needed it to grow faster at that temperature.
   A student who thinks cells produce the traits they need picks this. Mutation H arose by random mutation, also at 37 °C, and spread at 42 °C because cells carrying it left more descendants.
3. CMutation H spread to all of the cells at 42 °C within a single generation after it first arose.
   A student who thinks a favorable mutation spreads at once picks this. At 42 °C the percentage rose gradually, from 3% at generation 150 to 22%, 68% and then 94% at generation 300.
4. DMutation H appeared in both populations but became common only at 42 °C, where it was favored. Correct
   Cells with mutation H were first found in both populations at generation 150 (3% at 42 °C and 1% at 37 °C). At 37 °C the mutation stayed at about 1% or less, but at 42 °C it rose to 94% by generation 300, so the variation it produced was acted on by natural selection only at 42 °C.

CED 7.4.B.1.i · Read this in Fix

1. AThe population evolved, as the frequency of allele b rose from 0.20 to 0.60 over 15 years. Correct
   Allele b rose from 0.20 in 2000 to 0.60 in 2015, and B fell from 0.80 to 0.40. A change in allele frequencies from one generation to the next is evolution, so these data provide evidence that the population evolved; they do not by themselves show what caused the change.
2. BThe population did not evolve, as no new allele appeared in the population over the 15 years.
   A student who thinks evolution requires a new allele or trait picks this. The frequencies of the existing alleles changed, which is evolution.
3. CNatural selection favored allele b, as b became more common in each sample taken after 2000.
   A student who thinks every change in allele frequency is caused by natural selection picks this. The data show that allele frequencies changed, which is evidence of evolution, but not which process changed them; genetic drift or gene flow could also raise the frequency of b.
4. DIndividual birds evolved, as the alleles in each bird changed from B to b during the 15 years.
   A student who thinks individuals evolve picks this. A bird's alleles do not change during its life; the population's allele frequencies changed because of which birds left offspring.

CED 7.4.C.1 · Read this in Fix

1. AResistance arose by random mutation before exposure, as the same two colonies grew on both antibiotic plates. Correct
   On both antibiotic plates, cells grew only from colonies 3 and 7. If the antibiotic had caused resistance, resistant cells would have appeared at independent positions on the two plates. Matching positions show that resistant cells were already present in colonies 3 and 7, having arisen by random mutation before any contact with the antibiotic, which then allowed only them to grow.
2. BThe antibiotic caused resistance mutations in some of the cells it touched, so colonies grew where it acted.
   A student who thinks the environment causes the mutations that help organisms survive it picks this. Mutations caused on contact would arise at unrelated positions on the two plates; instead, the same two colonies grew on both.
3. CThe cells on the antibiotic plates became resistant because they needed resistance to survive it.
   A student who thinks cells produce the traits they need picks this. All cells on the antibiotic plates needed resistance, yet only cells from colonies 3 and 7 grew on both plates, which shows that resistance was already present in them.
4. DResistance did not arise at random, since random mutation would give colonies at different positions on each plate.
   A student who thinks 'random' means 'happening at the moment of exposure, with no pattern' picks this. Mutation is random with respect to need; a mutation that occurred before the transfer is inherited by the whole colony, so it shows up at the same position on every plate.

CED 7.4.A.1.i · Read this in Fix

1. AMutations arise more often in genes whose new alleles would help the organism survive in its environment.
   A student who thinks mutations are directed toward what is needed picks this. Mutation is random; it is not more frequent in genes where a new allele would help.
2. BMutations add new alleles to a population, but every new allele that is produced by mutation lowers fitness.
   A student who thinks mutations are always harmful picks this. A new allele can be beneficial, detrimental or neutral, depending on its effect and the environment.
3. CMutations arise at random with respect to an organism's needs, and they can create new alleles in a population. Correct
   Mutation is a random process: it does not occur because a new allele would be useful. It is the source of new alleles, so it adds new genetic variation to a population.
4. DMutations in any cell of an animal's body add new alleles that the animal can then pass on to its own offspring.
   A student who thinks every mutation is inherited picks this. In animals, only mutations in cells that give rise to gametes are passed to offspring.

CED 7.4.A.1.i · Read this in Fix

1. AThe allele increased in every small population, as random change makes a population better adapted.
   A student who thinks drift improves adaptation picks this. The allele rose in two small populations and fell in the other two, and the alleles had no effect on fitness.
2. BAllele frequency changed more in the small populations, as chance has more effect in small populations. Correct
   In the populations of 10, the allele reached 1.0 in one, was lost in another, and ended at 0.15 and 0.80 in the other two. In the populations of 500, it stayed between about 0.40 and 0.63. With no effect on fitness, these changes are genetic drift: chance changes in allele frequency, which are much larger in small populations and have no consistent direction.
3. CAllele frequency stayed close to 0.5 in all of the populations, as random changes cancel each other out.
   A student who thinks random events cancel out picks this. In the small populations the allele frequency moved far from 0.5, reaching 1.0 in one population and 0.0 in another.
4. DAllele frequencies in the small populations became alike, as alleles drifted from one population to another.
   A student who thinks genetic drift is the movement of alleles between populations picks this. The small populations ended at very different frequencies (0.0, 0.15, 0.80 and 1.0); drift is chance change within each population.

CED 7.4.A.1.ii · Read this in Fix

1. ABrown beetles in this population survive and reproduce better than green beetles do.
   A student who thinks the null hypothesis states the expected effect picks this. This is the alternative hypothesis of selection favoring brown beetles; a null hypothesis states no difference.
2. BThe frequency of the brown allele does not change from one generation to the next.
   A student who thinks allele frequencies cannot change without selection picks this. In a small population, drift alone changes allele frequencies, so 'no change' does not separate drift from selection.
3. CNo beetles move into or out of this population between one generation and the next.
   A student who thinks genetic drift is the movement of individuals between populations picks this. Movement between populations is gene flow; whether it occurs does not tell drift from selection.
4. DBrown and green beetles in this population do not differ in their survival or reproduction. Correct
   Natural selection on body color requires that brown and green beetles differ in survival or reproduction; drift does not. Testing the null hypothesis of no difference shows whether there is evidence for selection. If it is not rejected, the change in allele frequency could be explained by drift alone.

Working No calculation. Genetic drift is change by chance, without differences in fitness; natural selection requires that phenotypes differ in survival or reproduction. A test that separates them therefore takes as its null hypothesis that brown and green beetles do not differ in survival or reproduction. 'No change in allele frequency' is not the right null hypothesis, because drift alone changes allele frequencies in a small population.

CED 7.4.A.1.ii · Read this in Fix

1. AThe survivors carry alleles A and B because these alleles helped the animals survive the flood.
   A student who thinks bottleneck survivors are the fittest picks this. The flood killed animals regardless of their alleles, so which alleles remain depends on chance.
2. BThe survivors' allele frequencies match those before the flood, as survivors are a random sample.
   A student who thinks a random sample matches its source picks this. A sample of 6 alleles from 20 differs by chance: B rose from 0.30 to 0.67, and C and D were not sampled at all.
3. CAlleles C and D were lost and allele B became more common, as chance decided which animals survived. Correct
   Before the flood the frequencies were A 0.40, B 0.30, C 0.20 and D 0.10; among the survivors they are A 0.33 and B 0.67, and C and D are absent. Because the flood killed animals regardless of their alleles, these changes happened by chance: the bottleneck effect, a type of genetic drift, which reduced genetic variation.
4. DNew C and D alleles will soon arise among the survivors, as the population needs the variation it lost.
   A student who thinks populations produce the variation they need picks this. New alleles arise only by random mutation, which is rare; lost alleles are not restored because they are needed.

CED 7.4.A.1.iii · Read this in Fix

1. A40% Correct
   Percent decrease = (25 − 15)/25 × 100 = 40%. With only about 20 individuals for several generations, many rare alleles were lost by chance, and the number of alleles of every gene in the table fell.
2. B10%
   A student who reports the change in number, 25 − 15 = 10 alleles, as the percent change picks this. The change must be divided by the original number: 10/25 × 100 = 40%.
3. C67%
   A student who divides the change by the new value picks this: 10/15 × 100 = 67%. Percent change is calculated relative to the original value, 25.
4. D60%
   A student who gives the new number as a percentage of the original picks this: 15/25 × 100 = 60%. Today's number is 60% of the original, which is a decrease of 40%.

Working Gene P: 25 alleles before the bottleneck, 15 today. Percent decrease = (original − new)/original × 100 = (25 − 15)/25 × 100 = 40%. Distractors: the change in number, 10 alleles, written as a percentage gives 10%; dividing the change by the new value gives 10/15 × 100 = 67%; the new value as a percentage of the original gives 15/25 × 100 = 60%.

CED 7.4.A.1.iii · Read this in Fix

1. AAllele A will return to its mainland frequency of 0.10 on the island, as the island's founders came from the mainland.
   A student who thinks a founder population returns to its source's makeup picks this. Nothing pulls the island's frequencies back toward 0.10; they start at the founders' frequency, 0.33.
2. BAllele A must increase fitness on the island, as its frequency rose from 0.10 on the mainland to 0.33 among the founders.
   A student who thinks every change in allele frequency is caused by natural selection picks this. The storm carried birds regardless of their alleles; the higher frequency among the founders is a chance sampling effect.
3. CAllele A starts at 0.33 on the island, well above its mainland frequency, as the founders were a chance sample. Correct
   On the mainland, A is 2 of 20 alleles (0.10); the 3 founders happen to carry 2 A alleles of their 6 (0.33). The island population's allele frequencies start from those of its founders, so by chance A begins about three times as common as on the mainland: the founder effect, a type of genetic drift.
4. DAllele A rose to 0.33 because the island environment caused new A mutations to arise in the founders.
   A student who thinks a new environment causes useful mutations picks this. The founders carried their A alleles from the mainland; the model shows a sample of existing alleles, not new mutations.

CED 7.4.A.1.iv · Read this in Fix

1. ASeeds from another meadow land in this meadow but are all eaten by birds before they can germinate.
   A student who thinks any arrival of individuals is gene flow picks this. The seeds die before growing and reproducing, so none of their alleles enter the population's next generation.
2. BPollen carried by bees from a population in another meadow fertilizes some plants in this meadow. Correct
   The pollen brings alleles from another population, and the seeds it fertilizes carry those alleles into this population's next generation. Adding alleles by migration of individuals or gametes is gene flow.
3. CBy chance, a few plants in the meadow produce many more seeds than the other plants in the meadow.
   A student who confuses gene flow with genetic drift picks this. Chance differences in reproduction within a population can change allele frequencies by drift; no alleles enter from another population.
4. DA copying error during DNA replication in one plant produces a new allele that ends up in its pollen.
   A student who thinks any new allele is gene flow picks this. A new allele produced within the population arises by mutation; gene flow brings alleles in from another population.

CED 7.4.A.1.v · Read this in Fix

1. ATheir allele frequencies are likely to diverge, as frogs no longer mix between the ponds. Correct
   Gene flow between the ponds kept their allele frequencies alike. With it stopped, mutation, genetic drift and any selection act on each population separately, so their allele frequencies are likely to diverge, even though the ponds are similar.
2. BTheir allele frequencies are likely to stay the same, as both ponds contain frogs of one species.
   A student who thinks members of a species share fixed alleles picks this. What kept the populations alike was gene flow, not membership of one species; without it they can diverge.
3. CThey will become separate species as soon as the dam stops frogs moving between the ponds.
   A student who thinks a barrier immediately creates two species picks this. The dam stops gene flow, but differences between the populations build up only over many generations.
4. DTheir allele frequencies will stay alike, as the ponds are similar and so select for the same traits.
   A student who thinks populations change only through natural selection picks this. Mutation and genetic drift act on each population independently, so allele frequencies can diverge even in similar environments.

CED 7.4.B.1.iii · Read this in Fix

1. AIn each population, the most common alleles are those that best suit the particular habitat where it lives.
   A student who does not distinguish drift from natural selection picks this. Alleles matched to each habitat would be evidence for natural selection, not for drift.
2. BEach population carries new alleles, found in none of the others, that arose by mutation after it was founded.
   A student who thinks genetic drift is mutation picks this. New alleles show that mutations occurred; drift changes the frequencies of alleles already present.
3. CLizards often move between the six populations, carrying alleles from one population to another.
   A student who thinks genetic drift is the movement of individuals picks this. Movement between populations is gene flow, which would make their allele frequencies more alike, not explain their divergence.
4. DPopulations in nearly identical habitats differ in allele frequencies as much as populations in very different habitats. Correct
   If natural selection caused the divergence, populations in similar habitats would tend to have similar allele frequencies. Large differences between populations in nearly identical habitats, as large as between populations in different habitats, fit chance changes in small populations: genetic drift.

CED 7.4.B.1.ii · Read this in Fix

1. Aχ² ≈ 4.51, below 5.99, so the null hypothesis is not rejected; a change in frequencies is not shown.
   A student who uses the number of classes as the degrees of freedom picks this. With two phenotypes there is 1 degree of freedom, and the critical value is 3.84, which 4.51 exceeds.
2. Bχ² ≈ 4.51, above 3.84, so the null hypothesis is rejected; the phenotype frequencies have changed. Correct
   Expected counts are 150 dark and 50 light. χ² = 169/150 + 169/50 = 4.51. With 2 − 1 = 1 degree of freedom, the critical value at p = 0.05 is 3.84, and 4.51 > 3.84, so the difference from the 2010 frequencies is statistically significant. The phenotype frequencies of this single-gene trait changed, which indicates a change in the population's genetic makeup and is evidence that the population evolved.
3. Cχ² ≈ 4.51, above 3.84, so the null hypothesis is not rejected; the data fit the 2010 frequencies.
   A student who reads a χ² above the critical value as a good fit picks this. χ² measures the departure from expected counts; exceeding 3.84 means the null hypothesis is rejected.
4. Dχ² ≈ 4.51, above 3.84, so the null hypothesis is rejected; natural selection has changed the frequencies.
   A student who thinks a significant result identifies the cause picks this. The test shows that the frequencies changed, not whether selection, drift or gene flow changed them.

Working Expected counts from the 2010 frequencies: dark 0.75 × 200 = 150; light 0.25 × 200 = 50. χ² = (137 − 150)²/150 + (63 − 50)²/50 = 169/150 + 169/50 = 1.13 + 3.38 = 4.51. Degrees of freedom = 2 classes − 1 = 1; critical value at p = 0.05 = 3.84. Because 4.51 > 3.84, the null hypothesis is rejected: the phenotype frequencies of this single-gene trait have changed, which indicates a change in the population's genetic makeup and is evidence that it evolved. The test does not show which process caused the change. Distractors: using 2 degrees of freedom gives a critical value of 5.99 and the wrong decision; reading χ² above the critical value as a good fit reverses the rule; naming selection as the cause goes beyond what the test shows.

CED 7.4.C.1 · Read this in Fix

1. AIn the small population, more new mutations arise in each generation, and these change its allele frequencies.
   A student who thinks drift is caused by new mutations picks this. A small population gains fewer new mutations per generation than a large one; its frequencies change by chance sampling of existing alleles.
2. BIn the small population, plants change their own alleles more readily, as a small population needs more variation.
   A student who thinks organisms produce the variation they need picks this. Plants do not change their alleles because of need; the frequencies change by chance in which plants reproduce.
3. CIn the small population, chance differences in which plants reproduce affect a larger share of its alleles. Correct
   Each generation's alleles are a sample of the last generation's. In a population of 20, the chance success or failure of a few plants changes a large fraction of the 40 copies of the gene, whereas in a population of 20,000 such chance events are a tiny fraction and largely balance out. Random processes therefore change allele frequencies more in small populations.
4. DIn the small population, alleles drifting in from other populations make up a larger share of its alleles.
   A student who thinks genetic drift is the movement of alleles between populations picks this. Movement of alleles between populations is gene flow, and nothing in the stem describes any; the small population changes by chance in which of its own plants reproduce.

CED 7.4.B.1 · Read this in Fix

1. AGreen beetles became less common because brown beetles are better suited to their habitat.
   A student who thinks every change in a population is caused by natural selection picks this. The model shows the change was caused by where a branch fell, which does not depend on body color.
2. BNeither color became more common, as random losses tend to even out across the colors.
   A student who thinks random events affect all types equally and cancel out picks this. The branch crushed only green beetles, and green fell from 5 of 10 to 3 of 10.
3. CSome green beetles turned brown after the branch fell, so brown became more common.
   A student who thinks individuals change to suit events picks this. Generation 2 consists of the offspring of the survivors; no beetle changed color.
4. DGreen beetles became less common because of an event that did not depend on their color. Correct
   Green beetles fell from 5 of 10 in generation 1 to 3 of 10 in generation 2, because the branch happened to land on three green beetles. Where a branch falls does not depend on body color, so the change is a random occurrence (genetic drift), not natural selection.

CED 7.4.A.1 · Read this in Fix

1. AIndependent: proportion of red beads after 10 generations; dependent: number of beads in each sample
   A student who swaps the independent and dependent variables picks this. The sample size is chosen by the student before the trials; the proportion of red beads is the measured result.
2. BIndependent: number of beads drawn in each sample; dependent: proportion of red beads after 10 generations Correct
   The student deliberately compares samples of 4 beads with samples of 20, so sample size, which models population size, is the independent variable. The proportion of red beads after 10 generations is what is measured, so it is the dependent variable.
3. CIndependent: number of generations; dependent: proportion of red beads after 10 generations
   A student who treats time as the independent variable picks this. Every population is measured after the same 10 generations, so the number of generations is a controlled variable.
4. DIndependent: number of beads in each bag; dependent: proportion of red beads after 10 generations
   A student who confuses a controlled variable with the independent variable picks this. Every bag holds 100 beads, so this does not differ between the populations compared.

Working No calculation. The investigator varies the sample size (4 beads or 20 beads) between the populations compared, so sample size is the independent variable. The proportion of red beads after 10 generations is measured in response, so it is the dependent variable. The number of beads in each bag (100) and the number of generations (10) are the same for every population, so they are controlled variables.

CED 7.4.A.1.ii · Read this in Fix