7.11 Variations in Populations

1. APlant survival after the drought is higher in plots with 16 genotypes than with 1.
   A student who confuses the null hypothesis with the researchers' prediction picks this. Higher survival with more genotypes is the alternative hypothesis that the experiment is designed to test.
2. BPlant survival after the drought does not differ among plots with 1, 4 or 16 genotypes. Correct
   A null hypothesis states that the independent variable, here the number of genotypes, has no effect on the dependent variable, survival. The researchers then use the data to decide whether to reject it.
3. CPlants with 1 genotype per plot survive the drought better than those with 16.
   A student who thinks the null hypothesis is the opposite of the prediction picks this. The null hypothesis is no difference, not a difference in the other direction.
4. DThe drought does not reduce survival in any plot, whether it has 1, 4 or 16 genotypes.
   A student who states 'no effect' for the wrong variable picks this. Every plot received the drought, so the experiment cannot test whether drought reduces survival; its independent variable is the number of genotypes.

CED 7.11.A.1.ii · Read this in Fix

1. AThe frogs in the diverse population were probably stronger and larger, so the fungus could not infect them.
   A student who equates fitness with strength picks this. The diverse population also declined, so the fungus did infect and kill frogs in it; survival depended on heritable resistance, not size or strength.
2. BEach frog in the diverse population carried many alleles of every gene, so each frog resisted the fungus.
   A student who treats diversity as a property of each individual picks this. A diploid frog carries at most two alleles of a gene; diversity lies in the differences among frogs, and only some frogs survived.
3. CThe diverse population was more likely to include frogs with alleles for surviving the fungus. Correct
   Both populations started near 1000 and met the same fungus. The diverse population fell to about 700 and recovered to about 950 by year 10, while the other fell to about 95. A genetically diverse population is more likely to contain individuals that can withstand a new pressure; their survival and reproduction let the population recover.
4. DFrogs in the diverse population gained new resistance alleles during their lives after exposure to the fungus.
   A student who thinks individuals acquire alleles in response to their environment picks this. Exposure does not create alleles; frogs that already carried suitable alleles survived and reproduced.

CED 7.11.A.1.i · Read this in Fix

1. AIts frequency will stay the same, because once a population has adapted to a condition it remains adapted.
   A student who thinks adaptations are permanent picks this. Selection depends on the current environment; when the antibiotic is removed, the selective pressure on this allele is reversed.
2. BIt will disappear from the population at once, because the bacteria no longer need it to survive.
   A student who thinks traits vanish when they are no longer needed picks this. Need does not remove alleles; the frequency falls gradually as cells without the allele reproduce faster.
3. CIts frequency will probably keep rising, because resistant cells are the fitter cells in any environment.
   A student who thinks an advantageous allele is advantageous everywhere picks this. Without the antibiotic, resistant cells divide more slowly, so they are less fit than susceptible cells.
4. DIts frequency will probably fall over generations, as susceptible cells will outgrow resistant cells. Correct
   Without the antibiotic, the allele no longer helps cells survive but still slows division, so susceptible cells leave more descendants and the allele becomes less common. An allele that is adaptive in one environment can be deleterious in another.

CED 7.11.A.1.iii · Read this in Fix

1. AThe dark-wing allele is favored in both forests, since dark moths are the fitter moths overall.
   A student who thinks one allele is the better allele in every environment picks this. On light bark, dark moths survived less well (21%) than light moths (62%).
2. BThe dark-wing allele is favored on dark bark and disfavored on light bark, as bark-matching moths survived more. Correct
   On dark bark, 58% of dark moths were recaptured, compared with 18% of light moths; on light bark, 62% of light moths were recaptured, compared with 21% of dark moths. Moths that match the bark survived better in both forests, so the same allele is favored in one environment and selected against (disfavored) in the other, because the selective pressure differs; moths that contrast with the bark are probably found more easily by predators.
3. CThe dark-wing allele will become the more common allele in both of the forests, because it is the dominant allele.
   A student who thinks dominant alleles become more common picks this. Dominance describes how alleles interact in heterozygotes; on light bark the dark phenotype is selected against, so the dark allele is not expected to increase there.
4. DLight moths released on dark bark became darker during the week, so more of them survived there.
   A student who thinks individuals change their heritable traits to suit the environment picks this. An adult moth's wing color does not change; on dark bark, light moths survived less well (18%) than dark moths (58%).

CED 7.11.A.1.iii · Read this in Fix

1. AThe mean for six-variety fields is significantly lower than the mean for one-variety fields. Correct
   The ±2 SE bars approximate 95% confidence intervals: one variety 56–72%, six varieties 23–37%. These do not overlap, so the lower infection in six-variety fields is a significant difference: the more genetically diverse plantings withstood the disease better.
2. BThe mean for two-variety fields is significantly lower than that for one-variety fields.
   A student who treats any difference in means as real picks this. The two-variety bar (46–64%) overlaps the one-variety bar (56–72%), so the data do not show a significant difference.
3. CThe two-variety and one-variety fields have exactly the same true mean infection rate.
   A student who thinks overlapping error bars prove the means are equal picks this. Overlap means only that a difference has not been shown; the true means may still differ.
4. DEach plant in a six-variety field was itself more resistant than any plant in a one-variety field.
   A student who treats diversity as a property of each individual picks this. The data are field means; a mix of varieties lowers infection because not all plants are susceptible, not because every plant is more resistant.

Working Read each mean ± 2 SE as an approximate 95% confidence interval: one variety 64 ± 8 = 56 to 72%; two varieties 55 ± 9 = 46 to 64%; six varieties 30 ± 7 = 23 to 37%. The six-variety interval does not overlap the one-variety interval (37 < 56), so that difference is likely significant. The two-variety interval overlaps the one-variety interval (64 > 56), so these data do not show a significant difference between them, and they do not show that the two means are equal either.

CED 7.11.A.1.ii · Read this in Fix

1. AWith little variation, each condor has fewer genes in its cells, so each bird's body works less well.
   A student who treats diversity as a property of each individual picks this. Every condor has the full set of condor genes; low diversity means that the birds carry fewer different alleles across the population.
2. BBirds raised in captivity may have lost some ability to adapt, and they pass this loss on to their chicks.
   A student who thinks traits acquired during life are inherited picks this. Living in captivity does not change the alleles a bird passes on; the concern is the small number of alleles the founders carried.
3. CLow variation matters only while numbers are small; diversity returns as the population grows.
   A student who thinks population growth restores diversity picks this. Alleles lost when the population was small do not return as numbers grow; the growing population carries only the founders' alleles.
4. DWith little variation, few condors may carry alleles for surviving a new disease or change. Correct
   The ability of a population to respond to a change in its environment depends on its genetic diversity. Because all living condors descend from a few birds, the species carries few alleles, so a new disease or environmental change could find few or no individuals able to withstand it.

CED 7.11.A.1.i · Read this in Fix

1. AThe loop is an example of negative feedback, so the population will return to its original size soon.
   A student who thinks every feedback loop stabilizes a system picks this. In negative feedback a change triggers a response that reverses it; here each change makes the next change in the same direction larger.
2. BLosing diversity makes the population better adapted, so its size can soon start to increase again.
   A student who thinks a uniform population is better adapted picks this. Losing diversity reduces the chance that some individuals can withstand a new pressure, as the model's third box shows.
3. CEach change strengthens the next, so a small population with low diversity can keep on declining. Correct
   Following the arrows: a small population loses diversity through genetic drift; with less diversity, fewer individuals can withstand a new pressure; lower survival and reproduction make the population smaller still. Each step amplifies the next, a positive feedback loop that can drive a population toward extinction.
4. DLower survival causes new mutations to arise, which restores diversity and stops the decline.
   A student who thinks hardship causes the mutations a population needs picks this. Mutations arise at random and are rare; poor survival does not cause them, and the model has no arrow that restores diversity.

CED 7.11.A.1 · Read this in Fix

1. AWith more variation, some individuals are more likely to survive and reproduce, so a sharp decline is less likely. Correct
   When the environment changes, individuals whose heritable traits suit the new conditions survive and reproduce more. A population with more variation is more likely to include such individuals, so it is less likely to decline sharply and more likely to recover.
2. BWith more variation, more individuals are poorly suited to conditions, so the population declines faster.
   A student who thinks a uniform population is better adapted picks this. Variation increases the chance that some individuals can withstand a new pressure.
3. CWith less variation, new mutations arise faster, producing the variation that the population needs.
   A student who thinks need produces mutations picks this. Mutations arise at random and do not become more frequent because a population lacks variation.
4. DWith more variation, each individual can change its own traits more easily to suit the new conditions.
   A student who thinks individuals change their heritable traits in response to the environment picks this. Variation helps because different individuals already differ in their traits, not because individuals change.

CED 7.11.A.1 · Read this in Fix

1. AThe allele is harmful in every environment and has become common in some regions just by chance.
   A student who thinks mutations are always harmful picks this. In malaria regions, carrying one copy increases survival; the allele's frequency follows the distribution of malaria, which chance does not explain.
2. BInfection with the malaria parasite causes the HbS mutation in people who are exposed to it.
   A student who thinks the environment causes the mutations organisms need picks this. Malaria does not cause the mutation; people inherit HbS from their parents.
3. CPeople in regions without malaria lost the allele because they had stopped needing it to survive.
   A student who thinks unneeded traits are lost picks this. Alleles are not lost because they are not needed; without malaria, selection against the allele in people with two copies, with no advantage for carriers, keeps it rare.
4. DWhere malaria is common, carriers survive better, keeping HbS common; elsewhere it gives no benefit. Correct
   Where malaria is common, people with one copy of HbS are more likely to survive and reproduce, so selection keeps the allele common despite disease in people with two copies. Without malaria, the allele gives no survival advantage and still causes disease in people with two copies, so selection keeps it rare. The same allele is adaptive in one environment and deleterious in another.

CED 7.11.A.1.iii · Read this in Fix