7.2 Natural Selection

1. AIt acts on individuals, each of which changes its heritable traits to fit the environment it lives in.
   A student who thinks individuals evolve picks this. Selection does not change an individual's heritable traits; it changes which traits are common in the next generation.
2. BIt acts directly on the alleles in a population, whatever the phenotypes those alleles produce.
   A student who thinks selection acts on alleles directly picks this. An allele is favored or disfavored only through the phenotypes it helps produce; one with no effect on the phenotype is not exposed to selection.
3. CIt acts on the phenotypes of individuals, and the result is a change in the heritable makeup of the population. Correct
   Individuals survive and reproduce, or fail to, because of their phenotypes in their environment. The offspring of the more successful individuals inherit their alleles, so the population's heritable makeup changes over generations.
4. DIt acts on whole species, keeping the traits that are best for the long-term survival of the species as a whole.
   A student who thinks selection works for the good of the species picks this. Selection results from differences in reproduction among individuals within a population, not from what benefits a species as a whole.

CED 7.2.A.1 · Read this in Fix

1. AMinnows now need to be smaller, so the adults produce smaller offspring in response to the new predator.
   A student who thinks organisms produce the traits they need picks this. Adults do not change the heritable size of their offspring because of need; small size spreads because small minnows leave more offspring.
2. BEach minnow grows to a smaller size during its life, so that it can avoid being eaten by the predator.
   A student who thinks individuals evolve picks this. A selective pressure changes which individuals survive and reproduce; it does not change the heritable size of each minnow.
3. CThe minnow species gradually reduces its body size to protect the species from being wiped out by the predator.
   A student who thinks selection works for the good of the species picks this. Small size spreads because individual small minnows leave more offspring, not because it protects the species.
4. DMinnows of different sizes now differ in their chances of surviving to reproduce, so small size becomes more common. Correct
   The predator makes small minnows more likely than large ones to survive and reproduce. Because size at maturity is heritable, their offspring tend to be small, so small size becomes more common over generations: the change in the biotic environment applies a selective pressure.

CED 7.2.A.2 · Read this in Fix

1. AWhere malaria is common, people with one copy of HbS are likelier to survive and have children, some of whom inherit HbS. Correct
   In regions where malaria is common, the HbS allele increases the fitness of people with one copy, because they are less likely to die of malaria and so leave more children on average. Their children can inherit HbS, so it is kept at a higher frequency there than where malaria is absent.
2. BPeople in malaria regions developed the HbS allele because they needed protection against the disease.
   A student who thinks organisms produce the traits they need picks this. HbS arose by mutation, not because it was needed; it became common where malaria is common because people with one copy left more children.
3. CThe malaria parasites cause mutations that produce the HbS allele in some of the people they infect.
   A student who thinks an environmental challenge causes the mutations that help organisms survive it picks this. Malaria does not produce HbS mutations; it raises the survival of people who already carry one copy.
4. DPeople who survive malaria acquire the HbS allele during the illness, and they then pass it on to their children.
   A student who thinks characteristics acquired during life are inherited picks this. Surviving an infection does not change a person's alleles; only people who already carry HbS can pass it on.

CED 7.2.A.3 · Read this in Fix

1. AThe warmer water will cause mutations that change variant 1 into variant 2 in the fish.
   A student who thinks the environment causes the mutations that help organisms survive it picks this. Variant 2 is already present in the population; warming changes which fish leave more offspring, not which mutations occur.
2. BThe frequency of variant 2 will increase, as at 28 °C fish with it have the more active enzyme. Correct
   At 28 °C variant 2 works at about 97% of its maximum and variant 1 at only about half of its maximum. Fish with variant 2 are likely to be more active, so more likely to survive and reproduce, and the variant is inherited, so its frequency rises over generations.
3. CAll fish with variant 1 will die in the first warm year, leaving only the fish that carry variant 2.
   A student who thinks selection is all or none picks this. At 28 °C variant 1 still has just over half of its maximum activity; fish with it are at a disadvantage, not certain to die, so the change is gradual.
4. DVariant 1 will remain the more common one, as the commonest variant in a population is the fittest.
   A student who thinks the most common variant is the fittest picks this. Variant 1 was common when the lake was cold, where it is the more active variant; at 28 °C variant 2 is more active, so variant 2 is favored.

CED 7.2.B.1 · Read this in Fix

1. AWhite-flowered plants will switch to making purple flowers, so their w alleles will not be selected against.
   A student who thinks individuals change their traits to suit their environment picks this. A ww plant cannot produce purple flowers; its phenotype follows from its genotype. The w allele persists because most copies are in Ww plants.
2. BMost w alleles in the model are in Ww plants, which have purple flowers and so are not selected against. Correct
   The model has 6 Ww plants carrying 6 w alleles and 2 ww plants carrying 4, so 6 of the 10 w alleles are in purple-flowered plants. Selection acts on phenotypes: pollinators visit Ww plants as readily as WW plants, so the w alleles they carry are passed on, and w persists even though white-flowered plants make few seeds.
3. CWhite-flowered plants survive as well as purple-flowered plants, so the w allele is not being selected against.
   A student who thinks selection acts only through survival picks this. White-flowered plants produce few seeds, so ww plants are selected against through reproduction; w persists because most copies are hidden in purple-flowered Ww plants.
4. DPopulations tend to keep purple and white flowers in a 3:1 ratio, so the white phenotype and the w allele are kept.
   A student who applies the 3:1 ratio of a heterozygous cross to a whole population picks this. The model itself shows 10 purple and 2 white plants, not 3:1, and nothing keeps a population at that ratio; w persists because most copies are in Ww plants.

CED 7.2.A.1 · Read this in Fix

1. AGrow plants from the stored 1995 seeds and from new seeds side by side under identical conditions, and compare flowering dates. Correct
   Growing both sets of plants under identical conditions removes the effect of warmer springs. If plants from new seeds still flower earlier than plants from 1995 seeds, the difference is heritable, so the population has evolved; if they flower at the same time, the change in the wild is a response to warmth.
2. BRecord the population's flowering dates and spring temperatures for 10 more years, and test whether the two are correlated.
   A student who thinks more observations of a correlation can show its cause picks this. Flowering dates would track temperature whether the plants have evolved or are only responding to warmth, so this cannot separate the two.
3. CGrow plants from new seeds in a cool greenhouse and in a warm one, and compare the flowering dates of the two groups.
   A student who treats individuals' responses to the environment as evolution picks this. This shows how plants of one origin respond to temperature during their lives, not whether flowering time has changed between generations.
4. DGrow plants from new seeds in a warm greenhouse, and check whether they flower as early as plants in the wild do.
   A student who thinks one treated group is enough picks this. Without plants from 1995 seeds grown under the same conditions, early flowering could not be attributed to a heritable change.

Working No calculation. The two explanations make different predictions only when plants of different origin are grown under the same conditions: if heritable flowering time has changed, plants from new seeds will flower earlier than plants from 1995 seeds even in identical conditions; if the change is only a response to warmth, they will flower at the same time.

CED 7.2.A.2 · Read this in Fix

1. ALate flowering, which gives more seeds averaged over both kinds of spring, is favored in every year.
   A student who thinks a phenotype that does best overall is favored everywhere picks this. Averaged over the two kinds of spring, late flowering gives slightly more seeds (142.5 against 132.5), but in warm springs early flowering gives more.
2. BFlowering time has no effect on fitness, as plants of both types survive through the season.
   A student who thinks selection acts only through survival picks this. Fitness is measured by reproductive success, and the two types differ in seed production in both kinds of spring.
3. CAfter a single warm spring, the next generation will be made up only of early-flowering plants.
   A student who thinks selection is all or none picks this. In warm springs late-flowering plants still produce a mean of 110 seeds, and flowering time is heritable, so late-flowering plants still appear in the next generation; selection makes early flowering more common gradually, not all at once.
4. DWhich flowering time gives more seeds per plant depends on whether the spring is cool or warm. Correct
   In cool springs, late-flowering plants produce more seeds (175 against 100); in warm springs, early-flowering plants do (165 against 110). A change in spring temperature reverses which phenotype leaves more offspring, so the selective pressure on flowering time changes with the environment.

CED 7.2.A.2 · Read this in Fix

1. A0.75
   A student who confuses the survival rate of a phenotype with its frequency among survivors picks this: 60/80 = 0.75 is the proportion of resistant insects that survived. The frequency among survivors is 60/106.
2. B0.08
   A student who assumes one spraying leaves the population's makeup unchanged picks this: 80/1,000 = 0.08 is the frequency before spraying. Among the survivors it is 60/106 = 0.57.
3. C0.06
   A student who divides by the population before spraying picks this: 60/1,000 = 0.06. The frequency among survivors must be divided by the number of survivors, 106.
4. D0.57 Correct
   Among the 106 survivors (46 susceptible and 60 resistant), 60 are resistant: 60/106 = 0.57. Before spraying the frequency was 80/1,000 = 0.08, so a single spraying greatly increased the frequency of the phenotype with higher fitness in this environment.

Working Survivors: 46 susceptible + 60 resistant = 106. Frequency of the resistant phenotype among survivors = 60/106 = 0.566, which rounds to 0.57. Before spraying it was 80/1,000 = 0.08, so one spraying raised the frequency of resistance about sevenfold. Distractors: 60/80 = 0.75 is the proportion of resistant insects that survived, not their frequency among survivors; 80/1,000 = 0.08 is the frequency before spraying; 60/1,000 = 0.06 divides the resistant survivors by the population before spraying.

CED 7.2.A.3 · Read this in Fix

1. APlants in X gained the allele for the rare form when they needed it, which plants in Y were unable to do.
   A student who thinks organisms produce the traits they need picks this. New alleles arise by random mutation, not because they are needed; X persisted because some of its plants already carried the allele for the rare form before the soil changed.
2. BThe salt caused new mutations for the rare form in X's plants, and these plants then went on to survive.
   A student who thinks an environmental challenge causes the mutations that help organisms survive it picks this. The rare form was present in X before the soil became saltier; the salt changed which plants left more offspring.
3. CX already had plants with an inherited form suited to salty soil for selection to act on, whereas Y had none. Correct
   Natural selection can act only on variation that is present. In X, plants with the rare form were more likely to survive and reproduce in salty soil and passed on their allele, so X persisted and the rare form became common. Y had no plants with the rare form, so no plants were well suited to the new conditions.
4. DRare-form plants in X pumped out extra salt to protect the other plants, so the population as a whole survived.
   A student who thinks traits work for the good of the group picks this. The rare form helps the plants that carry it survive and reproduce; it became common because those plants left more offspring, not because it protected others.

CED 7.2.B.1 · Read this in Fix

1. AGreater resistance increases fitness where insecticide is sprayed but decreases it where it is not. Correct
   In the sprayed area, relative fitness rises from 0.2 to 1.0 as resistance increases; in the unsprayed area it falls from 1.0 to 0.7. The same phenotypic variation increases fitness in one environment and decreases it in another.
2. BGreater resistance increases fitness in both areas, as resistance helps insects whether or not they are sprayed.
   A student who thinks a favorable trait is favorable everywhere picks this. In the unsprayed area the model shows fitness falling as resistance increases, from 1.0 to 0.7.
3. CGreater resistance decreases fitness in both areas, as resistance alleles arise by mutation and are harmful.
   A student who thinks mutations are always harmful picks this. In the sprayed area the model shows fitness rising with resistance, from 0.2 to 1.0; whether an allele is harmful depends on the environment.
4. DOnly high-resistance insects reproduce where spray is used, and only low-resistance insects reproduce where it is not.
   A student who thinks selection is all or none picks this. The model gives low-resistance insects a relative fitness of 0.2 in the sprayed area and high-resistance insects 0.7 in the unsprayed area, so the less favored types still reproduce; they leave fewer offspring, not none.

CED 7.2.A.3 · Read this in Fix