7.1 Introduction to Natural Selection

1. AEach individual changes its heritable traits to suit the environment during its life, and the population changes as its members adjust.
   A student who thinks individuals evolve picks this. Natural selection does not change an individual's inherited traits; it changes how common those traits are in the population over generations.
2. BOrganisms produce new traits when they need them, so over time the population gains the traits that its environment demands.
   A student who thinks traits arise because organisms need them picks this. Variation arises at random with respect to need; selection acts on variation that already exists.
3. CIndividuals with heritable traits suited to their environment leave more offspring, so those traits become more common. Correct
   Natural selection works through heritable variation: individuals whose traits suit the environment survive and reproduce more, so in each generation a larger share of the population inherits those traits. The population evolves; no individual changes its inherited traits.
4. DEach generation improves on the one before, so the population advances toward a more complex and perfect form.
   A student who thinks evolution is progress toward a goal picks this. Selection favors whatever leaves more offspring in the current environment, which need not be more complex, and its direction can change.

CED 7.1.A.1 · Read this in Fix

1. ALight beetles darken during their lives, so that more beetles are dark in each generation that follows.
   A student who thinks individuals change to suit their environment picks this. Each row in the model is a new generation; the change is in the proportions of inherited colors among the offspring, not in the color of any individual beetle.
2. BThe proportion of dark beetles rises in each generation, as dark beetles survive and reproduce more. Correct
   The model shows 4 of 10 beetles dark in generation 1, 6 of 10 in generation 2 and 8 of 10 in generation 3, so dark beetles must be surviving and reproducing more than light ones, and their inherited color becomes more common in each generation.
3. CAll light beetles are eaten in each generation, so only dark beetles are left to reproduce.
   A student who thinks selection removes every individual without the favorable trait picks this. Light beetles are still present in generations 2 and 3; they are less likely to survive than dark beetles, not certain to die.
4. DBeetles become dark because they need camouflage, so more dark beetles appear in each generation.
   A student who thinks traits arise because organisms need them picks this. Dark and light beetles are both present from generation 1; birds change which of them survive and reproduce, and need does not produce the color.

CED 7.1.A.2 · Read this in Fix

1. AA small female that lived 3 years and produced 9 offspring that survived to breed Correct
   Evolutionary fitness is measured by reproductive success: the number of offspring that survive to reproduce. This female's 9 breeding offspring are more than any other individual's, so she contributes most to the next generation despite her small size and short life.
2. BA large male that won every fight he entered and fathered 2 offspring that survived to breed
   A student who equates fitness with size and strength picks this. Winning fights matters only if it leads to offspring; this male left 2 breeding offspring, fewer than the small female's 9.
3. CA female that lived 9 years, the longest of all, and produced 4 offspring that survived to breed
   A student who thinks fitness means survival picks this. A long life gave this female more chances to breed, but she left 4 breeding offspring, fewer than the female that lived only 3 years.
4. DA female that lived 4 years and produced 30 offspring, none of which survived to breed
   A student who counts all offspring produced picks this. None of the 30 offspring reproduced, so this female passed her genes to no later generation; her reproductive success is zero.

CED 7.1.B.1 · Read this in Fix

1. ABeak depth evolved steadily in one direction, toward the best depth for this island.
   A student who thinks evolution moves steadily toward a goal picks this. Mean beak depth rose after the drought and fell after the wet year; which depth was favored depended on the seeds available that year.
2. BNo evolution occurred, as mean depth at the end was about the same as at the start.
   A student who thinks only lasting, long-term change counts as evolution picks this. The heritable makeup of the population changed between generations twice; each change is evolution even though the second reversed the first.
3. CIndividual birds' beaks deepened after the drought and grew shallower after the wet year.
   A student who thinks individuals evolve picks this. Each point is the mean for the young hatched that year, a different group of birds; the change is in which beak depths the young inherited, not in any adult's beak.
4. DThe direction in which beak depth evolved changed when the island's conditions changed. Correct
   Young birds hatched after the drought had deeper beaks (about 9.41 mm in year 3 to 9.68 mm in year 4), and young hatched after the wet year had shallower beaks (9.65 mm to 9.44 mm). Beak depth is heritable, so the population evolved in one direction after the drought and in the opposite direction after the wet year: a fluctuating environment changed the direction of evolution.

CED 7.1.B.2 · Read this in Fix

1. AAll birds with beaks shallower than 9 mm died, and all birds with deeper beaks survived the drought.
   A student who thinks selection is all or none picks this. The graph shows 10% and 18% survival in the two shallowest classes and only about half survival in the deepest class: deeper beaks raised the chance of survival, but no class all died or all survived.
2. BBirds survived the drought by deepening their beaks as they cracked the large, hard seeds left.
   A student who thinks use changes a trait picks this. Beak depth was measured before the drought, and the data compare survival among birds that already differed in beak depth; they give no evidence that any bird's beak changed.
3. CThe birds most likely to survive were the largest and strongest, whatever their beak depth.
   A student who equates fitness with size and strength picks this. The data show survival changing with beak depth, rising from 10% to 52% across the classes, so beak depth was not irrelevant; body size was not measured.
4. DThe deeper a bird's beak before the drought, the more likely the bird was to survive it. Correct
   Survival rises steadily with beak depth, from 10% in the 7–8 mm class to 52% in the 11–12 mm class. When the remaining food was mostly large, hard seeds, birds with deeper beaks were more likely to survive the competition for it.

CED 7.1.A.2 · Read this in Fix

1. AA group of 60 birds with a similar range of beak depths, given a plentiful supply of the same large seeds Correct
   This group differs from the experimental group only in the amount of food, so competition for seeds is removed while the birds, the kind of seed and the other conditions stay the same. If survival depends on beak depth only when seeds are limited, the hypothesis is supported.
2. BA group of 60 birds with a similar range of beak depths, given no seeds at all during the 8 weeks
   A student who thinks the control is the group that receives nothing picks this. Birds given no food would starve whatever their beaks; this tests the effect of having no food, not the effect of competition for a scarce supply.
3. CA group of 60 birds with a similar range of beak depths, given the same limited supply of large seeds
   A student who thinks the control is set up exactly like the experimental group picks this. This group is a replicate: it shows whether the result repeats, but it still has scarce food, so it cannot show what happens without competition.
4. DA group of 60 wild birds of the species, observed in their natural habitat over the same 8 weeks
   A student who thinks the control is the population living naturally picks this. Wild birds differ from the enclosed birds in food, predators, weather and much else, so a difference in survival could not be attributed to scarce food.

Working No calculation. The factor being tested is the scarcity of food, which creates competition. The control must differ from the experimental enclosure only in that factor: the same kind of birds and the same large seeds, but a plentiful supply. A group with no food tests starvation, an identically treated group is a replicate, and wild birds differ in many uncontrolled ways.

CED 7.1.A.2 · Read this in Fix

1. ALong-limbed parents tend to have long-limbed offspring, and parents with lost tail tips tend to have short-tailed young.
   A student who thinks acquired characteristics are inherited picks this. The first part is right, but a tail tip lost to a predator is an injury, not a heritable difference, so it is not passed on.
2. BLong-limbed parents tend to have long-limbed offspring, and a parent's lost tail tip does not affect its offspring. Correct
   Limb length is heritable, so the offspring of the long-limbed lizards that survive tend to have long limbs, and long limbs become more common. Losing a tail tip is an injury acquired during life; it does not change the genes passed on, so the offspring have complete tails.
3. COffspring grow long limbs during their lives when predators are common, whatever the limb length of their parents.
   A student who thinks organisms develop the traits they need picks this. Limb length is heritable, so offspring resemble their parents; predators change which lizards survive, not the limbs offspring grow.
4. DOffspring all have long limbs, as predators remove every short-limbed lizard before it can reproduce.
   A student who thinks selection removes every individual without the favorable trait picks this. Longer-limbed lizards escape more often, but some short-limbed lizards also survive and reproduce, so the change is in proportions, not a complete switch.

CED 7.1.A.2 · Read this in Fix

1. ABirds crack hard seeds → their beaks deepen with use → their offspring inherit the deeper beaks → mean beak depth rises
   A student who thinks characteristics acquired through use are inherited picks this. A change in an adult's beak caused by use would not be passed on through its genes; the change comes from which birds reproduce.
2. BBirds vary in beak depth → seeds are scarce → each bird's beak deepens over its life → mean beak depth rises
   A student who thinks individuals evolve picks this. Natural selection changes the population by changing which birds leave offspring, not by changing each bird's beak.
3. CDrought begins → the drought causes new deep-beaked variants to arise → these birds then survive → mean beak depth rises
   A student who thinks the environment creates the variants it favors picks this. Variation in beak depth is already present before the drought; the drought changes which birds survive and reproduce.
4. DBirds vary in heritable beak depth → seeds are scarce → deep-beaked birds leave more offspring → mean beak depth rises Correct
   Natural selection needs heritable variation, competition for limited resources, and differences in survival and reproduction. Deep-beaked birds leave more offspring, which inherit deep beaks, so the mean beak depth of the population rises over generations.

CED 7.1.A.1 · Read this in Fix

1. ABlack females have the highest fitness, as they survive for longer than red or yellow females do.
   A student who thinks fitness means survival picks this. Black females live longest but leave the fewest surviving offspring (mean 2.1), so their fitness is the lowest of the three.
2. BRed females have the highest fitness, as their mean is higher than the yellow and black means.
   A student who treats any difference between means as real picks this. Red females are fitter than black females, but the red and yellow bars overlap (both cover 3.6–4.4), so the data do not show that red females are fitter than yellow ones.
3. CRed and yellow females have higher fitness than black; a difference between red and yellow is not shown. Correct
   Fitness is measured by reproductive success. The black females' bar (1.7–2.5) does not overlap the red (3.6–4.8) or yellow (3.0–4.4) bars, so black females are likely to leave fewer surviving offspring. The red and yellow bars overlap, so the data do not show that one of them is fitter.
4. DRed and yellow females have higher fitness than black, and their overlapping bars show equal fitness.
   A student who thinks overlapping error bars prove that means are equal picks this. Overlap shows only that a difference has not been established; the red and yellow means (4.2 and 3.7) could still differ.

Working No test statistic is calculated; the decision rests on the ±2 SE error bars. Red: mean 4.2, bar 3.6–4.8. Yellow: mean 3.7, bar 3.0–4.4. Black: mean 2.1, bar 1.7–2.5. Black against red or yellow: the bars do not overlap (2.5 < 3.0), so black females are likely to leave fewer surviving offspring. Red against yellow: the bars overlap (3.6–4.4), so a difference between them is not shown, and overlap does not show they are equal. Fitness is measured by reproductive success, so the black females' size and long life do not make them the fittest.

CED 7.1.B.1 · Read this in Fix

1. AThin-shelled snails will become more common, as they produce more offspring now that crabs are gone. Correct
   With no crabs, a thick shell no longer improves survival but still costs resources, so thin-shelled snails leave more offspring. Shell thickness is heritable, so the proportion of thin-shelled snails rises: the change in the biotic environment reverses the direction of selection.
2. BThick-shelled snails will keep becoming more common, as a thick shell is the favorable trait for these snails.
   A student who thinks a favored trait stays favored in every environment picks this. A thick shell was favorable while crabs were common; without crabs it lowers the number of surviving offspring, so it is no longer favored.
3. CThe proportions will stay the same, as without crabs neither shell type is more likely to die.
   A student who thinks selection acts only through death picks this. Survival may now be similar, but thin-shelled snails leave more surviving offspring, and differences in reproduction change the population too.
4. DSnails will build thinner shells now that they no longer need thick ones and will pass this change on.
   A student who thinks acquired characteristics are inherited picks this. A snail does not change its heritable shell thickness through disuse, and any change it made during its life would not be passed on; the shift comes from which snails leave more offspring.

CED 7.1.B.2 · Read this in Fix