Natural Selection

9.92 − 9.40 = 0.52 mm. The 1976 bar runs 9.28-9.52 mm and the 1978 bar 9.78-10.06 mm; they do not overlap, so the increase is unlikely to be chance alone.

• Correct: Mean beak depth rose by about 0.5 mm, and the error bars do not overlap, so the difference is likely real.: Correct: a 0.52 mm rise with non-overlapping ± 2 SE bars.
• Mean beak depth rose by about 0.5 mm, but the error bars overlap, so the data do not show a real difference.: The bars end at 9.52 and start at 9.78 mm, so they do not overlap.
• Mean beak depth rose by about 5 mm, which is larger than the year-to-year change at any later time.: The rise is about 0.5 mm, not 5 mm; the axis is in tenths of a millimeter.
• Mean beak depth stayed about the same, because the change is smaller than the size of the error bars.: The bars are about ± 0.1 mm wide, far smaller than the 0.5 mm change.

Beak depth is heritable and affected survival during the drought, because only deep beaks could open the remaining seeds. Survivors were mostly deep-beaked, so their offspring had deeper beaks: directional selection.

• Correct: In the drought, deep-beaked birds cracked the hard seeds left, survived and bred more, and their chicks inherited deep beaks.: Correct: differential survival in the drought, then inheritance by the next generation.
• Chicks hatched after the drought grew deeper beaks because their parents passed on the beak growth they gained by cracking hard seeds.: Changes gained during life are not inherited; the parents' alleles, not their experience, set the chicks' beaks.
• The drought caused mutations that made beaks deeper in the birds that needed to crack large, hard seeds to survive.: Mutations arise at random, not because birds need them; the drought sorted birds that already differed.
• Birds hatched in 1978 ate harder seeds as chicks than birds hatched in 1976, and the extra work made their beaks grow deeper.: The stimulus says chicks resemble their parents in beak depth whatever they are fed, which rules out diet as the cause.

In the drought, deep beaks were favored; after the wet year, abundant small, soft seeds favored smaller beaks, which handle them efficiently, and smaller birds need less food. A trait's value depends on the environment.

• Correct: The favorable beak depth depends on the food supply, so selection reversed when the seeds changed.: Correct: changing food changed which beak was favorable, and selection followed.
• Selection in the drought was too weak to last, so beak depth returned to its starting value after a few years.: The 1978-1982 means stayed high for years; they fell only after the food changed in 1983.
• Wet years cause mutations for smaller beaks, which undo the mutations for deeper beaks caused by the drought.: Weather does not cause specific mutations; it changes which existing beak depths survive and breed.
• The population has one fixed best beak depth, and the 1984-1985 values show it drifting back toward that value.: There is no fixed best value; the favored depth changed with the seeds available.

Shortened and lengthened birds were cut and glued. The cut-and-reglued birds had the same handling with no change in length, so any difference between them and the other treatments can be attributed to tail length.

• Correct: To test whether the handling itself affects mating success, with tail length unchanged: Correct: it controls for the procedure itself.
• To provide birds with the longest possible tails for comparison with the lengthened group: Their tails were the normal length; the lengthened group had the longest tails.
• To measure how much a male's tail feathers grow back in the four weeks after being cut: Regrowth was not measured; the dependent variable is the number of new nests.
• To give females a choice between males with natural tails and males with glued tails: Every group lived on its own territory; the reglued group was not set up as a choice test.

This is sexual selection by mate choice. Fitness is offspring, not survival: a male with a long tail may die sooner but fathers more chicks, so long-tail alleles increase.

• Correct: Females prefer long-tailed males, and the extra matings outweigh the survival cost, so long-tail alleles spread.: Correct: mating success can outweigh a survival cost.
• Long tails help males escape predators by distracting them, so long-tailed males live longer than other males.: The stimulus gives no evidence that tails help escape; the data concern mating success.
• Females have long tails too, so the trait is favored in both sexes for the same reason.: The stimulus says males' tails are several times longer than females': a sexual dimorphism.
• Males grow longer tails during the breeding season because they need them to attract a female.: Need does not produce traits, and individual growth in one season does not explain why the trait evolved.