Question

Bird Beak Traits



Scientists collected data about one bird population for five generations. The scientists determined the percentages of birds in the population that had specific beak shape traits, 1-6. The data are represented by the graph shown below.



The line graph titled 'Percentages of Beak Shape Traits in a Population' shows Generation from 0 to 5, in increments of 1, on the horizontal axis. The percentage of population with trait is shown on the vertical axis from 0 to 70, in increments of 10. A solid line labeled 'Beak shape Trait 1' starts at about 15%, gradually falls to 10% in generation 1, slightly rises then falls to below 10% in generation 3, then remains steady through generation 5. A solid line labeled 'Beak Shape Trait 2' starts at about 15%, gradually falls to 10% in generation 2, remains steady at 10% in generation 3, and falls to 0% in Generation 5. A solid line labeled 'Beak Shape Trait 3' starts just below 20%, falls to 10% in Generation 2, rises sharply to about 15% in Generation 3, then falls to slightly above 0% in Generation 5. The dashed line labeled 'Beak Shape Trait 4' starts at 20%, falls to below 10% in Generation 3, and continues to gradually fall to about 5% in Generation 5. Two dashed lines labeled 'Beak Shape Trait 5' and 'Beak Shape Trait 6' start slightly above 20%, rise to above 30% in Generation 1, then gradually rise to about 40% in Generation 3. At generation 3, the beak shape traits diverge. Beak shape trait 5 remains steady at about 40% in Generation 4, then falls to about 35% in Generation 5. Beak Shape Trait 6 sharply rises to 70% in Generation 5.



Why did the beaks in this bird population MOST LIKELY change over time?


Predators caused the change by only consuming birds with certain beak types.


Environmental changes caused the bird population to adapt through natural selection.


Natural selection caused the environment to change and the bird population to adapt.


The birds caused the change by only mating with birds that had certain beak types.

Answer 1

Environmental changes caused the bird population to adapt through natural selection.

The changing frequencies (one beak type rising to dominance while others decline) show differential survival/reproduction of birds with advantageous beaks—consistent with natural selection driven by changing environmental conditions. The other choices either reverse cause/effect or describe narrower mechanisms (predation or mate choice) that are less likely to explain the overall population shift shown.