Evolution of Galapagos Finches
In the Galapagos Islands, a species of finch known as Geospiza magnirostris is found primarily on the island of Santa Cruz, where its main food source is the seeds of the plant Tribulus cistoides. A volcanic eruption on a neighboring island created a land bridge to a previously isolated island, which the finches colonized. This new island has a similar plant, Tribulus terrestris, which produces seeds of slightly different sizes and has a different flowering season. Over time, two distinct populations of Geospiza magnirostris evolved: one that continues to feed on Tribulus cistoides and another that exclusively feeds on Tribulus terrestris. Researchers have noticed that the beak size of the finches has diverged significantly between the two populations. The finches feeding on Tribulus cistoides have retained the ancestral beak size, while those feeding on Tribulus terrestris have developed larger beaks to handle the larger seeds. There is minimal interbreeding between the two populations.
Context on Galapagos finches
Question 1
The divergence between the two populations of Geospiza magnirostris is likely to have occurred due to which of the following factors?
A change in the weather pattern that affected the flowering season of Tribulus cistoides.
The introduction of a new food source in the form of Tribulus terrestris seeds on the neighboring island.
An increase in predators on the original island of Santa Cruz, forcing finches to migrate.
The eruption and subsequent land bridge allowed for gene flow between previously isolated islands.
Question 2
Which of the following isolating mechanisms is most likely responsible for the limited interbreeding between the two populations of Geospiza magnirostris?
Geographic isolation since the finches are now separated by water.
Behavioral isolation due to differences in feeding behavior.
Gametic isolation due to incompatibility of gametes from the two populations.
Temporal isolation due to different flowering seasons of their respective food plants.
Question 3
Suppose the finches from the two populations do interbreed and produce offspring with intermediate beak sizes. These offspring, however, have lower survival rates because they cannot effectively process the seeds of either Tribulus cistoides or Tribulus terrestris. What kind of selection is illustrated by this scenario?
Directional selection favoring specialization in beak size.
Disruptive selection favoring either the ancestral or larger beak size.
Stabilizing selection favoring intermediate beak sizes.
Sexual selection based on beak size attractiveness.
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