Speciation (College Board AP Biology)

In 1981 a single immature male Geospiza conirostris finch flew more than 1000 kilometers from the Galåpagos island of Espanola to the Galåpagos island of Daphne Major, where no G. conirostris finches were living. The immigrant finch bred with a female G. fortis, a species of finch common on Daphne Major. The F₁ finches and later generations interbred only within their lineage. By 2012 scientists counted 23 individuals, including eight breeding pairs, within this hybrid lineage on Daphne Major. The hybrid lineage became known as Big Bird.

Birds with different beak shapes and sizes eat different of food. The dimensions of the Big Bird beaks relative to the beaks of the major competitor finch species on Daphne Major are shown in figure 1.

Figure 1. The dimensions of the beaks of the Big Bird lineage and of its major competitor species in 2012 on Daphne Major. Each symbol represents the beak dimensions of a single bird.

The Big Bird lineage became reproductively isolated from G. fortis. Describe one prezygotic mechanism that likely contributed to the reproductive isolation of the Big Bird lineage from G. fortis.

Based on the data in figure 1, explain why the Big Bird population has been able to survive and reproduce on Daphne Major.

A virus infects and kills all G. magnirostris on Daphne Major but does not affect the other finch species.

Assuming food type and availability stay the same, predict the most likely change in the beak phenotype of the Big Bird population after six more generations.

Provide reasoning to justify your prediction in part (c).

Existing isolated brook trout populations in Newfoundland, Canada, were once part of a larger population that was fragmented at the end of the most recent glaciation period about 10,000 to 12,000 years ago. Researchers investigated 14 naturally separated stream populations of brook trout. They found that the populations are all genetically distinct and show differences in morphology.

Describe the prezygotic barrier that results in these genetically distinct populations.

Brook trout with longer fins are able to swim faster than brook trout with shorter fins. In one of the Newfoundland streams, the main prey of the brook trout evolved to move faster. For brook trout living in this stream, explain the difference in fitness between longer-finned individuals and shorter-finned individuals.

If two morphologically and behaviorally distinct populations of brook trout remain isolated for many generations, predict the likely impact on both populations.

Researchers claim that there are more genetic differences between any two current brook trout populations than there are between any single current population and the ancestral brook trout population from which all the trout are descended. Provide reasoning to justify their claim.