0. In the following scenario, at which step do we know that speciation has occurred?
A. A volcano erupts on an island, splitting a population of black birds into two separate populations.
B. Over time, population 1 develops dark red feathers on its back through the process of sexual
selection, but population 2 does not develop these feathers. On occasion individuals from the
populations meet and are able to interbreed to produce fertile offspring.
C. As time increases, population 2 develops longer beaks for digging bugs out of holes in trees.
Population 1 does not develop this trait because they dig in the ground for food with their feet.
On occasion the populations still interbreed and produce fertile offspring.
D. As even more time passes population 1 develops smaller bodies for burrowing in the ground to
hide from predators. Population 2 does not develop this trait because they live in trees. On rare
occasion the populations cross paths and interbreed to produce sterile offspring.

Habitat isolation occurs when populations of a species move to a new habitat, preventing interaction with the original population. Over time, natural selection, mutation, and genetic drift lead to divergence and eventually speciation.

In the scenario described:

Option A: The split caused by the volcano eruption initiates geographic isolation.
Option B: The populations still interbreed, indicating speciation has not yet occurred.
Option C: The development of different traits, like longer beaks, is a sign of divergence but not full speciation.
Option D: Smaller bodies and sterility in offspring suggest reproductive isolation, a key indicator of speciation.

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