Consider a hypothetical animal species, Species X, that can neither fly nor swim. Separate populations of Species X live in the same geographical area and often interbreed. After a period of time, a new species, Species Z, arises from Species X. The following list describes some barriers that may have contributed to the formation of Species Z:
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- A newly formed river separated one population of Species X from the other populations.
- One population of Species X developed a new mating ritual that was not recognised by members of the other populations of Species X.
- One population of Species X began to breed several weeks later than the other populations of Species X.
- A volcanic eruption created a mountain that separated one population of Species X from the other populations of Species X.
The barrier(s) that could have led to the formation of Species Z by sympatric speciation is
- either barrier 1 or barrier 4 .
- either barrier 2 or barrier 3 .
- barrier 1 only.
- barrier 2 only.