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Interspecies hybridization

Explanation:

Darwin's observation of the limited number of similar bird varieties was due to interspecies hybridization .

The phenomenon called hybridization occurs through genetic crossing involving two animals of different species, in the case of the question above, two birds of different species. The result of this hybridization is that the offspring generated from this crossing, in most cases, do not have organisms with the ability to reproduce. This is because the countries of this offspring, being of different species, have incompatible genes, making the offspring sterile, in most cases, but there are exceptions.

This phenomenon causes the emergence of new species, which will end up always having a small population, due to the inability to reproduce. Thus generating a limited number of similar varieties within the species.