An attempt to look at “isolation” from a different perspective…

If an original population is separated into two sub-populations through geographical isolation so that no more gene flow can take place, we speak of allopatric speciation. The gene pools of the two subpopulations now develop differently, since different environmental factors have an impact in the geographically isolated habitats and therefore randomly occurring mutations that occur randomly are passed on with different succes.
 
As a result, both gene pools diverge from each other in a way that no fertile off springs result after mating events of two individuals from each subpopulation. A new species is created.
By chance (e.g. through a storm or through driftwood) some specimens (at least two - male and female, or one fertilized female) of a finch species came to the geographically very isolated Galapagos Islands. Initially, the songbird species reproduced very strongly because, in addition to the excessive food supply, there were no predators on the island. At some point, however, the pressure of intraspecific competition on the finches increases because  of limited habitat and food.. Geographical separation raises the possibility of the development of the parent species in different directions. From the island of origin, single individuals can settle on to another island by chance. The process of rapid reproduction is repeated, but now under different conditions (different environmental influences).
 
Thus, different selection factors act on the subpopulation than on the starting population. Since only a small part of the alleles of the original population was included in the subpopulation, the gene pool is limited. Random genetic mutations of these individuals are more likely to get fixed in the new subpopulation. The inter-specific differences increase, we speak of allopatric speciation. In addition, the finches found a habitat on the Galapagos Islands with dozens of previously unoccupied niches. By occupying those, and dozens of subspecies initially emerged from the original subpopulation.
 
Darwin's finches are about 20 cm tall and differ mainly in the shape and size of their beak as well as in their different ways of living, especially feeding. Their chants also differ among the species.