It is a form of reproductive isolation through which a new species evolves within one population. The one condition that is needed for this to happen is for newly formed species to be able to reproduce and to exchange genes.
It is done without geographic barriers (both species live in the same location), as opposed to allopatric speciation.
It is NOT the case here because G. conirostris and G. fortis come from different areas to begin with and not from the same species.
What is reproductive isolation ?
There are two types of reproductive isolation :
pre-zygotic
post-zygotic
Pre-zygotic isolation happens where there are discordances in terms of space and time, behavior and mechanics (reproductive organs that are different).
It IS the case here because there was a spatial barrier between the 2 species : it is said that the male finch did NOT originate from the Galapagos.
Therefore, even though the species are able to physically mate, they wouldnât have been able to without the immigration of the male finch.
What is post-zygotic isolation ?
Like we just said, it is a type of reproductive isolation.
When two animals of 2 different species reproduce, their offspring is considered âhybridâ and is, more often than not, unable to reproduce.
It is not the case in this question because the different offsprings are able to reproduce.
But if the two birds (G. conirostris and G. fortis) were from different species, then how were they able to produce fertile offspring in the first place? Arenât those two characteristics (reproduction & fertile offspring) indicators of members of the same species?
Reproduction & fertile offspring are not always indicators of members of the same species; in this case the lack of post-zygotic isolation + lack of a subset of reproductive isolation (behavioral isolation) was already established. Basically its not like ââdifferent species cannot mate and have fertile offspringââ is a rigid rule, it just speaks to species separation mechanisms probably.
