IMAT 2020 Q29 [Isolation and Speciation]

In 1981, an immature male finch (Geospiza conirostris) arrived on an island in the Galapagos.

This male finch was similar to finches of the species (G. fortis) living on the island.

However, G.conirostris was 70% larger than G. fortis and had a distinctly different song.

The diagram shows how, after observing further generations, a new species has evolved.


The success of the new species is due to its larger bill and body size and its unique song.

Which of the following terms is/are illustrated by this information?

  1. sympatric speciation
  2. reproductive isolation
  3. post-zygotic isolation

A. 3 only
B. 2 only
C. 1 and 2 only
D. 2 and 3 only
E. 1 only


The correct answer is B.

What is sympatric speciation ?

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 wouldn’t 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 “hybrid” 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.