Of the 11 extant sub-species of tortoise in the Galápagos, 5 are located on one island
(Isabela). Each of these sub-species is located at a higher elevation of 5 separate volcanoes, kept apart by features that make
it impossible for the tortoises to cross (mainly rough lava fields). Identify and discuss this method of speciation. Find one
other example of this type of speciation in the Galápagos. Find an example and discuss the contrasting type of speciation
(without separating barriers). If possible, use organisms you have directly observed.
Galápagos mockingbirds (Nesomimus)
are another example of geographically isolated speciation both as a group and as individual species
and subspecies. The genus Nesomimus includes four species and six subspecies, all of them are endemic to the Galápagos archipelago.
Initially, when the mockingbird first arrived in the Galápagos it diverged from its continental cousin to become a new genus. Then
after establishing themselves on the islands a closer scale geographical isolation occurred multiple times producing the multiple
species and subspecies.
Being a land bird it would be rare for this species to fly long distances over water. So, having arrived 1,000 km from its nearest
relative, isolation would occur preventing the mixing of the two population's genes. Then by going through the process of adaption
to their new environment in isolation, speciation occurred. Moreover, individuals of this species over time populated other islands
becoming geographically isolated from each other allowing speciation to occur differentiated adaptation strategies in the various
climates of each of the islands.
Damselfish in the genera Abudefduf found in the shallow rocky waters around the islands have diverged to form separate species. The
panamic sergeant major (A. troschelii)
night sergeant (A. concolor)
both occupy the same area but have carved out
differing ecological niches. A. troschelii primarily feeds on plankton near the surface of the water while A. concolor primarily
feeds on sessile bottom dwelling mollusks and invertebrates. Both species also graze on algae at the bottom, however, this difference
of feeding habits causes an ecological isolation allowing speciation to take place. Furthermore, A. troschelii feeds during the day
and A. concolor feeds during the night increasing the isolation between the species further isolating the two species ecologically.