24.2 speciation can take place with or without geographic separation ow that we have a clearer sense of what constitutes a unique species, let's return to our discussion of the process by which species arise from existing species. Speciation can occur two main ways, depending on how gene flow is interrupted between populations of the existing species(Figure 24.5). Allopatric("other country") speciation. allopatric speciation(from the Greek allos,other, and homeland), gene flow is interrupted when a population is divided into geographically isolated subpopulations or example, the water level in a lake may subside resulting two or more smaller lakes that are now home to separate populations (see Figure 24.5a). Or a river may change course and divide a population of animals that cannot cross it. Allopatric speciation can also occur without geologic remodeling, such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population. the flightless cormorant show in Figure 24.1. most likely originated in this way from an ancestral flying species that reached the Galapagos Islands. The Process of allopatric speciation. How formidable must a geographic barrier be to promote allopatric speciation? The answer depends on the ability of the organisms to move about. Birds, mountain lions, coyotes an cross rivers and canyons as can the windblown pollen of pine trees and the seeds of many flowering plants. In con