How can geographic isolation lead to speciation?

Geographic isolation leads to speciation by interrupting gene flow between populations, a process known as allopatric speciation.12

When a physical barrier—such as a mountain range, river, or desert—separates a population, the two groups become genetically isolated. Over time, these populations adapt to their unique environments, causing their gene pools to diverge. As they become increasingly different, reproductive isolation often arises as an accidental by-product of this genetic divergence.12

If the populations remain separated long enough, they may become so genetically distinct that they can no longer interbreed, even if the physical barrier is removed. In some cases, natural selection may further promote the development of reproductive isolating mechanisms if hybrid offspring prove less fit than non-hybrids. This process is considered the dominant form of speciation in organisms that reproduce sexually.12