Speciation can happen when two related species are isolated geographically.
One way to study speciation indirectly is to examine geographical variation, or how the characteristics of organisms differ between different locations. We can then infer from the variation how speciation occurs. During the early 20th century, biologists such as David Starr Jordan3 and Ernst Mayr4 used this approach and noticed that, in most cases, two closely related species do not occur at the same location nor are they distantly separated. Rather, they usually occur in geographically adjacent regions that are separated by a geographical barrier such as a mountain range or a body of water. The biologists concluded from this pattern that:
- Speciation often begins when a single species becomes geographically separated into two populations. Individuals cannot travel between the populations, preventing the two populations from interbreeding.
- Because the two populations cannot exchange genes, and because they may be subject to different environmental conditions, they slowly evolve differences.
- Eventually the two populations become different enough that they do not interbreed even if they come into contact (in other words, they are ‘reproductively isolated’), and are therefore separate species.
These conclusions were based on broad patterns in the distribution and relationships of many species. But determining how speciation occurs in any particular case can be difficult, because we are usually only presented with the outcome of the process (two species) and we often have no record of their common ancestor or the intermediate forms that occurred during speciation.
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