Q: A very heavy rainstorm floods a mountain river, changing its course and digging a deep canyon through the soft soils of the meadow in the valley below. What happens to the mice on either side of the valley?
When the flooding of the river occurs, it acts as a geographical barrier between two groups of mice that were of a single lineage forcing them to split into two. These mice are now two different populations geographically separated from each other by the canyon and through the genetic waft and the differential population pressures the two groups will go through and they will develop into two different species over a span of time (Futuyma, 1986). This is the process of allopatric speciation that makes two different species develop from the two groups. This process is referred to as Cladogenesis, where a lineage splits into two.
Allopatric isolation (geographic isolation) between the two groups of mice on either side of the valley leads to reproductive isolation in the two groups (Wessells, 1988). When the two populations of mice will be reproductively isolated, they will be free to follow the diverse evolutionary paths. These populations will differentiate for two important reasons:
- The different geographic areas on either side of the valley might have different population pressures such as differences in rainfall, predators, temperature and competitors causing the two populations to differentiate (Rosenzweig, 1995).
- The two regions on either side of the valley might not be very different but the two populations might differentiate due to mutations and genetic combinations that may occur in each (Rosenzweig, 1995).
The differences in the two populations will also depend on the power of selective pressures causing rapid change. Given selection and time, the two populations will finally develop into different species.