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?

Question

agreene · Answer

They will have minor speciation, because they will adapt to their new environment and the two populations will not be able to mate with one another. In other words, it will force what was one population of mice to become two populations.

trancenova · Answer

yep, its called Allopatric speciation: http://en.wikipedia.org/wiki/Allopatric_speciation