An illustration is given with a drawing in the shape of 'Y.'

1. The bottom part of 'Y' consists of purple dots, representing members of a species sharing a common gene pool. Over time, genes are shared by interbreeding.
2. At intersection point, reproductive isolation occurs.
3. The left and right side piece of 'Y' consists of blue and pink dots respectively showing that over time, isolated gene pools diverge into separate species.