The diagram represents speciation caused by temporal isolation. Multiple populations of a species reproduce during particular breeding times in a year. Populations can become temporally isolated over time, eventually leading to speciation. The y-axis represents, not three years, but hundreds of generations of a hypothetical species. The x-axis represents time over 12 months of the year. Gradual isolation of populations due to differing breeding seasons eventually lead to allochronic speciation.
<a href="//commons.wikimedia.org/wiki/User:Azcolvin429" title="User:Azcolvin429">Andrew Z. Colvin</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>