The squares and circles are different shapes of the same species, polygons.
The Next Generation
Predators have found the population of circles and squares and attempt to consume them
When the predator comes to eat the circles and squares the circles are able to roll away, but the squares are not. The predator eats one of the squares.
Since the circles were able to roll away from the predators more of them survived to reproduce and passed their genes onto their offspring and less squares survived to do the same.
As more generations of circles and squares are born less and less squares survive and less of the p, or square gene, is passed onto the next generation there are more circles that are born in the next generation
The reproductive advantage for the circles was that they were able to move and therefore able to escape from predators and that allowed for a higher chance of survival which slowly became more prevalent in the polygon population