Shape Matters More Than Size

I read an article on LiveScience titled, Evolution of Genitals: Shape Matters More Than Size. They use data from scarab beetle to demonstrate that shape of genitals is significant. It is important not only for the parents to produce offspring, it also plays a role in evolution of the species. If a population of species has been isolated from another physically, the two populations might gradually and indecently develop so different shape of genitals that even when they are merged again, they cannot breed with each other. Therefore allopatric speciation occurs. However, the size of genitals does not show such significance in this case.