Biological diversity? Look to the genitals

[O]f all the organs than an animal is provided with, the greatest differences between species are not in their brains or beaks, or in their kidneys or guts, but in their genitals. This applies to cave beetles, bumblebees, and elephant shrews, as well as to velvet worms, land slugs, water and rove beetles, small ermine moths, daddy longlegs spiders, banana and hover flies, egg parasitoid wasps, aquatic annelid worms, hoofed mammals, sharks and rays, primates, guppy fish, damselflies, land planarians, nematode worms, trombidiform mites, and harvestmen. To name but a few.

Menno Schilthuizen, Nature's Nether Regions: What the Sex Lives of Bugs, Birds, and Beasts Tell Us Abut Evolution, Biodiversity, and Ourselves (New York: Penguin Books, 2015), p. 34