I’ve talked about how evolutionary, dudes don’t matter all that much. There are a handful of animals (including spider mites, bdelloid rotifers, and whiptail lizards) that are LADIES ONLY. They make their babies with a few different methods that all result in a chickload of chicks at the end. What we’ve learned is fellas are sometimes not that useful and that you can’t have a species without the fillies.
BUT APPARENTLY THAT IS NOT THE CASE.
In some species of Corbicula clams, they are ALL DUDES. (Yes, I LOVE the irony of an all-boy species being also all-clams.)
BUT HOW?
These guy-clams are born hermaphrodites (can make both sperm and eggs) and so they selfie in the nature way – meaning they make bebes with their own sperm and eggs. Which isn’t that weird. BUT here’s the weird part. They somehow kick out all of the egg genes during fertilization so they are just sperm clones of each other. All dudes. So these chaps would just be copying themselves forever which is not always the best choice.
Asexuality isn’t that unusual in nature, but one of the big benefits of sex (besides, um, orgasms) is it helps living things have more diversity in their genes, which is generally a plus.
So what these special gentlemen do is they STEAL EGGS FROM OTHER CLAMS. Clam boinking is usually more of a spewing of lady- and dude-spunk into the abyss (or the ocean) and hoping they find each other.
Sometimes the Corbicula clam might find itself with a foreign egg. Instead of scrambling up an omelette, the clam might go the sexy route and fertilize this egg. He might not successfully eject all of the egg’s genes and POW. New genetic diversity and a possibly weird hybrid-clam that is still mostly Corbicula.
Sources
Sexy Science Facts 
