Now I'm no Tricky Dicky Dawkins - I've never watched dogs 69ing, for starters - but I do know the definition of, and delineation between, species is hazy. Consider the case of Ark: Survival of the Fittest [official site]. The multiplayer Battle Royale 'em up started as a mode for crafty survival sandbox Ark: Survival Evolved. A subspecies, right? Then it was spun off into a standalone free-to-play game. Separate species? Not quite: Survival of the Fittest is now merged back into Survival Evolved so it can use the Development Kit mod tools. Mutation merging to spawn new mutations? I don't know, man. 6,000 years old and this Earth is still full of surprises.
Ark: Survival of the Fittest is once again part of Survival Evolved. Well, almost. It's still a standalone game but no longer free-to-play, only available with purchases of Survival Evolved once more. However, folks who played when it was free-to-play do get to keep on surviving.
Why the merge? Two things, according to developers Studio Wildcard: modding and monetisation.
Wildcard say that they bringing Survival of the Fittest back into Survival Evolved means no pesky licensing issues or content restrictions with using SE stuff in SotF mods. "We wanted to ensure that modders would have freedom when working with the kit to create the mods that they wanted to see in SotF," they say.
If you fancy modmaking yourself, you can download the SotF Development Kit here.
As for monetisation, Wildcard say they wouldn't be good at monetising SotF and didn't want to learn, so running it F2P would just be a drain. Their words:
"To ensure that SotF would remain a success and a sound choice, Wildcard would have had to put a lot of resources and time into learning how to become a ‘free to play' developer. We would have had to learn how to monetize a free game, notoriously through the use of in-app purchases such as skins and other cosmetics. Ultimately, it's clear this is not who we are, we like to make games, and to make the gameplay fun. We don't know much about monetization, and quite frankly we aren't interested in hiring an economics team to take over that process, it is much more in our and your best interest for Wildcard to solely focus on the development of a game. We just aren't cut out for free to play mechanics. With SotF being part of ARK: Survival Evolved, it'll be able to draw from Survival Evolved resource pool directly, and enable us to continue operating the title."
I might suggest an addendum to that second point: Ark's notoriously bad performance limits its playerbase - not to mention overwhelms its Steam player reviews - so they'd have trouble attracting enough players to support it as F2P. There's a reason why most free-to-play games run on decade-old hardware.