This is a bit frustrating, but I couldn't find it through Google or through scanning GOG (though I was fairly sure the game had been released here).

I remember a science fiction game where you would land on a planet and then have to sample or collect DNA from the critters around you in order to produce your own. It's not Impossible Creatures, though, it's an older game that leaned more heavily on the science fiction elements.

It was released on the PC and I remember the game being quite dark (both visually and thematically) and challenging. For some reason, I associate it with SEGA, but I'm not sure how reliable that is.

So, yeah, any help would be much appreciated!

Sounds a little bit like
https://www.mobygames.com/game/genewars

Not sure the dark elements would qualify with this one but

https://www.gog.com/game/spore_collection

Evolva?

on gog? hmm Dungeon Town?

Vaguely reminds me of Evolution wars i think (probably got the name wrong, might be cyborg fighter or something), a Genesis game where you could choose the pieces of your character so you could have an insect head and mechanical arms/legs.

My jaw would pop out of Sri Lanka if Sega touched GOG with a 10-foot pole.

Better grab your jaw, then:

https://www.gog.com/game/the_deadly_tower_of_monsters

Check out the company that released this game.

This is it! I think I may have smushed two games together.

The other one I was thinking was Dark Colony, I believe.

Thank you so much!
Thanks for the replies everyone! In my fuzzy nostalgia brain, I must have mixed Genewars and Dark Colony together into one game.

Glad to hear it, Genewars and Dark Colony are definitely two instabuys for me if they ever end up here :)

Well concerning everything else they do, WHAT is TAKING SO LONG?! I have a hankering for Mega-Man, Resident Evil, Maximo, Dead Rising, Alpha Protocol, and about 3 Alien games. Actually, where's Maximo for Steam while I'm at it?

I'm pretty sure Sega has never released a game on GOG. They acquired the publishing rights to that one and at least one other sometime after their releases here. The fact that the other one(s?) has since been delisted here speaks volumes. (No, I don't remember the name of the other one, or whether there were more.)