You are Keith, a 18 year old college student going for your degree in biology. You just have been given a research task to study frogs, and how feeding them various insects alters their health. It is a pretty easy task, but that is to be expected from an introductory level course.
You just finished setting up the aquarium in which you will house your frogs when the greatest mass evolution event hit the planet. The first phase was excruciatingly painful as your body destroyed then completely rebuilt itself all the way down from a genetic level and up to your actual bones and tissues over the course of just 20 minutes.
However after those 20 minutes, the woman you now see in your reflection is more beautiful than any woman you have ever seen before. Your looks are absolutely immaculate with curves in all the right places, but with a figure that just screams runway model. Even your voice and manner of speaking has changed and is now pure, elegant, and quite beautiful.
You are so completely enraptured by your new self that you have forgotten all about your frogs and accidentally knock over the bucket they were being temporarily kept in, letting them roam free around your room.
"Ah, shoot! Wait a second little froggies!" You beg as you awkwardly stumble after them. Your body is now so unfamiliar that even standing is difficult. For one, your FF cup breasts are not helping you in maintaining your balance. Actually, since obviously you aren't wearing a bra, their constant bouncing and jostling around is the main culprit to your lack of balance. You manage to finally corner a frog when the second phase rips through the world.
The changes to your body, while not anything noticable from the outside, yet, are still rather apparent to you. Mainly it is how your goal quickly changes from "securing the frogs and returning them to their now finished aquarium", to "securing the frogs so at least one of them can become my mate". You do not understand why the sight of the male frogs is turning you on so much, but you simply cannot fight your urge to mate with it.
You squat down onto all fours in front of him and to your surprise, call to him. In doing so, both of your breasts suddenly swell out to ludicrous proportions and quickly shrink back down in size as you begin croaking, hoping to get his attention. Meanwhile, your feet and legs have already begun reshaping into frog legs, permanently locking you into your current posture forever. You somehow just know that you never will be able to stand on 2 legs again, but that isn't important right now, as your horniness continues to rise. To your delight, the other frogs seem more than interested in you and all begin to make noise as well.
You begin to strip the rest of your clothes as your body begins coating itself in a slimy mucus. Finally, the numerous male frogs jump onto you and begin the process of taking turns cumming inside of your now changing pussy. By the time your orgy is over with, your transformations have all settled down. The back half of your body is identically shaped to that of a frog, from your large webbed feet to just below your ass, while also including your pussy too. Your pair of vocal sacs have taken up resistance inside your breasts, your tongue is now super long and sticky, your hands are webbed, and your fingers have merged into just 3, including thumbs, per hand. The only other significant change to you, is the fact that you are now well beyond heavily pregnant. Due to your size difference, you were able to be impregnated by every single frog that fucked you. Your countless eggs inside of you all have been successfully fertilized by at least one of your now many mates.
'Being a frog slut doesn't sound so bad actually' you think to yourself. The sex is fantastic, and using your vocal sacs is so damn pleasurable that you likely could easily make yourself cum just by using them enough. It was a bit unusual for female frogs to have vocal sacs, but you just chalked it up to being originally a guy. You certainly wasn't complaining about having them. You decide to start going by Keira, as Keith no longer fits your current self nearly as well.
As you lay about the house, thinking about what just happened to you. A quick bit of movement catches your eyes and you instinctively lash out your tongue at it. To your credit, you manage to squarely hit the fly, but you truely wish you had missed as your tongue brings it right back into your mouth. It tastes better than you had been expecting, but thinking about what you are eating still makes the whole thing a pretty awful experience overall, but you manage to swallow it anyways despite your mental unwillingness to do so.
Suddenly the comes a knock at your door. However, as you are now, your realize that opening it would be near impossible as your hands are not meant for using door mobs anymore. You could probably open it eventually, but only after wiping off all the mucus you would put on it just by touching it. Luckily for you though, the door was unlocked, and the woman just opened it and walked inside.
You do not recognize her voice, but she seems to know you enough to know that you live here, as she calls for you using your male name. You sigh, which sounds somewhat like a croak, and leap down from the couch and hop into the other room to see who is in your home. Your reverse harem of frog suitors hop after you as well. You do not mind the escort, as it makes you feel like you are their queen or something, which you have found is kind of a fun concept to think about.
You turn the corner to find another gorgeous woman, though you personally feel that you are still more attractive than her, and proudly push forward your larger breasts to show off a bit. This causes the woman to click her tongue at you with an irritated look on her face. "So it did get you too. And stop doing that. So what if your tits are bigger than mine? I am still taller than you!"
You know exactly who they are now. There is only one person you know of who would ever use their superior height like that.