The book was much clearer on this: Jurassic Park did their gene splicing on Xenopus frogs, whose principle "tell the kids about it" trait is their ready ability to change sex. I mean, Edmund Scientific sold their eggs as "X-Ray Frogs Mysteriously Change Gender" in the back of Boy's Life and Popular Mechanics for fifty years. They're great for research because you don't even really need to worry about breeding them as they pretty much auto-select. That whole "Nature finds a way" thing was David Koepp blithely erasing the bit where Jurassic Park failed through abject stupidity because it stretched credulity that scientists could simultaneously be so brilliant and so bone-headed.
At least it did before gain-of-function research on highly-contagious pathogens.