This sounds like it could be the Ousters in Rise of Endymion, by Dan Simmons.
The book was written in 1997 so it would have come before your earliest remembrance. While the book is about a lot of things, one of the more memorable moments for me took place in the last quarter of the book, where the two main characters Raul and Aenea are in conversation about the direction of evolution for humans, which is controlled by the Church.
It doesn't have to be just plants. Life adapts . . . birds, men and women in flying machines, you and me in paraglider, people adapted to flight . . .
They then encounter an antagonist, Nemes, who has fingers that grow out into blades and seriously injures Raul.
Nemes held up her hands and her fingernails - already pale and long - extended another ten centimeters, flowing into streaming spikes. Names reached dow with those sharpened nails and peeled back the skin and flesh of her right forearm, revealing some sort of metallic endoskeleton that was the color of steel but which looked infinitely sharper.
After this encounter, Raul wakes up with another character, and it feels like they are able to survive in space at first:
We were in space. The surrounding pod had simply disappeared. We were floating in space - seemed to be floating in space, except for the presence of air to breathe - and we were far out on a branch of a . . .
They then encounter the Ousters, people who have evolved to survive in space, and have certain animal characteristics such as webbed hands, wings, and also exist with other alien races who have evolved to survive in space.
These colonists, our ancestors - most traveling in cold sleep deeper than cryogenic fugue - were among the best ARNists, nanotech, and genetic engineers Old Earth System had to offer. Their missions were to find habitable worlds and - in the absence of terraforming technology - to bio-engineer and nanotech the millions of Old Earth life-forms frozen aboard their ships into viable adaptations for those worlds
I particularly like this description where Raul sees the Ousters in flight:
Everywhere was life and motion: Ouster angels with hundred-klick wings not only flitted among the branches and beyond the leaves, but were hurled deeper into space - inward toward the sun, more quickly outward past the ten-thousand klick root systems; a myriad of smaller life forms shimmered in the blue envelope of atmosphere.
Raul and Aenea don't actually engineer themselves to live in space, but at certain moments in the story it certainly feels like they could have lived among the other Ousters in this way. While the focus of the book isn't on genetic engineering, it's a very memorable section and is a strong theme in this latter part of the book.