Their genetic variation is small enough that they are all capable of interbreeding. This doesn't mean, though, that they all look the same. They vary in terms of body size, body plan, lifestyle, habitat. It's just that all of these things are controlled through epigenetic and environmental factors. Due to epigenetics, the offspring of one animal tends to look similar to its parent, but compared to terrestrial evolution, things can radically change within a few generations.
The way this works, roughly, is that they have thing similar to hox genes in terrestrial animals, but at a more powerful and fine-grained level, and epigenetically toggle-able. Want to become carnivorous? Get wings? Become smaller or bigger? Sure, switch on the right genes and off we go!
So basically, this is a planet where Lamarckian evolution is pretty much true. Classical Darwinian evolution still happens, of course, but it's subtler and more rare, as most environmental changes can be dealt with epigenetically long before selection really has a chance to kick in.
The interesting thing to note here, too, is that these creatures have basically gone up an additional layer of abstraction - of "scripting" - compared to earth-based ones. RNA and DNA are already an abstraction layer above proteins. They are very much the programming language to the machine code of proteins. And much like with programming, where higher-level languages get built on top of lower-level ones, trading power for safety, gene activation is yet another layer on top of that. And it is less powerful and more safe. As a human, if you get a mutation in the wrong place and you're dead or terribly messed up, but get unfortunate gene activation patterns, and the worst kind of thing that can happen is that you're just not very healthy or well-adapted to your environment.