The genre has nothing to do it, or it shouldn't. If it was fantasy, maybe. If anything sci-fi should be way up there in terms of realism, because it attracts people who pick things apart. Halo in particular has a history of in-depth analysis.

: On a fun side note, my wonderful and lovely significant other is
: currently finishing up her phD in Biology, with a specialization in
: Functional Morphology. So trust me when I say I have some pretty decent
: access to a wealth of rational scientific hooks on which to try and build
: relatively plausible justifications for regarding phenotypic variation.

That's interesting. Did you guys consult a biologist on any of the redesigns?