This one feels rather empty of substance.
The Farscape Wiki has a summary here.
First of all, I just need to get one science-y nitpick out of the way:
The terms 'plant' and 'fungus' are not interchangeable. Plants have chlorophyll and produce their own food. Fungi must absorb nutrients from some other source. We're talking two completely different kingdoms here! AAAAAARRRRGH!
Okay, now -- the fact that I noticed the above mistake is actually a pretty good indicator as to the overall quality of this episode. As I remarked to SABR Matt in a recent chat session, I'm willing to put aside messed up and/or implausible science so long as the story is engaging. Absent a good plot, however, I immediately start carping on all the little things the writers get wrong -- and Natural Election doesn't really have a good plot.
Given that Moya's life is imperiled once again, this episode should've elicited at least some emotional response. Alas, the treatment of Moya's suffering here really pales in comparison to, say, the similar situation that arose in the Liars, Guns, and Money trilogy. Indeed, Moya is almost completely ignored in favor of the soap opera unfolding between Aeryn and John. Don't get me wrong: I have nothing against the show's central relationship. But this is the stretch of the series in which I completely lose interest in their issues. Just get the paternity test already!
This script wasn't scraped from the absolute bottom of the barrel, but it is pretty boring.
The performances were thoroughly average.
There's nothing worth reporting on this front.