SG-1 and the Prometheus are recruited by Thor to battle replicators that have taken over a planet in the Asgard home galaxy, but are soon taken captive by a new line of sentient, human-form replicators who intend to use the team's knowledge to help them achieve their goal of conquering other worlds.
"Unnatural Selection" provides examples of the following tropes:
- Admiring the Abomination: Jonas finds the idea of replicators "fascinating" before (and to an extent, after) he actually comes face to face with them.
- Big Eater: O'Neill catches Teal'c and Jonas hanging out in one of the storage rooms on board the Prometheus, gorging themselves on ice-cream before it can have a chance to melt. Teal'c in particular becomes very possessive after O'Neill tries to steal some, and then takes O'Neill's carton after finishing his own.
- Blue and Orange Morality: Teal'c describes the replicators as this.Teal'c: Replicators exist merely to increase their numbers and do so without prejudice. They are no more evil than a virus.
O'Neill: A really evil virus.
- Blunt Metaphors Trauma: Possibly. It's either that, or extremely deadpan sarcasm.Thor: If you wish to reconsider —
O'Neill: No, no. I full well expected the other shoe to drop eventually.
Thor: We can only hope that this will be the last footwear to fall.
- Call-Back: O'Neill asks if Thor needs the team to come up with another dumb idea.
- Continuity Nod: Thor mentions that the Asgard have been studying the android Reese for a means to combat the replicators.
- Faux Affably Evil: The human-form replicators, especially First, who remains polite and charming even while mind-raping the team.
- For the Evulz: There's no real reason for the human-form Replicators to make the Mind Probe experience as painful as it is, or even for them all to force their way into each mind - with the exception of Fifth, who has to be bullied into it by First and only do so after Carter lets him (and does so considerably more gently), they mostly just seem to want to do it because they feel like it.
- Hive Mind: Fifth states that the human-form replicators have a shared consciousness and can mentally control the regular replicators.
- Mind Probe: The human replicators do this by shoving their hands into the heads of the team (though as Fifth shows, they don't strictly have to), creating dreamlike worlds from which they can extract the required information.
- Muggles Do It Better: Thor cites this as his reason for requesting the help of the team, due to their possessing physical strength and military expertise that the Asgard lack.
- My Greatest Failure: First uses the death of O'Neill's son against him, threatening to make him relive the memory every time he refuses to cooperate.
- Nice Job Breaking It, Hero!: The Asgard attempt to trap all of the replicators inside a time dilation field, but don't count on the replicators using the technology against them and altering the machine so that time inside the field is sped up rather than slowed down, allowing them all the time they need to create more and more of themselves.
- No, Mr. Bond, I Expect You to Dine: The replicators invite the team for a meal and insist that they eat, though they make no pretenses about the fact that they're only doing so to ensure that they have the energy to withstand repeated bouts of Mind Rape.
- No-Sell: After First drops the bombshell that he and the others are replicators, O'Neill, Teal'c and Jonas open fire while they endure the hail of bullets without a flinch.
- Product Placement: The ice cream that Teal'c, Jack, and Jonas enjoy — and that Teal'c briefly fights with Jack over, swapping Jack's full pint for his nearly-empty one — is clearly Ben & Jerry's; no attempt is made to conceal or obscure the logo, and it is all the same brand.
- Ridiculously Human Robot: The human-form replicators.
- Shoot the Dog: O'Neill and Carter betray Fifth, promising to take him with them and then abandoning him in the time dilation field with the others after he helps them escape. O'Neill insists that they didn't have a choice, however, given what the consequences could be if any of the replicators were allowed to escape.
- Shout-Out: O'Neill apparently wanted to call the X-303 the Enterprise.
- Skewed Priorities: Hammond is revealed to have his grandchildren listed above the President on his speed dial.O'Neill: You might wanna get upstairs and punch one on the old speed dial.
Hammond: My grandchildren?
O'Neill: ...Two, then. I think the President might wanna know what the Asgard have in mind for our new ship.
- The Social Darwinist: First has shades of this, comparing a replicator conquest of the galaxy to humanity's position as the dominant species on Earth, claiming that it only makes sense for the strong to subjugate the weak.
- Token Good Teammate: Fifth is the only one of the human-form replicators to demonstrate empathy and compassion as a result of First attempting to correct the "design flaw" in Reese's programming during his creation. Carter even says that he seems "almost human" compared to the others.
- What Did You Expect When You Named It ____?: O'Neill objects to Earth's first starship being called Prometheus for precisely this reason.O'Neill: It's a Greek tragedy, who wants that?
- Year Inside, Hour Outside: The Asgard attempt to trap the replicators using a Time Dilation machine to invert this, but the replicators manage to hack the device so that it plays the trope straight, requiring the team to go in and fix the problem.