WMG: Species

Sil was an attempt to make a Cute Monster Girl of a Xenomorph.
It's just about Canon that the Xenomorphs were created for some reason, so it stands to reason that eventually the Powers that Be (like the Space Jockeys) (or one of their researchers) would try to go in this direction. However, a few problems cropped up:
  1. The formula was incomplete, needing a local planetary framework to build on. Therefore, they had to send it someplace with intelligent life so that it would be well-suited for its new home.
  2. They set the sex drive wayyyyyyy too high.
  3. They overestimated how resilient the local males would be. Modern humans are remarkably squishy compared to nearly every other species.

The [1] that sent the signal are actually the Engineers.
And the sample they sent is a fragment of Xenomorph DNA. It was their plan B in case the LV-223 facility WMD wouldn't work. So they sent a piece of genome of their most successful(according to EU successful enough to rebel against them and win) bioweapon and let our stupidity do the rest. Also, Sil and her ilk look an awful lot like a perfect mix of human and xenomorph, in appearance, mindset and abilities. The fact they can be tame and reasonable when treated like people and not like lab rats was not a problem for the Engineers, they know humanity all too well...

Sil and Patrick are not the same type of creature.
It would explain certain inconsistencies. Like why Patrick's true form doesn't look anything like that of Sil's son. And why Sil took much longer to develop from fertilized egg to baby than seems normal for her species (she was presumably implanted in a human surrogate mother, and her species doesn't have whatever mechanisms allowed Patrick's offspring to just suck 7-9 pounds of material out of their hosts in minutes, so her early growth was dramatically slowed down). They are related but distinct species (or subspecies - they're obviously interfertile). This theory is also appealing because it removes the third movie's implication that any offspring Sil had would probably have been sickly and sterile so she could never have been a serious threat to humanity. And it would provide a handy excuse for any fanfic writers who might want to throw out some of the things we learned about the creatures in the second and third movie.
  • Since they seem to be artificial constructs to begin with, this makes a fair bit of sense.
  • Remember that Sil's son was still a baby. It may well be that once they enter the pupa stage for turning into adults they may well turn into the more quadruped form as Patrick.

The aliens are a seed/specialized unit of a superorganism.
The aliens are fast-maturing, fast-breeding creatures which seem to operate largely on hard-wired instinct, which isn't a life strategy that one would typically associate with selection for high intelligence. It would make sense, however, if they're specialized autonomous units designed to serve the purposes some larger superorganism, something like the Tyrannids or Zerg.

The aliens were just misunderstood.
They didn't plan Genocide by Supermodel at all. Perhaps they're very keen on genetic modification and given how resilient alien Sil is, they thought they are giving us a gift. For them it was like sending a machete and a hockey mask, because, hey, a machete is a useful tool, and everybody likes hockey, right? Right?
  • Well, according to Fitch they did first give us an infinite-energy methane formula, but when they gave their "friendly instructions" about how to construct a hybrid, they really didn't send their intentions.
  • Going off this, the reason the aliens are so crazy it not because of the alien DNA, but because of the Human DNA. Maybe the combination of both is what drives their aggressiveness and need to mate Up to Eleven and pure aliens are actually a peaceful, benign race. Of course, for this theory to work you have ignore what the scientists said about Mars in the second film, but given that: A) He was crazy and B) The movie was crap, we can just ignore it.
    • On the other hand, the "pure" example of alien DNA grew at a hyperfast rate, devouring every living thing it could reach. This may just be the way it normally spawns, including its fantastic metabolism, and it slows down once it's glutted itself. All the same, this is still dangerous for anything in its way, suggesting the species will actively hunt when it needs to.

The aliens were curious as to what we would do, to see if we really were a threat.
The aliens wanted to see how we'd react to the organism they sent along with the methane formula. They may have reasoned that their first gift would make us curious to try out the second, so we would naturally try to create and raise Sil and her kind. What would happen after that was the critical thing. Remember, one of the scientists speculates if the aliens consider humanity an "intergalactic weed" to be dealt with, and if Sil is meant to confront us. How we deal with her, whether we fight her, tame her, or fall to her, will reveal to them what we're capable of as a sentient species (and possibly how to be further considered.)
  • It's possible Sil's original species is not the same one that sent the message to Earth. The "pure" alien species grew abnormally fast, nearly killing the scientists next to it; it may be some predatory creature on the aliens' home world. There may have been more than one reason for sending its DNA here:
    • The aliens simply want to see what we do with it.
    • On their planet, "super-plants" have run wild, and the aliens are watching how we deal with one, hoping for a solution to a real problem on their home world.
    • Sil's home world is dying, and this is how the species colonize new worlds. The methane formula may be for their own later use, like sending supplies in advance of landing.
    • Sil and her kind are a test; if earthlings can confront her and survive, we've proven ourselves capable of overcoming threats.
    • Sil is still a test, but defeating her proves how dangerous we are.

Under this scenario, the Mars colony could be from some piece of space debris, the equivalent of a rat washing up on a deserted island, not really intentional. The alien species may have just been keeping an eye on our solar system because it looked interesting.

The aliens grow using the same method as plants.
As You Know plants absorb CO2 from the air and release oxygen. They use the retained carbon to grow and thus they acquire most of their mass from the air itself rather than absorbing it through the roots, as one might expect. The aliens may gain mass and regenerate with a similar process, just sucking carbon right out of the air as they grow. This also explains any other conservation of mass problem with aliens in sci-fi.

The hot chick who boinked the guy to death in the men's room in Species 3 was the daughter of the 2nd woman in Patrick's threesome in Species 2
Look at them and tell me they aren't related.