Okay, so you know how Lavos' Core is a culmination of all the genetics contained by the planet's forms of life, right? Pretty much the way I see it, Lavos CHOSE which genetic traits it wanted, ultimately manipulating itself to be the strongest lifeform possible.
Well my thought is the Destructive Rains From the Heavens is a thermonuclear attack (as discussed in another forum), potentially causing widespread thermonuclear destruction. As such, it kills off all the weak lifeforms, and only the STRONGEST lifeforms are able to survive the longterm effects of the attack.
Now going on my own imagination, having destroyed most of the weak species, Lavos is then able to retrieve OPTIMAL genetic data, not only collecting it for it's own strengthening, but to also PASS IT ON to it's Spawns. That way, the Spawn's contain the strengths of it's parent, plus some. Then, when the Lavos Spawns leave and repeat the process, the Lavos species as a whole continuously betters itself.
Did that make sense?
I'm currently writing a thesis on Lavos, and some of that information is detailed within.