Remember in Jurassic Park when Ian was nervous about the whole dinosaur-in-a-theme-park deal because of chaos theory? He said "life finds a way." He was referring to the fact that John Hammond (dinosaur-cloner, owner of the park) thought he was in control of the situation because they had genetically engineered all the dinosaurs to be female, so there was no way they could reproduce. And what did they find later? A nest with eggs in it.
Weeds are a prime example that nature does prevail, and always will.
Consider this: if you have a bare area in your lawn, you will get weeds there -- even if you've had a lawn care service for years, or even if you've been treating weeds yourself.
If parts of your lawn are thin (due to encroaching shade, poor mowing habits, no fertilizer, etc.) you will get weeds in it.
Nature sees that bare dirt going to waste, and makes something grow there.
Conclusion: A thick, healthy lawn is the only thing that really keeps the weeds out. Weed control materials by themselves will not do the job. Mow often, water properly, and don't forget the fertilizer! Remember, though, that weed control must be ongoing. Weed seeds are everywhere: lying dormant, being spread by mowers and weed eaters and people's feet, and squirrels and birds pooping. Everywhere!
Now consider this: I had a virtual crop of purslane come up in my flowerbed this summer. Where did they come from? Did someone explode a purslane-seed bomb? I'd never seen purslane in my flowerbed before!
The fact is, most weeds are prolific seeders, assuring the continued procreation of their species.
A single dandelion plant may produce from 2,000 to 12,000 seeds. Individual plants left undisturbed may survive up to 13 years.
A single crabgrass plant can produce 150,000 seeds.
A study in Minnesota found as many as 130 million weed seeds per acre at a 6" depth. (That's 2,948 seeds per square foot.)
And it gets worse! These seeds can last for decades! Jimsonweed, for example, can germinate at a 90% rate after 40 years in the soil.
Conclusion: When I dug around and tilled the soil to plant flowers this year, I stirred up a bunch of weed seeds that had been in the dirt for some time. This is why we get weeds in our flowerbeds, because we're always digging around in there, disturbing old weed seeds!
Trivia: The oldest known seed to have germinated was a Judean date palm seed, which was discovered in Israel while excavating Herod the Great's palace. The seed was carbon-14 dated at 2,000 years and germinated in 2005.
A 1,300 year old carbon-14 dated sacred lotus seed found in a dry lake bed in China was also successfully germinated in 1995. Not exactly from the Jurassic period (150-200 million years ago,) but really old, just the same! (However, plants from the Jurassic period, like cycads, are still around today, which proves my point about the procreation thing...)
Other claims have been made about much older seeds - 3,000 year old seeds from Egyptian tombs, or 10,000 year old seeds taken from Russian permafrost -- but none of the claims have been confirmed with radiocarbon dating.