What a joy it is to watch a comedy like “Evolution,” where these rules are followed:

• The main characters are funny and even frantic, but never cloying, hammy or over-eager.

• The story is not adhered to religiously, but neither is it thrown to the wind for the sake of some non-sequitur jokes or pop-culture references.

• The humor dabbles in crudeness, but generally eschews it, preferring to remain in the realm of good old-fashioned nuttiness.

For as loose and crazy as “Evolution” is, there is an underlying intelligence that speaks volumes about the filmmakers’ level of respect for the audience: We can amuse them without speaking down to them.

Not that it’s afraid to get down in the dirt, because it does stoop to ground level when necessary. The difference is, when Orlando Jones is face-down on a gurney having an alien life form removed from his tuckus, it’s not just the grossness of the situation that makes audiences hoot. It’s the fact that the scene has some actual funny dialogue and dedicated performances in it. The film doesn’t lean on outrageous situations for its laughs; it creates outrageous situations and then continues to work, mining real comedy from those circumstances.

Orlando Jones plays Harry Block, a science teacher and women’s volleyball coach at Glen Canyon Community College in Arizona. He and his fellow instructor Ira Kane (David Duchovny) are first on the scene when a meteor crashes to the Earth, and they discover living organisms on the rock. These organisms begin not just to multiply, but to actually evolve — from single cells to simple life forms to fish to amphibians to reptiles, and so on. In a matter of days, they do what it took mankind 250 million years to do.

The government takes over, naturally, and ruins everything, led by Ida’s former military acquaintance Dr. Woodman (Ted Levine) and CDC representative Allison Reed (Julianne Moore). But in the end, it is the expertise of the regular guys, aided by local drop-out Wayne (Seann William Scott) and a firetruck full of Head & Shoulders shampoo, who save the day from the rapidly advancing alien civilization.

The director is Ivan Reitman, who also helmed “Ghostbusters,” to which this film has a few similarities (including the presence of a droll Dan Aykroyd, this time as Arizona’s no-nonsense governor). Jones and Duchovny together fill the Bill Murray role, remaining glib while things become hysterical around them.

It’s the deadpan tone that keeps the film hopping. Rather than lunge at the jokes, everyone stays cool about it, letting the humor flow naturally. Duchovny is expert at this, of course, from his years on “The X-Files,” but it’s refreshing to see Jones (and Scott, who usually appears only in teen dumbbell comedies) doing respectable, comically dignified work.

Yes, I said dignified. No matter how extreme things get — and there is a moment when Jones’ character gets revenge on an alien for the aforementioned back-end trouble — the performers remain under control. Julianne Moore’s clumsiness seems out of place, in fact. You wouldn’t think someone falling down a lot would be an ill fit for a sci-fi comedy, but it is.

Some of the ideas don’t pan out, most notably Scott’s character’s attempts at being a fireman and the obligatory romance between Kane and Reed. But the laughs are solid far more often than they’re weak, and when it’s over, you know you’ve been entertained.

B+ (; PG, a lot of mild profanity, some sci-fi.)