WE'VE SEEN SO MANY nature shows on PBS lately that we've formulated a Darwinian theory of restaurants. Like any species fighting to survive, those restaurants most likely to flourish in a competitive arena are the ones that can locate and exploit a vacant niche in a particular locale. And a restaurant's niche is highly specific -- expense-account posh, say, or economy-Chinese. The odds of success are diminished if the niche isn't clearly defined ("What kind of restaurant is this?"), or if it's already occupied ("Another barbecue place?")

If this theory holds, then the future of the Flaps restaurant in Potomac should be secure. The niche is neighborhood-relaxed-versatile, and, given the paucity of restaurants in Potomac, it's been wide open. But it isn't just a lack of competitors that recommends this place: Much of the food is very good, the selection is broad, and the prices are moderate. Above all, Flaps sets a friendly, low-key mood that's just right for a restaurant catering to a local trade. This is a small, dimly lit place with the look of an English country pub. It's dominated by dark wood, brass accents and bare, polished tables, and there's a wonderfully inviting little bar at the back that reminded us of an upscale Cheers. The young servers are unfailingly friendly and solicitous, but they can also be a bit klutzy. Sometimes it is endearing, as when three people struggle mightily to open an ordinary bottle of wine. Sometimes it is exasperating, as when soups, salads and entrees all arrive within a period of 20 minutes, or when no one seems sure of whether or not reservations are accepted or whether they'll serve brunch next Sunday.

The menu really consists of two menus: a standard selection that includes hamburgers, steaks, barbecued ribs and chicken and is probably identical to what's served at the other two Flaps restaurants in the Washington area; and a separate list of about a dozen specials that change daily and include fresh fish, shellfish, veal and a sprinkling of Cajun dishes. The specials are unique to the Potomac Flaps, and it's among these that the more imaginative items are to be found.

The standout among the regular appetizers is the clam chowder, creamy but not overly rich, intensely flavored with clams and cleverly accented with what tastes like thyme and bay leaf. The rest of the regular list is pretty unremarkable -- rather heavy chicken fingers, mushrooms stuffed with mushy, overdressed crab, well-flavored but gooey fettuccine Alfredo, routine onion soup. There's more uncertainty, but also more excitement, among the appetizer specials. Look especially for the outstanding whole shrimp cocktail (heads, eyes, the works), made with immense, fresh prawns, a dish that sums up everything shrimp should be. Take a chance on the alligator, too, tender, thin-sliced and somewhat similar to pork loin. For something a little less exotic, there's sometimes a very good, peppery seafood chowder. But beware the utterly bland seafood bisque (could this soup have come from the same kitchen as that lively clam chowder?).

The jewel of the regular entrees is shrimp Barcelona, served with a marvelous sauce of butter, lemon, wine, herbs, garlic, capers and what tastes like a bit of mustard. Love at first taste. In lots of restaurants the "specials" are just a second menu of dishes that appear every night, but at Flaps they really do change -- the kitchen, in other words, generates some excitement, some dynamite dishes -- and, inevitably, some flops. Among the successes are the crawfish dishes, including one in a heavenly marinara sauce, and another, crawfish Diane, in a first-class sauce with garlic, spring onion and shallots. Soft-shell crabs, in a beautiful shallot-lemon-parsley-garlic sauce, have been an elegant delight. For something more robust, look for the peppery monkfish-tomato casserole, a zinger of a dish.

Among the less successful experiments have been chicken bayou, a good try that's spoiled by an overpowering breaded seafood stuffing; blackened redfish overdoused in spices and too long on the heat (the blackened beef dish is a far better variant); and grilled swordfish that's been crumbly and dry. A close call is chicken and shrimp amaretto, which verges on cloying sweetness but is narrowly rescued by the bitter tang of orange peel.

There are more mundane offerings, too: decent if unremarkable steak; big, juicy hamburgers (keep that Bass ale in mind); fresh, generous salads; and a nice rack of barbecued ribs that's a good buy at $8.95. (The barbecued chicken is cheaper but dry.) Desserts are not a strong suit here. The peanut butter pie is like a classy Reese's Cup, the chocolate mousse-brandy pie is good but overly sweet, and the cheescake somehow misses the flavor of cheese.

A sure mark of success in both a species and a restaurant is the ability to adjust to change. Should the Cajun Age cool down, the Flaps kitchen seems versatile enough to adapt. Let's hope the next era's menu is as interesting as this one.