1. In a 3- to 4-quart pan over high heat, bring 1 1/2 to 2 quarts water to a boil. Add pasta and cook until barely tender to bite, 9 to 11 minutes. Drain and rinse with cold water.
2. In a large bowl, mix pasta, beans, corn, tomato, cilantro, onions, lime peel, 1/3 cup lime juice, 1/2 teaspoon pepper, and salt to taste.
3. In a small bowl, mash avocado with 1 tablespoon lime juice and sour cream. Add salt and pepper to taste.
4. Equally mound pasta salad on dinner plates. Top with dollops of the avocado mixture. Surround or accompany with tortilla chips.