What is the muscular valve between the esophagus and the stomach

The lower esophageal sphincter, or gastroesophageal sphincter

Explanation:

The esophagus is a structure, part of the alimentary canal (gastrointestinal tract) that connects the pharynx to the stomach and through which the food passes. The esophagus is a fibromuscular tube that is involved in the food transport via peristaltic contractions (radially symmetric muscle contractions and relaxation).

the answer to this question is a

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.