Which of the following is not considered a formal taxonomic rank in biological classification?

Explanation

An ecotype refers to a genetically distinct population within a species that adapts to specific environmental conditions, but it is not an official taxonomic rank. The primary hierarchical categories in biological classification include Kingdom, Phylum (or Division), Class, Order, Family, Genus, and Species. While there are additional subcategories and supercategories such as sub-species or super-family, ecotype is considered a subgroup within species rather than a formal classification level.