A Small World of Exceptions?

When rules are mentioned in biology, the opposite concept of exceptions immediately arises. In this chapter, we analyze the existing definitions of what an exception is in general and in life sciences. We give examples that invite us to reflect on the richness and complexity of exceptions since it is not just an absolute concept, but there can be gradients of types of exceptions depending on the type of rule to which they are linked. We mention what exceptions mean in everyday life and what value they have in the scientific field. We also comment on the concept of exception, which is extended to what is considered exceptional in terms of biological characteristics. In this context, single and multiple trait exceptions and the implications of population outliers are discussed.