The folding of RNA sequences into secondary structures is a simple yet biophysically grounded model of a genotype–phenotype map. Its computational and mathematical analysis has uncovered a surprisingly rich statistical structure characterized by shape space covering, neutral networks and plastogenetic congruence. I review these concepts and discuss their evolutionary implications. BioEssays 24:1164–1177, 2002. © 2002 Wiley‐Periodicals, Inc.