The developmental and genetic bases for the formation, plasticity and
diversity of eyespot patterns in butterflies are examined. Eyespot pat
tern mutants, regulatory gene expression, and transplants of the eyesp
ot developmental organizer demonstrate that eyespot position, number,
size and colour are determined progressively in a developmental pathwa
y largely uncoupled from those regulating other wing-pattern elements
and body structures. Species comparisons and selection experiments sug
gest that the evolution of eyespot patterns can occur rapidly through
modulation of different stages of this pathway, and requires only sing
le, of very few, changes in regulatory genes.