Data and analysis from: Two centuries of monarch butterfly collections reveal contrasting effects of range expansion and migration loss on wing traits

Migratory animals exhibit traits that allow them to exploit seasonally
variable habitats. In environments where migration is no longer
beneficial, such as oceanic islands, migration-association traits may be
selected against or be under relaxed selection. Monarch butterflies are
best known for their continent-scale migration in North America but have
repeatedly become established as non-migrants in the tropical Americas and
on Atlantic and Pacific Islands. These replicated non-migratory
populations provide natural laboratories for understanding the rate of
evolution of migration-associated traits. We measured >6,000 museum
specimens of monarch butterflies collected from 1856 to the present, as
well as contemporary wild-caught monarchs from around the world. We
determined (1) how wing morphology varies across the monarch’s global
range, (2) whether initial long-distance founders were particularly suited
for migration and (3) whether recently-established non-migrants show
evidence for contemporary phenotypic evolution. We further reared
>1,000 monarchs from six populations around the world under
controlled conditions and measured migration-associated traits. Historical
specimens show that (1) initial founders are well-suited for long-distance
movement and (2) loss of seasonal migration is associated with reductions
in forewing size and elongation. Monarch butterflies raised in a common
garden from four derived non-migratory populations exhibit
genetically-based reductions in forewing size, consistent with a previous
study. Our findings provide a compelling example of how
migration-associated traits may be favored during the early stages of
range expansion, and also the rate of reductions in those same traits upon
loss of migration.