Perspective piece: Comparison of eastern and western North American monarch butterflies

Monarch butterflies are a species of conservation priority due to
declining overwintering populations in both eastern and western North
America. Declines in western overwintering monarchs—more than 99.9% since
monitoring began—are especially acute. However, the degree to which
western monarchs are a distinct biological entity is uncertain. In this
review, we focus on phenotypic and genetic differentiation between eastern
and western monarchs, with the goal of informing researchers and
policy-makers who are interested in monarch conservation. Eastern and
western monarchs occupy distinct environments and show some evidence for
phenotypic differentiation, particularly for migration-associated traits,
though population genetic and genomic studies suggest that they are
indistinguishable from one another. We suggest future studies that could
improve our understanding of differences between eastern and western
monarchs. We also discuss the concept of adaptive capacity in eastern and
western monarchs as well as non-migratory populations outside of the
monarch’s primary North American range. Finally, we discuss the prospect
of completely losing migratory monarchs from western North America and
what this entails for monarch conservation.