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Abstract The diversification of many lineages throughout natural history has frequently been associated with evolutionary changes in life cycle complexity. However, our understanding of the processes that facilitate differentiation in the morphologies and functions expressed by organisms throughout their life cycles is limited. Theory suggests that the expression of traits is decoupled across life stages, thus allowing for their evolutionary independence. Although trait decoupling between stages is well established, explanations of how said decoupling evolves have seldom been considered. Because the different phenotypes expressed by organisms throughout their life cycles are coded for by the same genome, trait decoupling must be mediated through divergence in gene expression between stages. Gene duplication has been identified as an important mechanism that enables divergence in gene function and expression between cells and tissues. Because stage transitions across life cycles require changes in tissue types and functions, we investigated the potential link between gene duplication and expression divergence between life stages. To explore this idea, we examined the temporal changes in gene expression across the monarch butterfly (Danaus plexippus) metamorphosis. We found that within homologous groups, more phylogenetically diverged genes exhibited more distinct temporal expression patterns. This relationship scaled such that more phylogenetically diverse homologous groups showed more diverse patterns of gene expression. Furthermore, we found that duplicate genes showed increased stage-specificity relative to singleton genes. Overall, our findings suggest an important link between gene duplication and the evolution of complex life cycles. 

Abstract 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.