"What came first: inversions or local adaptation?"Peischl, StephanThe classical Kirkpatrick and Barton model (KB model) suggests that inversions proliferate by linking locally adapted alleles and suppressing the formation of locally unfit recombinants. In support of this hypothesis, numerous empirical examples of chromosomal inversions are linked to traits that contribute to local adaptation and show clinal variation across space. However, identifying the specific targets of selection within inversions has proven difficult and the exact evolutionary forces that drive inversion evolution are still elusive. In particular, the fundamental question of which came first, inversions or locally adapted polymorphism, remains unresolved. A key assumption of the KB model is the existence of exactly two locally optimal haplotypes, which leads to strong linkage disequilibrium (LD) between inversions and locally adapted alleles. Here I extend the KB model to allow for multiple locally adaptative traits, creating a complex landscape of natural selection across space. I show that inversions can still evolve in this model, even when recombinant haplotypes are selectively favored in some habitats. Two important conclusions from this model are that (i) strong LD between inversions and locally adapted alleles does not necessarily establish and (ii) the temporal sequence of inversion and locally adapted polymorphism establishment can result in distinct equilibrium states. The latter observation opens new avenues for theoretical and empirical research on the temporal dynamics of inversions and local adaptation. Recent empirical observations in the well-studied inversion 3 R Payne in Drosophila melanogaster corroborate our modeling predictions, highlighting the relevance of theoretical frameworks in understanding inversion dynamics. |
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