Spatially heterogeneous gene flow may hinder linking phylogeographic data to macroevolutionary patterns

How microevolutionary processes translate to macroevolutionary patterns is a central question in evolutionary biology. Macroevolutionary and biogeographic studies often rely on species trees to make inferences about diversification, but gene flow can lead to incorrect phylogenetic inference. Under a range of biogeographic conditions, gene flow may not occur uniformly across space during diversification, which could lead to macroevolutionary misinferences. For example, whenever a "peripheral" population is relatively isolated and inferred as sister to a "core" clade of populations that exchange migrants, it might solely reflect gene flow, as opposed to macroevolutionary processes such as the order of biogeographic dispersal events. We use simulations of the hypothetical case of spatially heterogeneous gene flow described above and found that relatively low levels of gene flow led to monophyly of the adjacent populations, with longer branches for peripheral taxa, regardless of the true divergence history. We highlight an empirical example wherein lowland antbirds (Family: Thamnophilidae)-a clade known for dynamic gene flow in Amazonia-tend to be young in Amazonia, but older at Amazonias periphery. Although it is challenging to know if the simulated bias applies here, our work suggests a distinguishability problem for any nodes stemming from radiations in geographically heterogeneous environments.