The hidden landscape: Evidence that sea-level change shaped the present population genomic patterns of marginal marine species.

Oftentimes, to understand the genetic relatedness and diversity of today's populations requires considering the ancient landscape on which those populations evolved. Nowhere is this clearer than along Earth's coastline, which has been in its present-day configuration for only about 6.5% of the past 800,000 years (Dolby et al., 2020; Miller et al., 2005). During ice ages when glaciers expanded in the Northern Hemisphere, they stored enough of the planet's water to drop global sea level by ~120 m below present levels ("lowstand", Figure 1a), and there have been at least eight of these 100,000-year cycles preceding today. When glaciers melted, ocean water reflooded shorelines, shifting and re-forming marginal marine habitats globally and shaping the relatedness of populations (Dolby et al., 2016). In a From the Cover article in this issue of Molecular Ecology, Stiller et al. (2020) integrate population genomic analysis of leafy seadragons in southern Australia with estimates of available seabed area to reveal that the expansion of habitat that accompanied this reflooding led to strong demographic expansions. With statistical models, they also show that western populations were eliminated and then recolonized because the continental shelf there is narrow, leaving little available habitat when sea level was low (Figure 1b). Their results document the dynamic and interrelated nature of a hidden, changing landscape and the evolution of species inhabiting it.