Niche differentiation confers coexistence prior to the species boundary in an aquatic plant

Abstract

Despite the prevailing view that ecological divergence drives speciation, we know little about when or how nascent lineages evolve the ecological differences needed to coexist upon secondary contact. Here, we apply ecological coexistence theory to quantify the potential for coexistence among 126 allopatric lineages of the globally distributed duckweed Spirodela polyrhiza. Using competition experiments simulating secondary contact, we found that nascent lineages were more likely to coexist as they become genetically distinct. This occurred through the rise of niche differences, but not competitive differences, early in lineage divergence. Competition assays against its sister-species Spirodela intermedia further showed that most niche differences may already be in place to promote coexistence well before timescales at which speciation is complete. Our findings suggest that early coexistence may thus create the ecological conditions for sympatry among nascent lineages and broaden opportunities for gene flow during speciation, serving as an important but overlooked link between the ecological controls that maintain biodiversity and its evolutionary origins.