A genetically based ecological trade-off contributes to setting a geographic range limit

<p>Understanding the ecological factors that shape geographic range limits and the evolutionary constraints that prevent populations from adaptively evolving beyond these limits is an unresolved question. Here, we investigated why the euryhaline fish, <i>Poecila reticulata</i>, is confined to freshwater within its native range, despite being tolerant of brackish water. We hypothesized that competitive interactions with a close relative, <i>Poecilia picta</i>, in brackish water prevents <i>P. reticulata</i> from colonizing brackish water. Using a combination of field transplant, common garden breeding, and laboratory behavior experiments we find support for this hypothesis, as <i>P. reticulata</i> are behaviorally subordinate and have lower survival in brackish water with <i>P. picta</i>. We also found a negative genetic correlation between <i>P. reticulata</i> growth in brackish water versus freshwater in the presence of <i>P. picta</i>, suggesting a genetically based trade-off between salinity tolerance and competitive ability could constrain adaptive evolution at the range limit.</p>