Maintenance of a Genetic Cline in the Barnacle Balanus glandula.

The barnacle Balanus glandula is a broadly distributed species in the temperate northeastern Pacific that is notable for a robust genetic cline between about 36° and 40° N latitude. Prior work established the evolutionary origins of this pattern and proposed that it is maintained by environmental selection. In recent years, "climate velocity" studies in marine habitats have shown dramatic distributional shifts for many species as they track their preferred temperature range in a warming ocean. We re-sampled B. glandula across its entire geographic range to determine whether there has been any shift in this genetic distribution, a development signaling that temperature or other climate factors are maintaining this genetic cline. Additionally, we asked whether the spatially distributed mitochondrial lineages also vary in reproductive output with latitude, using location as a proxy for temperature and other coastal environmental factors. Here we show that although the distribution of the genetic cline has not appreciably changed, there is a notable association of decreased reproductive output at lower latitudes of the distribution in the "northern" lineage of B. glandula.