A few studies published DNA sequence data from barnacles (B. glandula) collected 15-20 years ago; the best example is here:
What these studies showed is that, despite the high potential for dispersal/gene flow (the larvae are drifting in the water for many weeks), there is distinct genetic diversity dominant in southern California (the southern end of the distributional range) than in the northern part of the range. Here is a plot that includes data from the two studies with greatest spatial coverage.
The color variation (grey, black) separates data from two different time points (late 90s, early 2000s) from two papers (Sotka et al 2004, Wares and Cunningham 2005) that collected data from the same gene region (mitochondrial cytochrome oxidase I) but from different ends of that gene. So you see that in southern California (south of Point Conception), there is one 'type' that is at 100% frequency; however, the cline never goes to completion, as the southern mtDNA type can still be found at low frequency all the way into Alaska!
It has been 20 years since some of these barnacles were collected. There have been estimates that many species are moving northward (well, or following local climate velocity) at rates of 20km or more per decade. So we have a slight chance of picking up some signal on this movement. Wait, did I say movement? Of course not: they are barnacles! Our question is whether there is differential growth/fitness (reproduction) of the two primary mitochondrial lineages. If the southern type is doing better as the water warms, its frequency should increase. This means either the cline will shift northward, or will flatten out since northern sites already had a low frequency of "southern" individuals.
We will collect 50-60 Balanus from outside the FHL labs, from diverse microhabitats, and along with samples already taken at 2 other locations we will use PCR and sequencing to see how this cline looks 20 years later.