Ecological Consequences of Genetic Breaks
Descriptions of phylogeographic breaks and cryptic species abound in the literature, but explorations of the ecological and evolutionary consequences of such breaks are far less common. Some past and ongoing work in our lab has focused on morphological and ecological differences coincident with genetic discontinuities.
Shell shape in Acanthinucella
The fossil record documents repeated shifts in the geographic range of coastal marine species associated with Pleistocene climatic change. By transiently isolating populations and driving range contractions and expansions, these shifts may profoundly effect evolutionary differentiation, helping to determine levels of genetic variation and population similarity within species as well as providing an incubator for novel morphologies, ecological associations and even new species. In addition, range expansions documented in the fossil record may offer the sole opportunity to directly observe natural dispersal in marine organisms. However, no study to date had simultaneously detailed ranges shifts revealed by the fossil record with studies aimed at detecting the genetic consequences of such shifts. Working with Kaustuv Roy (UCSD), we conducted genetic surveys of the muricid gastropod Acanthinucella spirata. The fossil record suggests that this species developed a novel shell morphology during an episode of geographic restriction caused by climatic change. We found more mtDNA sequence variation in the region of restriction (the southern California Bight) than in recolonized populations north of Point Conception, providing support for the rapid evolution of novelty associated with climate-driven changes in geographic range (Hellberg et al. 2001).
Bacterial defense symbiosis in Bugula
The broadly distributed bryozoan Bugula neritina was known to consist of at least two cryptic species, only one of which harbors the symbiont that produces the cytotoxin, bryostatin 1 (Davidson and Haygood 1999). In the course of trying to find out which of these cryptic species inhabits oil platforms in the Gulf of Mexico, we identified an additional cryptic species occurring along the north Atlantic seaboard. The geographic break we found between cryptic Bugula species correlated with geographic variation in the palatability of the bryozoan larvae (Lopanik et al. 2004). This change appears to be underlain by a shift in the bacterial symbionts of the Bugula species (McGovern and Hellberg 2003).
Cryptic coral species, cryptic ecological divergence
What was thought to be Porites lobata along the tropical Pacific coast of the Americas turns out to include P. evermanni, which in this region is nearly identical morphologically. Reproduction in P. lobata is mainly sexual, but in P. evermanni there is much clonal division, facilitated by the high number of boring clams in its skeleton and the triggerfish that break off fragments in pursuit of their molluscan prey. The two cryptic species also differ in their resistance to bleaching, despite the apparent identity of their algal symbionts (Boulay et al. 2014).