Descriptions of phylogeographic breaks abound in the literature, but explorations of the ecological and evolutionary consequences of such breaks are far less common (see Sanford et al. 2003 Science 300: 1135-1137 for a notable exception). Some past and ongoing work in our lab has focused on morphological and ecological differences coincident with geographic genetic discontinuities.
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 Science 242: 1707-1710)
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 (McGovern and Hellberg. 2003). 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. Mol. Ecol. 12: 1207-1215).
Davidson, S. K. and M. G. Haygood. 1999. Identification of sibling species of the bryozoan Bugula neritina that produce different anticancer bryostatins and harbor distinct strains of the bacterial symbiont "Candidatus Endobugula sertula". Biol. Bull. 196: 273-280.
Lopanik, N., N. Lindquist, and N. Targett. 2004. Potent cytotoxins produced by a microbial symbiont protect host larvae from predation. Oecologia 139: 131-139.
Blennies of the genus Acanthemblemaria are tightly associated with the tube that they live in, which provides them with protection from predators, access to food, and a place to brood their larvae. Of the dozen or so species found in the Caribbean, two (A. spinosa and A. aspera) are broadly distributed. These two species potentially face competitive interactions with an array of morphologically variable congeners. Both also exhibit substantial phylogeographic structure. For his Ph.D. thesis, Ron Eytan will be looking at whether this genetic differention matters ecologically by measuring geographic variation in ecomorphology and correlating observed differences with changes in community composition and microhabitat availability. (Photo: A. spinosa by Paul Humann)
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