Michael S. Taylor
Southeast Missouri State University
Population Structure and Diversity
I have provided evidence of extremely strong population genetic structure in Elacatinus evelynae, the sharknose goby (Taylor & Hellberg 2003). This goby is widely distributed throughout the Bahamas and Caribbean, yet nearly every island population was genetically distinct from all others (φST as high as 79%). This result suggests that larvae of these gobies to not disperse away from their natal populations. Strong population structure is also evident for other species of Elacatinus, which suggests a propensity in this genus for genetic subdivision to evolve (Taylor & Hellberg 2006), which may explain the high species richness in Elacatinus.
The evidence for strong population structure and high species richness in Elacatinus raises a broader question: Does species richness within a genus correspond to realized population genetic structure within species? Marine populations characterized by low genetic connectivity may be able to adapt to local ecological conditions, which could facilitate the evolution of reproductive isolation. In contrast, populations with high genetic connectivity may be unable to adapt to the local environment due to a regular influx of non-adapted genes.
I am investigating this question by examining population genetic structure across the Caribbean region. Specifically, I will attempt to determine whether the relative richness of different genera of common reef fishes corresponds to realized genetic structure. My predictions are that populations of species-rich genera would demonstrate strong genetic subdivision, which would indicate a propensity for speciation, while populations of species-poor genera would exhibit weak or non-existent genetic structure. I would test these predictions with different genera of blennies and gobies that have similar life history characteristics and similar potentials for larval dispersal. The capacity for larval dispersal potential will be estimated by counting daily growth rings from otoliths.
Geographic barriers that limit the movement of individuals between populations may create or maintain phylogenetically discrete lineages. Such lineages are evident for Elacatinus evelynae, which correspond to two previously proposed biogeographic breaks near Exuma Sound in the Bahamas, and at the Mona Passage between Puerto Rico and Hispaniola. I tested whether these putative barriers influenced the formation of discrete genetic lineages for other species of Elacatinus (Taylor & Hellberg 2006). I sequenced portions of mitochondrial cytochrome b and nuclear rag1 for nine species and color forms that span the proposed breaks. My results showed that Mona Passage separated cytochrome b and rag1 lineages, with no genetic exchange between populations separated by just 23 km. However, the Central Bahamas barrier was only weakly supported by my data. Importantly, neither barrier coincided with deep genetic splits. This suggests that these two barriers did not initially isolate regional populations, but instead disrupt ongoing gene flow between regions. The inferred relationships further suggested a division of the Caribbean region into northwestern and southeastern regions, a pattern reflected by some freshwater and terrestrial vertebrates. These results, coupled with genetic and demographic data from other reef fishes and corals, provide robust support for the Mona Passage as a long-term biogeographic barrier for Caribbean animals.
This work is among the first genetic evidence that supports the presence of biogeographic barriers within the Caribbean Sea. However, my results apply only to a single genus. Is gene flow in other species of reef fishes governed by the Mona Passage? Distributional and morphological evidence suggests that the answer is yes: Species not extending east of the passage include the serranids Gramma melacara and some species of Hypoplectrus. Species as diverse as a tonguesole and a pipefish are more common west of the passage than to the east, while the widespread clinid fish, Malacoctenus triangulatus, shows distinct morphometric differences between individuals sampled across this region.
I am using mitochondrial and nuclear markers to investigate whether the Mona Passage disrupts gene flow for other benthic reef fishes (e.g., Malacoctenus and Starksia blennies, and other gobies).
Malacoctenus boehlkei photo copyright © Jim Christensen with uwphoto.net.
Starksia photo copyright © Keri Wilks with ReefNet, Inc.