Project: Evolution in the North Atlantic Basin

In the early 1990s, we proposed the first explicit model of population differentiation and speciation in the deep-sea fauna, the depth-differentiation hypothesis (Etter and Rex 1991). According to this theory, the potential for population differentiation decreases with depth because the bathyal zone (200-4000 m) has stronger selective gradients and more opportunity for geographic isolation to impede gene flow than does the more environmentally uniform abyssal plain (>4000 m). To determine whether depth-related variation is genetic, and therefore a consequence of evolutionary change, we developed new methods to extract and sequence mitochondrial DNA from archived deep-sea molluscan species collected in earlier expeditions that had been fixed in formalin and preserved in alcohol. These genetic studies, summarized in Etter et al. (2005) and reviewed in this proposal, support the depth-differentiation hypothesis. More importantly, they have revealed the limitations of using preserved material, and have resulted in a much more specific research agenda for the future.

Here we propose the first deep-sea sampling program specifically directed at answering fundamental evolutionary questions. We describe 3 hypotheses about evolution in the deep sea that emerged from our previous work. 1) The depth differentiation hypothesis suggesting population divergence decreases with depth. 2) The strong break in population structure at 3300 m may represent an unrecognized phylogeographic barrier. 3) Abyssal populations may be sinks that suffer chronic local extinction from being too rare to mate successfully, and are maintained by continued immigration from more abundant bathyal source populations. Our plan is to test each of these hypotheses using deep-sea protobranch bivalves, but for rigorous tests we need multiple independent loci. Nuclear loci are essential as independent measures of population structure, gene flow and historical influences, but are also critical to establish whether some of the remarkable divergences we have documented represent cryptic species. The formalin fixed tissues we have now are too degraded to obtain nuclear loci, so we are proposing to collect fresh samples to develop the nuclear loci (introns). The primary focus of this proposal (first three years of work) will be to collect the samples and develop nuclear markers from those samples that are sufficiently variable in deep-sea protobranchs to test each of the hypotheses and distinguish intra versus interspecific variation.

The deep-sea supports one of the most diverse and unique marine communities, the evolutionary and historical development of which are virtually unknown. The proposed research will contribute very significantly to answering the two most basic question about evolutionary diversification in this vast environment: Where does it occur, and how? It will also create a solid conceptual and methodological context for future evolutionary studies in the deep sea. The source-sink hypothesis of abyssal biodiversity is the most synthetic and comprehensive explanation of large-scale patterns of species diversity in the deep ocean. If proven correct by the proposed study of population genetic structure, it will greatly simplify our understanding of both evolutionary and ecological causes of species diversity patterns.