All populations are NOT created equal (2 Comments)

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Past glaciation events have had an enormous impact on Atlantic marine species

It’s no surprise that climate fluctuations influence the geographic distributions of many species.  I myself traded in my heavy winter coat for a coat of SPF 70 when I made the move from Rhode Island to Texas.  And we know from previous studies that climate-induced shifts are not isolated to terrestrial redheads.  Many Atlantic marine populations have survived in lower latitudes during periods of glaciation, while their northern counterparts were not so fortunate.  What is understudied, however, is how changes in climate affect the genetic diversity of a species.  After all, these refuge populations were left to recolonize higher latitude habitats once the waters warmed.  Such distributional shifts, occurring multiple times over earth’s history, could potentially result in an uneven distribution of genetic variation across species’ ranges.  A recent article by Dr. Jim Provan from Queen’s University in Belfast examined phylogeographic studies of North Atlantic marine species for hotspots of genetic variability.  He found evidence that nine different taxa, from seaweed to bryozoans to fish, currently display higher levels of intraspecific genetic variation at their rear-edge populations than throughout the rest of their range.

Genetic diversity is often seen as an indicator of adaptive potential within a species.  Populations with a greater diversity of genetic material tend to respond better, in the long run, to environmental threats than those with a more homogenous pool.  Since an organism’s genetic makeup is sort of like its utility belt, it would make sense that one with plenty of different gadgets would be preferred to one with just batarangs and band-aids when facing off against the villains of climate change, habitat destruction and predation.  What Dr. Provan’s article says is that there is a unique pattern, at least in Northern Atlantic marine species, of low-latitude populations having more genetic diversity than the rest of their species’ populations.  This is in contrast to the long-held abundant-center hypothesis, which stated that genetic variability is greatest where population densities are high.  Therefore, most distribution models that incorporated this hypothesis assumed that the center of a species’ geographic range held the most genetic variability, not its rear-edge populations.

This has serious implications for future conservation efforts.  Our climate is changing; where it used to be relatively stable, it’s now getting heated and making headlines with its overall unpredictability (we call this the Kanye effect).  Increases in sea temperature can have fatal implications for marine populations, especially those at rear-edges.  Further, if these threatened populations are the same that harbor most of a species’ genetic diversity, their demise will adversely affect the future of the species as a whole by limiting its adaptive potential.  In some circumstances, specific rear-edge populations may never have migrated away from their refugia to colonize northern habitats.  If this were the case, the loss of these populations would also mean the loss of unique genotypes found nowhere else in the world.

Conservationists should therefore understand the distribution of genetic diversity within a managed species.  Though not all species of marine life maintain most of their variability in rear-edge populations, it certainly seems to hold true for a diverse group of them.  Our best shot at protecting such organisms is to maintain these populations, since they will ultimately provide the best utility belt for their future bat-kids.

 

25 February 2014

February 25, 2014

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  • Great post Nate! I’ve noticed a small number of articles have been published on this topic lately and they seem to be seaweed dominated (yeah!). I think the take home message is spot on – that the abundant-center hypothesis doesn’t always hold and that conservation groups may need to take this into consideration when conserving for genetic diversity.

    In regards to conservation however, I think there are two key assumptions that are not addressed well in these papers. If these rear-edge populations are to be conserved for their genetic diversity the assumptions are then 1) these populations *will* contribute (genetically) to future populations (i.e., there is potential for successful migration/dispersal to other areas within the species range) and 2) the genetic diversity in these rear-edge populations have adaptive potential. I think these are touched upon but not well addressed even though they are central to effective conservation.

  • Thanks Kylla, you’re right in that there are a couple of assumptions being made for conservation purposes. I think on a broad scale most conservationists consider genetic variation to imply adaptive potential, though usually they don’t specifically test this link. With regards to this study, the article mentions that even if these populations are not contributing their genes to a wider range of intraspecies populations, their unique genotypes are reason enough to protect these habitats (which seems to get into a tricky argument for the intrinsic value of populations or species, though I could be reading too much into it). I agree that conservationists should thoroughly test these two assumptions in order to create a more effective management plan, I think the reason most do not may simply have to do with a lack of funding or resources for that sort of study.

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