Why is genetic diversity important?

By Abigail Lynch 

The Chicago River turns green every St. Patrick's Day.  Many Irish Americans are descentants who migrated because of the potato famine

The Chicago River turns green every St. Patrick’s Day. Many Irish Americans are descentants who migrated because of the potato famine.

You could almost blame the greeness of the Chicago River on lack of genetic diversity.

Well, at least, indirectly…

If it weren’t for the Irish potato famine, the Windy City, along with many other American cities, may not have had the influx of Irish immigrants in the mid 19th century bringing their Celtic traditions, affinity for fine whiskey, and that frolicking holiday every March 17th celebrating their good ol’ patron Saint Patrick with parades, leprechauns, green beer, and even a green river.  The potato famine is an extreme example of the consequences of loss of genetic diversity.  Ireland relied so heavily on monoculture of potatoes that when blight caused the potatoes to rot, the Irish lost their primary food source.  Famine – and mass migration – ensued.

So, while the Irish potato famine can also be tied back to many social and economic causes (e.g., absentee English land tenancy), lack of genetic diversity is certainly one contributing factor.  What can this tale of Irish woe demonstrate for fish conservation?  The value of genetic diversity.  This applies to fish species and aquatic communities just the same as it applies to agricultural crops.

The Portfolio Effect

If you can think of genetic value like you think of your retirement plan, a diversified portfolio minimizes risk and often provides the most reliable returns.  In fact, the ‘portfolio effect’ has often been cited with respect to biodiversity and ecosystem services (see Figge 2004).  Essentially, by having greater genetic diversity within a fish species (e.g., many discrete populations with different life history strategies rather than a single homogenized population), the species is more apt to withstand variable conditions.  Perhaps one year, one population does well and in another year a different population does well.  Over time, they all even out.  By having that variation built in, the species minimizes its risk of complete collapse (e.g., what happened during the potato famine).  The portfolio effect is particularly valuable when considering harvested species because it helps fisheries maintain stable catch levels through the boom and bust cycles of particular populations (see Schindler et al. 2010).

Global change and genetic diversity

Genetic diversity and the portfolio effect can help buffer species against global change.  Take Pacific salmon as one example.  Salmon have a wide range of life history strategies, perhaps most evident in the variation in spawning migration timing.  Evolutionarily, different spawning runs arose because in certain years environmental conditions favored the success of offspring spawned at certain times.  The stability of salmon abundances in southeast Alaska is often attributed to this high level of diversity.  With climate change, certain salmon life history strategies are frequently more favorable.  Indeed, there is already genetic evidence of long-term changes in migration timing of adult salmon in the region (see Kovach et al. 2012).

Sockeye Salmon may be resilient to climate change because of high genetic diversity.

Sockeye Salmon may be resilient to climate change because of high genetic diversity. (photo credit: Jonny Armstrong, University of Washington)

The greater the genetic diversity, the greater the opportunity for resiliency to future climate change.  When it comes to fish conservation in an era of global change, genetic diversity is like that diversified financial portfolio.  If you bet everything on one strain, you could end up with another potato famine.  But, if you prioritize conservation of diversity (as in the previous Fisheries Blog post on Gila Trout), you can pave the way for natural selection to promote evolutionary response to change.  While this may not turn any rivers St. Patty’s Day green, it can keep rivers red with salmon returning to spawn.

References:

Figge, F. 2004. Bio-folio: applying portfolio theory to biodiversity. Biodivers. Conserv. 13(4): 827–849.

Kovach, R.P., Gharrett, A.J., and Tallmon, D.A. 2012. Genetic change for earlier migration timing in a pink salmon population. Proc. R. Soc. B Biol. Sci. 279(1743): 3870–3878

Schindler, D.E., Hilborn, R., Chasco, B., Boatright, C.P., Quinn, T.P., Rogers, L.A., and Webster, M.S. 2010. Population diversity and the portfolio effect in an exploited species. Nature 465(7298): 609–612.

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