Species adrift: What do European bison and a rare flower have in common?

Twelve. There are twelve individuals to whom all European bison (Bison bonasus) alive today trace their lineage (Slatis 1960). After the species was hunted to near extinction in the late 19th century, the European bison, historically abundant across Europe, is now made up of 3,200 individuals between the wild and captivity. The subsequent population bottleneck has led to the fragile persistence of the species at rates far from the vastness of their ancestral counts (Olech 2008).

Simply put, when a population undergoes a traumatic event resulting in the loss of many individuals, the gene pool that propagates from the survivors has undergone a bottleneck. The bison are only one such example of this effect. In a study by Lee, Kim, and Kim, published this past June in the journal Ecology and Evolution, the Hallym University researchers concluded that bottlenecks in the evolutionary history of a species can, and will, lead to widespread genetic similarities within isolated populations, such as those observed in the rare Plagiorhegma dubium plant. Such likenesses are a phenomenon caused by genetic drift, a process that explains how some genotypes become more prominent than others within a given population.  

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A close relative of Plagiorhegma dubium, Jeffersonia diphylla, also known as “Twinleaf,” can be found across Eastern North America. Picture: Gould, Anita. “Twinleaf”, Taken 18 Apr 2009

P. dubium is a prime example of a species suffering from genetic drift, brought about in two key steps: an initial population bottleneck, followed by a lack of reasonable gene flow. The combination of these factors made the various populations, though physically near one another, genetically distinct across microhabitats.

Researchers didn’t initially know there was a bottleneck in P. dubium, but observed unexpectedly low genetic variation between individuals within a given population. This cannot be chalked up to inbreeding, as we’ll discuss later, rather the authors point to an historic bottleneck as the source.

Likewise, limited inter-population gene flow contributed heavily to the observed genetic drift in P. dubium plants. In the study, Lee, Kim, and Kim identified ten populations of P. dubium across the Korean peninsula, each varying in genetic distinctness from the other nine. If individuals of a species were unable to interact between populations, for cross-breeding or otherwise, the populations were considered ‘isolated’ and prone to drift, on the sole basis of founder effect.

Founder effect, along with a population bottleneck, acts as a prime cause for genetic drift. A founder effect occurs when individuals of a species are isolated from other populations, leaving any future progeny to come from the genes present in the founding members. This presents a limit on the variation available to the group, outside of any mutations that may arise.

Ultimately, this is what happened to the European bison and the existing population continues to struggle. Low genetic diversity will always make them prone to inbreeding, opening the doors for ruinous dormant traits and other genetic concerns.

P. dubium, however, has not yet reached that fate. Since there are multiple populations, albeit in low numbers and each one genetically distinct, it would not be difficult to simply cross-breed members of different populations and introduce the new progeny into each of the groups. By cross-breeding the populations, we would reintroduce genetic diversity where it was lost, a model that would suit any endangered species with multiple sub-populations.

The way they are now, the lack of genetic diversity across P. dubium populations makes each group more rare and more threatened. Each tiny population ritually breeds out any adaptive power they may have, as they get closer and closer to uniformity with each new progeny.

In this way, the study confirms that, by identifying population bottlenecks and other spatial discrepancies in genetic variation, we can designate species most at risk of extinction. Understandably, if we know that a population suffered a significant bottleneck in the past, conservationists can make sure to protect species’ habitats, counteracting an inherently limited ability to adapt to any future changes in the environment. If we can protect the ones most at risk, the rest of our world should fall in line.

 

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Citations:

Gould, A. (2009). Twinleaf [Photograph]. Retrieved from https://www.flickr.com/photos/ anitagould/3597092106/in/photolist-T4ZYUH-bsiVtC-6tS3aq-4BqwTX-Gz2XEN-6gu3Mg-4BuP8d-4BuPcU-JNYHo-4BqwQB-bRr15t-6kWkVJ-e8AAys-Tx9yid-263rhhL-6gq8Ur-9uywm4-SxVZvd-CJURLT-DfbYiG-eeCuUM-D8Pn88-FX4bb3-tbapuM-bFdPnZ-sTy35S-FrHhSN-TCeMiY-TCeKY3-TNz67Q-ULQEAM

Lee, S.-R., Kim, B.-Y., & Kim, Y.-D. (2018). Genetic diagnosis of a rare myrmecochorous species, Plagiorhegma dubium (Berberidaceae): Historical genetic bottlenecks and strong spatial structures among populations. Ecology and Evolution, 0(0). https://doi.org/10.1002/ece3.4362

Olech, W. (IUCN SSC Bison Specialist Group) (2008). Bison bonasus. The IUCN Red List of Threatened Species 2008: e.T2814A9484719. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T2814A9484719.en.

Slatis, H. M. (1960). An Analysis of Inbreeding in the European Bison. Genetics, 45(3), 275–287.

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