Not Out of the Woods, Yet–Bison Genetic Extinction

The topic of genetic extinction of the wild bison genotype is rather extensive.  So, the plan is to discuss this issue in several parts, beginning with the mechanisms contributing to the loss of genetic information. Then follow with an exploration of how to avoid those mechanisms and a review of particular issues involved.

Part I: Mechanisms

Imagine a river running endlessly in both directions (nevermind the logical inconsistency) with only one bridge crossing the river, the only means to the other side. Furthermore, coming up to that bridge is a 12-lane highway, but the bridge only permits one lane with no merging lanes allowing access to that one lane.  Instead the other 11 lanes all end with a wall at the river’s shore.  But this is unknown until the ends of those lanes are reached.  Only those traveling on the lane accessing the bridge are able to cross the river.  This situation presents a bottleneck.  By the 1900s the North American bison entered a genetic bottleneck.   From a population of many millions only less than a 1000 were able to cross over our imagined bridge.  A genetic bottleneck occurs when a population is reduced to a small subset of the original population.  The last remaining individuals, then, represent the remaining genetic heritage of the entire initial population. But they do not represent the overall genetic diversity of the population before the bottleneck.  Such circumstances are detrimental to the viability of a species because of the loss of genetic diversity [1].

                Other than out-right extermination of a species, Bailey has identified five processes contributing to the genetic extinction of the wild bison genotype [2]:

  • Founder Effects (Initiating herds with few individuals having limited genetic diversity)
  • Genetic Introgression (Crossbreeding with cattle genes–hybridization)
  • Inbreeding depression in small herds
  • Genetic drift in small populations
  • Artificial selection by human intervention (domestication)

Before going further the concept of wildness needs definition.  According to Bailey, wildness refers to the impact humans have on an animal population or an ecosystem.  A species or an ecosystem is considered wild if it exists and functions with no human intervention [3].  Of course, in the case of bison, Native Americans have interacted with the bison.  But perhaps it could be argued their intervention had no more impact than that of natural, random events; certainly not the impact our culture has had. 

Founder Effects:

Founder effects involve initiating herds with few individuals. Today’s herds have been initiated with the few bison left from the slaughter that took place in the latter half of the 19th century.  Because of this, and as illustrated in the thought experiment above, with the loss of genetic diversity the potential for effective natural selection has been reduced.  In addition, with few founders the stage is set for other resulting mechanisms contributing to genetic extinction—for instance, inbreeding depression and genetic drift experienced in small herds.

Genetic Introgression and Hybridization:

Bison were saved from extinction primarily by 5 ranchers and the small remnant in Yellowstone National Park.  The ranchers during the end of the 19th century and early 1900s experimented with crossbreeding bison with cattle in an effort to raise a hybrid for meat production.  This endeavor quickly ended since bison-cattle hybrids almost always resulted in female offspring and no viable male offspring.  Hybridization that results in one sex being absent, rare or sterile indicates evolutionary incompatibility between the two species [4].  Even though hybridization was a dead-end, the attempt introduced cattle genes into bison herds, known as genetic introgression. Many of the bison from these early hybridization efforts were used to initiate or grow other herds, injecting traits related to domestication into the bison herds and effecting physiology.  One study by Derr found bison with cattle-gene introgression tend to be smaller at an early age and never grow as large as more pure, wild bison [5].

Inbreeding:

Inbreeding involves the breeding of closely-related individuals and occurs in small herds or in herds maintained with few bulls [6], limiting the genetic diversity. Bison bulls will mate with as many cows as is possible, and dominant bulls will father more calves while less dominant bulls may not father any calves [7].  In small herds the genetic material of the dominant bulls will tend to be concentrated and passed on with genetic material of others bulls lost. The negative effects of inbreeding replace natural selection in determining the future genetic make-up.

Genetic Drift:

A change in the relative frequencies of alleles (one of two or more alternative forms of a gene found at the same place on a chromosome) is known as genetic drift. This process occurs in a population due to random events during survival and reproduction [8]. Random chance determines which genes or animals survive and reproduce, causing genetic change in a population.  For instance, a bison could break through the ice when crossing a river and drown. But the major source of changes in allele frequency lies in reproduction during the production of ova and sperm.  When ova and sperm are formed in cell division, chromosomes split leaving the reproductive cells with only half the chromosome set. Thus, during reproduction some alleles are discarded both from the bull and the cow. In large populations random events effects are relatively unimportant because opportunities are present for natural selection to work, mitigating any loss of genetic information. However, in small populations genetic drift may cause some genes to disappear, reducing the genetic diversity and evolutionary potential [9].

Artificial Selection—Domestication:

Domestication results from the replacement or weakening of natural selection by artificial, human-managed selection.  Thus domestication is eradication of wild bison by modification. For example, aurochs were continually domesticated eventually leading to modern domesticated cattle.  These efforts were so extensive aurochs no longer exist.  The essence of selective breeding involves humans deciding which individuals will produce the next generation which will better serve human goals [10].

One of the goals involves handling. Wild bison are difficult to handle, causing harm to the animal, causing potential damage to shoots and pens, and involving more time and effort on the part of the ranchers.  To mitigate handling issues, bison ranchers and farmers have found that by increasing the level of serotonin and lowering the levels of dopamine bison become more docile.  Over time selecting those animals with increased serotonin and lower dopamine for breeding will artificially select the more manageable bison, moving from wildness to domestication. 

Additionally, since ranching and farming are bottom-line businesses, each bison is seen as a productive unit.  Under this perspective management of bison will increase the number of cows altering the natural sex ratio. Bulls can breed many cows, whereas a cow will only have one, possibly two calves per year. Maximizing the commercial herd requires few bulls but many cows.  Besides, cows are also easier to handle than bulls. A biased sex ratio shifts the breeding behavior.  Cows that do not incite competition between bulls will more likely be bred.  Thus the traits associated with competition between bulls become artificially selected out.  Unfortunately, this management perspective not only occurs with private herds.  Public herds are seen as a revenue source, and consequently, subjected to the same practice [11].

               Given the above mechanisms pushing us toward the genetic extinction of wild bison, the question becomes:  how do we mitigate or prevent these processes?  Complete avoidance may not be possible. The means and objectives involved may be in conflict with each other and may require trade-offs.

Prevention and mitigation of the genetic extinction mechanisms contributing to the loss of wildness in bison will be explored in the next post.

Endnotes:

[1] Bison Bellows. Retrieved 09-Sep-2019 from https://www.nps.gov/subjects/bison/bison-bellows-12-13-15.htm.

[2] Bailey, James A. 47.  2013. American Plains Bison: Rewilding an Icon. Helena, MT. Sweetgrass Books.

[3] Bailey, 73.

[4] Hedrick, Paul W. “Conservation of Genetics and North American Bison (Bison bison)”. Journal of Heredity 2009: 100(4): 411-420.

[5] Bailey, 48.

[6] Bailey, 76.

[7] Lott, Dale F. 194. 2002. American Bison: A Natural History.  Berkeley. University of California Press.

[8] Bailey, 78.

[9] Bailey, 49-50.  Also for a full discussion of genetic drift see Bailey, 78-80.

[10] Lott, 196-198.

[11] Lott, 198-200.