Editing American bison (section)

=== Texas State Bison Herd ===
The Texas State Bison Herd (TSBH), also known as the Goodnight herd, was established by [[Charles Goodnight]] in the mid-1880s with five wild-caught calves. In 1887, the herd consisted of 13 individuals; in 1910, the population consisted of 125 individuals; and in the 1920s, the population ranged from 200 to 250 individuals. In 1929, Goodnight died and the herd switched hands multiple times, leaving the population of the herd unknown from 1930 until the herd was donated to the State of Texas in 1997, with a population of 36 individuals, solely descended from the original five calves.<ref name="Hedrick-2009" /> By 2002, the population of the TSBH consisted of 40 individuals and had concerningly low birth rates and high rates of calf mortality. This led to extra attention being given to this herd by conservationists who then performed significant amounts of genetic testing.

Goodnight was an advocate for the hybridization of bison with cattle, in the hopes of creating a stronger and healthier breed. When the herd was donated to the State of Texas, genetic testing revealed that 6 out of 36 individuals still carried cattle mitochondrial DNA.<ref name="Texas" />

Researchers found that the average number of [[allele]]s per [[Locus (genetics)|locus]] and the [[Zygosity#Heterozygous|heterozygosity]] levels (a measure of [[genetic diversity]], where high heterozygosity is representative of high genetic diversity) for the TSBH were significantly lower than that of the Yellowstone National Park bison population and the [[Theodore Roosevelt National Park]] bison population.<ref name="Texas" /> Additionally, of the 54 [[Microsatellite|nuclear microsatellites]] that were examined, the TSBH had 8 monomorphic loci (i.e., each loci had only one allele), whereas in both the Yellowstone and Theodore Roosevelt herds there was only one monomorphic locus, indicating a much lower level of genetic diversity in the TSBH.<ref name="Texas" /> The Yellowstone herd had an average number of alleles per locus of 4.75, the Theodore Roosevelt National Park herd had an average of 4.15 alleles per locus, but the TSBH only had an average of 2.54 alleles per locus, statistically significantly lower than the others.<ref name="Texas" /> The heterozygosity level of the Yellowstone, Theodore Roosevelt, and TSBH populations were 0.63, 0.57, and 0.38 respectively, with the TSBH again having a statistically significantly lower value.<ref name="Texas" /> This low genetic diversity found in TSBH is likely due to the critically low starting population, several additional bottlenecks throughout the herd's history–leading to [[inbreeding depression]]–,<ref name="Texas" /> and a continuously low population allowing for [[genetic drift]] to have a large effect. Before any addition of new individuals, the rate of loss of genetic diversity was estimated to be between 30 and 40% over the proceeding 50 years.<ref name="Texas" />

The inbreeding depression resulting from the multiple extreme population bottlenecks in the TSBH led to a coefficient of inbreeding of 0.367, equal to the level of inbreeding that results from two generations of full-siblings mating.<ref name="Hedrick-2009" />

The Texas State Bison Herd is also a useful example of the deleterious effects of extreme population bottlenecking, with an average natality rate of 0.376 offspring per female and a 1st-year mortality rate of 52.6% from 1997 to 2002, compared to an average natality rate of 0.560 offspring per female and a 1st-year mortality rate of 4.2% for the other bison herds.<ref name="Texas" />

Additionally, if it were not for the intervention of conservationists, the Texas State Bison Herd would have most likely gone extinct, as the population bottleneck would have proven to be too severe. Multiple population models based on the genetics of the TSBH in the early 2000s predicted a 99% chance of extinction of the TSBH in less than 50 years, with an estimation in 2004 giving the TSBH a 99% chance of extinction in 41 years without the introduction of any outside individuals (Halbert et al. 2004). Importantly for conservation, another simulation predicted that the addition of multiple (3–9) outside male bison into the herd would increase genetic diversity enough to give the herd a 100% chance of surviving for another 100 years.<ref>{{cite journal |last1=Halbert |first1=Natalie D. |last2=Grant |first2=William E. |last3=Derr |first3=James N. |date=2005-01-20 |title=Genetic and demographic consequences of importing animals into a small population: a simulation model of the Texas State Bison Herd (USA) |url=https://www.sciencedirect.com/science/article/pii/S0304380004003710 |journal=Ecological Modelling |language=en |volume=181 |issue=2 |pages=263–276 |doi=10.1016/j.ecolmodel.2004.02.022 |bibcode=2005EcMod.181..263H |issn=0304-3800}}</ref>

Conservation efforts have led the current TSBH population to be at the carrying capacity of their habitat, at around 300 individuals.