Bison - Evolution and Genetic History

Evolution and Genetic History

The bovine family (Taurids and Bisonids) diverged from the common ancestral line with Water Buffalo and African Buffalo about 5 to 10 million years ago. Thereafter, the family lineage of bison and taurine cattle does not appear to be a straight forward ‘tree’ structure as is often depicted in much evolution, because there is evidence of interbreeding and crossbreeding between different species and members within this family, even many millions of years after their ancestors separated into different species. This cross breeding was not sufficient to conflate the different species back together, but it has resulted in unexpected relationships between many members of this group, such as Yak being related to American Bison, when such relationships would otherwise not be apparent.

A 2003 study of mitochondrial DNA indicated four distinct maternal lineages in tribe Bovini:

1. Taurine cattle and zebu,
2. Wisent,
3. American bison and yak, and
4. Banteng, gaur, and gayal.

However, Y chromosome analysis associated wisent and American bison. An earlier study using amplified fragment length polymorphism (AFLP) fingerprinting showed a close association of wisent and American bison and probably with yak, but noted that the interbreeding of Bovini species made determining relationships problematic.

The Steppe Bison (Bison priscus) diverged from the lineage that led to cattle (Bos taurus) approximately 2 to 5 million years ago. The Bison genus is clearly in the fossil record by 2 million years ago. The Steppe Bison spread across Eurasia and was the bison that was pictured in the ancient cave drawings of Spain and Southern France

The European Bison or Wisent arose from the Steppe Bison, without fossil evidence of other ancestral species between the Steppe Bison and the European Bison, though the European Bison might have arisen from the lineage that led to American Bison if that lineage backcrossed with the Steppe Bison. Again, the web of relationships is confusing, but there is some evidence that the European Bison is descended from bison that had migrated from Asia to North America, and then back to Europe, where they crossbred with existing Steppe Bison.

At one point, some Steppe Bison cross bred with the ancestors of the modern Yak. After that cross breeding, a population of Steppe Bison (Bison priscus) crossed the Bering Land Bridge to North America. There is evidence of multiple crossings of bison to and from Asia starting before 500,000 years ago and continuing until at least 220,000 years ago. The Steppe Bison spread through the northern parts of North America and Steppe Bison lived in Eurasia until approximately 11,000 years ago and North America until 4,000 to 8,000 years ago.

Bison latifrons (Giant Bison or Longhorn Bison) is thought to have evolved in midcontinent North America from Bison priscus, after the Steppe Bison crossed into North America. Giant Bison (Bison latifrons) appeared in the fossil record appoximately 500,000 years ago. B. latifrons was one of many species of North American megafauna which became extinct during the transition from the Pleistocene to the Holocene epoch (an event referred to as the Quaternary extinction event). It is thought to have disappeared some 21,000–30,000 years ago, during the late Wisconsin glaciation.

The Bison latifrons (Giant Bison or Longhorn Bison) species was replaced by the smaller Bison antiquus. Bison antiquus appeared in the North American Fossil record approximately 250,000 years ago. Bison antiquus in turn evolved into the Bison occidentalis, then into the yet smaller Bison bison — the modern American bison — some 5,000 to 10,000 years ago. Some researchers consider Bison occidentalis to be a sub-species of Bison antiquus

During the population bottleneck, after the great slaughter of American Bison during the 1800s, the number of bison remaining alive in North America declined to as low as 541. During that period, a handful of ranchers gathered remnants of the existing herds to save the species from extinction. These ranchers bred some of the bison with cattle in an effort to produce “cattleo”. Accidental crossings were also known to occur. Generally, male domestic bulls were crossed with buffalo cows, producing offspring of which only the females were fertile. The crossbred animals did not demonstrate any form of hybrid vigor, so the practice was abandoned. The proportion of cattle DNA that has been measured in introgressed individuals and bison herds today is typically quite low, ranging from 0.56 to 1.8%. In the United States, many ranchers are now utilizing DNA testing to cull the residual cattle genetics from their bison herds. The U.S. National Bison Association has adopted a code of ethics which prohibits its members from deliberately crossbreeding bison with any other species.

Some cattle breeds are intentionally bred with bison to produce, for instance, Beefalo hybrids. Wisent-American bison hybrids were briefly experimented with in Germany (and found to be fully fertile) and a herd of such animals is maintained in Russia all the time. A herd of cattle-wisent crossbreeds (Zubron) is maintained in Poland. First-generation crosses do not occur naturally, requiring caesarean delivery. First-generation males are infertile.