by Jane Harris-Zsovan
Does the classification system used by biologists accurately reflect the path of natural selection through the generations? And does it trace the differentiation of species? Not necessarily.
Consider the Bovoids, genus bison (for example, the North American buffalo ) and genus bos (for example, domesticated cattle).
According to most theories of speciation, a cross between two genera (such as genus bison and genus bos) after a geographical separation of many thousands of years is unlikely. But tell that to a rancher in Western Canada or the United States where buffalo bison, raised on ranches, are interbred with cattle. (These animals have been known to interbreed since the 18th century.) The resulting offspring are called beefalo or cattalo. But there's more ...
Who are the bovoids?
Both domestic cattle and American bison can interbreed with their cousin, the European bison (the wisent), as well as with yaks and other members of the bos genus. A cattle/wisent cross is called a zubron. A yak/cattle hybrid is known as a dzo.
Yaks, domestic cattle (a descendent of the extinct aurochs ), European bison (wisent), and North American bison (both wood and plains) can interbreed with each other and the resulting hybrids. Cattalo, zubron, and dzo, are usually fertile, with the exception of some first generation hybrid males. Second generation hybrid males are fertile.
Hybridization of European and American bison does not appear to cause fertility problems even in first generation males. Some taxonomists argue that the wisent and the American bison are not separate species at all.
Who are the bison?
Ancestors of American bison and European bison are thought have descended from an ancient relative in Southern Asia over 400,000 years ago in the Pliocene epoch. These animals spread northward through Eurasia. Some herds headed westward into Europe, becoming the ancestors of the European bison.
The ancestors of the American bison crossed the Bering Strait in several waves, with fossilized evidence of their presence in North America dating to the Pleistocene epoch. According to the fossil record, the last migration of bison via the Bering Strait occurred over 10,000 years ago.
But, after millennia of separation, European and North American bison are still recognizable as bison.
The Bovoid family tree
Darwin’s theory of speciation through natural selection would predict that the hybridization of cows and yaks with bison is quite unlikely.
To see why it is unlikely, let's take a look at the family tree of cows, bison, and yaks. For simplicity, we’ll exclude some of the lesser known members of the bos genus, like the gaur (although they apparently interbreed with cattle):
Family: Bovidae
Sub Family: Bovinae (bison, buffalo, cattle and relatives)
Genus bison:
Species: American Bison: Bison bison
Sub-species
Bison bison bison (Plains bison)
Bison bison athabascae (Wood bison)
Species: European Bison (wisent) Bison Bonasus
Sub-species
Lowland wisent - Bison bonasus bonasus
Hungarian (Carpathian) wisent*
Caucasus wisent**
Genus: bos (oxen and true cattle)
Species
aurochs (bos Taurus )***
domesticated cow (bos Taurus)
yak (bos grunniens)
* Bison bonasus hungarorum - extinct
** Bison bonasus caucasicus - extinct with some hybrid survivors
***so named by ITIS or (Bos primigenius Taurus) as per the International Commission on Zoological Nomenclature
The conventional “Tree of Life” puts cattle and yaks in the genus Bos. Cattle, thought to be descendants of the aurochs, diverged from the Bovidae family thousands of years before the first bison walked over the Bering Straight.
It's messy, but why is it a problem?
The existence of the beefalo and its cousins, the dzo and zubron, show us that - after millennia of separation - the gene pool of individuals in the genus bison and genus bos has not changed enough to make interbreeding impossible. And, in the case of European bison and American bison, there is debate as to whether speciation has fully occurred.
Clearly, the Darwinian theory of speciation by natural selection is not the whole story. Maybe it’s not the true story at all.
Reference
University of Michigan Museum of Zoology. www.ultimateungulate.com/Artiodactyla/Bison_bison.html
Hinterland Who's Who.
Alberta Bison Association www.cfwf.ca
Scoops Newletter of the Alberta Farm Writers Association.).
From www.sfsu.edu/~geog/bholzman/courses/
Nowak, R. M. [editor]. 1991. Walker's Mammals of the World (Fifth Edition). Baltimore: The Johns Hopkins University Press.
Canadian Species at Risk September 2007, Committee on the Status of Endangered Wildlife in Canada COSEWIC.gc.ca http://www.cosewic.gc.ca/eng/sct0/rpt/dsp_booklet_e.htm Animal Diversity.
ummz.umich.edu/site/acct.classification Nowak, R. M. [editor]. 1991.
Walker's Mammals of the World (Fifth Edition). Baltimore: The Johns Hopkins University Press.
http://www.ansp.org/museum/jefferson/otherFossils/bison.php Academy of Natural Sciences Thomas Jefferson Fossil collection.
http://www.nps.gov/cavo/historyculture/people.htm Capulin Volcano National Monument, National Park Services, U.S. Department of the Interior
www.ultimateungulate.com/Artiodactyla/Bison_bison.html -
http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180706 Biodiversty and Conservation: A
Hypertext book by Peter J. Bryant, http://www.dbc.uci.edu/~sustain/bio65/lec04/b65lec04.htm