Ancon sheep

Ancon sheep (also called "Otter" sheep) are a grouping of domestic sheep with long bodies and very short legs, with the fore-legs being crooked. The term is generally applied to a line of sheep bred from a single affected lamb born in 1791 in Massachusetts, USA. The breed was artificially selected and maintained for its desirable inability to jump over fences. It was allowed to go extinct in 1876 when it was no longer required.^[1]

The name "Ancon" has also been applied to other strains of sheep arising from individuals with the same phenotype, such as a Norwegian stock bred from a single individual born in 1919, and a Texan, USA stock bred from a single individual born in 1962. These lineages were also allowed to go extinct after scientists no longer needed them for genetic research.^[1][2]

Excavations in Leicester, UK have also revealed metacarpals, metatarsals and phalanges characteristic of Ancon sheep that date to approximately AD 1500, thereby demonstrating that the phenotype has arisen independently at least four times.^[1]

The unique features of ancon sheep are caused by a recessive dwarf mutation, which often results in crippling. The specific effect of the mutation is to cause chondrodystrophy.^[1][3]

Significance to the history of biology

Despite the Ancon's small numbers the sheep's contribution to the history of biology has been substantial. Several chapters in Charles Darwin's On the Origin of Species references the Ancon as an example of his argument that offspring inherit independent characteristics from their parents. The notion is perhaps best understood by the claim it denounced: the long-held belief among naturalists and breeders that inherited traits were a "blending" of the traits of each parent.^[4] If that were true, the offspring of an Ancon and a common sheep would have a height in-between that of each parent. Instead, the resulting offspring were either Ancon or common, thus supporting Darwin's contention.

Darwin's review of the Ancon sheep helped lead to the establishment of an early consensus view of the sheep's role in understanding biology. Darwin referred to the Ancon as a "sport", or a new species or breed born suddenly from a prior species (as a form of phylogenetic variation, not to be confused with the "sporting plant" referred to in the study of botany) as opposed to a new species developing gradually over a long period of time through the natural selection of multiple traits.

Darwin believed that this was possible because the Ancon sheep was born and bred in captivity. In his 1868 work The Variation of Animals and Plants under Domestication, Darwin argued that sudden mutations such as the Ancon are rare to non-existent outside of captivity. In his view, species change through the mutation and natural selection of minor traits which, when added up over time, gradually establish new species.^[5] Later scholars supporting Darwin's view also pointed out that the Ancon sheep had essentially a single trait - their short legs - which also made rapid mutation unusually likely, since other species and breeds are set apart from their progenitors by many traits. ^[6]

References

1. 1 2 3 4 Gidney, Louisa (May–June 2007). "Earliest Archaeological Evidence of the Ancon Mutation in Sheep from Leicester, UK". International Journal of Osteoarchaeology. John Wiley and Sons. 17 (3): 318–321. doi:10.1002/oa.872. ISSN 1099-1212. 
2. ↑ Shelton, Maurice (1968). "A recurrence of the Ancon dwarf in Merino sheep" (PDF). Journal of Heredity. John Wiley and Sons. 59 (5): 267–268. PMID 5753237. Retrieved 2014-03-07. (registration required (help)). 
3. ↑ Chang, T. K. (1949). "Crippling in chondrodystrophic (Ancon) sheep". Growth. 13 (3): 299–307. PMID 18142372. 
4. ↑ Schwartz, Karlene; Vogel, Jane (Dec 1994). "Unraveling the Yarn of the Ancon Sheep". BioScience. Oxford University Press. 44 (11): 764–768. doi:10.2307/1312585. ISSN 0006-3568. Archived from the original on unknown. Retrieved 2014-07-03. (subscription required (help)). Ancon sheep are important to the history of biology because Charles Darwin used them to support his argument that animals inherit parental traits without blending: "When turnspit dogs and Ancon sheep, both of which have dwarfed limbs, are crossed with common breeds, the offspring are not intermediate in structure, but take after either parent." Before Darwin's day, many plant and animal breeders assumed that parental traits would mingle in the offspring.  Check date values in: |archive-date= (help)
5. ↑ Schwartz, Jeffrey (January 13–16, 2005). "A Cultural History of Heredity III: 19th and Early 20th Centuries" (PDF). Darwinism versus Evo–Devo: a late–nineteenth century debate. A Cultural History of Heredity III: 19th and Early 20th Centuries. Max Planck Institute for the History of Science. pp. 67–84. Retrieved 7 March 2013. 
6. ↑ Eigenmann, C.H. (1895). "The Study of Variation". Proceedings of the Indiana Academy of Science. 5: 265–278. Retrieved 2014-03-07.