This project is about the recurring incidence of individuals with primitive sheep characteristics in Australian Merino sheep flocks. Why does it occur? There is no accepted mechanism in quantitative genetics to explain why a purebred domesticated population would revert toward its wild ancestors.
There is a manuscript ( see the evol.pdf file in the mev subdirectory) which was written in 1989 and has remained unpublished. It shows that neo-primitive individuals occur in some Merino stud flocks, in some selection experiments, and in unselected flocks. There is a rewrite of this manuscript being undertaken by Dr Philip Moore, and the rewrite files are available in the mev-rewrite subdirectory.
It is generally accepted among Australian Merino breeders that a flock will decline in productivity and wool quality, if it is not culled at all, or if it is selected in an inappropriate manner. The practice of rogueing is an accepted procedure among plant breeders. It would seem that practical breeders accept atavism, but geneticists have a problem with the concept.
We would welcome communication from anyone who has observed this phenomenon in other domesticated species. What do breeders of other domesticated species think would happen under an inappropriate selection regime, and is it a movement in the direction of their wild ancestors?
The phenomenon was known to Darwin '.. our varieties certainly do occasionally revert in some of their characters to ancestral forms.' [ Origin of Species, Chapter I, page 37 ]
- A major gene segregating in Merino flocks
- Migration - that is introgression of the Merino with genetic material from other breeds
- Canalisation of wool follicle development being stronger in the wild type than in domesticated populations
- Something special about the sheep's fleece and fitness, possibly involving birthcoats
- Phenodeviants or developmental noise
- Dominance can explain simple one-locus cases of reversion or atavism
If you have an opinion or some data or some ideas on a genetic basis for this phenomenon, please submit a pull request. We would like to assemble a collection of data, breeder experience, and possible genetic explanations.
In one word, an explanation. Any unexplained phenomenon is a challenge. We need a theory of the quantitative genetics of extreme individuals.
Darwin, C (1859) The Origin of Species. Mentor, New York, 1960