Ovis aries: More about Mary's Little Lamb

Sheep through the Ages

     The different species found within the genus Ovis have different numbers of chromosomes.  Phylogenists believe that those with a higher chromosome count diverged earliest within the sheep lineage. Ovis vignei (Urial sheep) have a diploid count of 58, and Ovis ammon (Argali sheep) have a diploid count of 56. Even earlier forms of Ovis could have had a 60 diploid count. The majority of Ovis have diploid number 54, including Ovis aries, Ovis musimon, Ovis orientalis, Ovis dalli, and Ovis canadensis. Ovis nivicola bring up the rear with a diploid count of 52.
     More specifically, Ovis aries is believed to have branched off of Ovis musimon, but there is no fossil evidence to support this theory. My phylogenetic tree places the sheep in order of their chromosome count. The furthest up within the Ovis genus represents the oldest species and the lowest represents the youngest species. However, it is unlikely that each species gave rise to the species below it in this linear fashion. There was probably a division of two or three groups of sheep who separately evolved into the various species we see today.

     Although this phylogenetic tree doesn't contain all of the Ovis species, it better depicts how the sheep may have diverged into two lineages. Ovis ammon and Ovis aries make up the first division. Ovis ammon was probably the ancestor of Ovis musimon who eventually gave rise to Ovis aries, the domestic sheep that we see today.
    Developing a phylogenetic tree is no easy task. There are multiple ways in which different phylogenists can group organisms. While one phylogenist may use a method like morphology, another phylogenist may use shared derived characteristics. A third phylogenist may use a combination of the two techniques. The possibilities are limitless, and no one tree can be completely verified.

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