Evolution and Speciation

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The brine shrimp Artemia comprises a group of bisexual and parthenogenetic, morphologically similar, species very likely to have diverged from an ancestral form living in the Mediterranean area some 5.5 million years ago (Abreu-Grobois and Beardmore, 1982; Abreu-Grobois, 1987; Badaracco et al. 1987), though this estimate, based on allozymes, could be somewhat conservative as compared to that based on mitochondrial DNA (Perez et al. 1994). At that time the area was, according to geological indications, the only place in the world exhibiting for significant periods of time the very high salinity required for Artemia to thrive (Abreu-Grobois, 1987; Badaracco et al. 1987). The hypothesis of the Mediterranean as the centre of radiation for Artemia is also supported by the diversity of Artemia types currently found in the area, i.e. bisexuality and parthenogenesis on the one hand, together with diploidy and polyploidy on the other (see below).