Eligmodontia

Eligmodontia belongs to the subfamily Sigmodontinae and the tribe Phyllotini. Eight[6] species of Eligmodontia have been described, three of these containing 2 subspecies each. In a 1962 revision of the tribe Phyllotini,[7] Philip Hershkovitz synonymized all 10 named forms of Eligmodontia known by then into a single species with two subspecies. The lighter and larger northern populations were known as Eligmodontia typus puerulus, and the darker and smaller southern ones as E. typus typus. For nearly 30 years, Hershkovitz's approach was followed until karyotypes and molecular data became available. Today, five distinct karyotypes have been described, and as many distinct clades have been found.[8]

The following 5 species can be unequivocally recognized:[9]

• Monte Gerbil Mouse or Monte Laucha, Eligmodontia moreni
• Andean Gerbil Mouse or Altiplano Laucha, Eligmodontia puerulus
• Eligmodontia hirtipes (Thomas, 1902) (recently separated from E. puerulus)
• Morgan's Gerbil Mouse or Western Patagonian Laucha, Eligmodontia morgani
• Eastern Patagonian Laucha, Eligmodontia typus
• Highland Gerbil Mouse, Eligmodontia bolsonensis Mares, Braun, Coyner & van den Bussche, 2008

The case of the newly proposed species E. bolsonensis is quite interesting. Phylogenetically it is part of the same clade as E. typus. Yet there seems to have been reproductive isolation between these two parapatric populations – the population separated as bolsonensis occurs where the range of E. typus extends northwards and upslope into the Andes. And while splitting E. bolsonensis from E. typus would leave the latter non-monophyletic as regards because of incomplete lineage sorting, the two differ weakly but consistently in several molecular and morphological characters.[5]

Altogether, this seems to represent a case of ongoing parapatric speciation, with a population of E. typus becoming separated at the northern and upper limit of its range not more than a few 100,000 years ago. Whether they are to be treated as species or subspecies is essentially a matter of what species concept one prefers. Additionally, it appears that the karyotype reported for E. typus originates from the upland population, and that the karyotype of E. typus proper is unknown.

A crude molecular clock – uncalibrated due to the absence of fossil Eligmodontia – has been applied to this genus. However, it agrees well with the emergence of key geographical features in the region. The data suggests that the genus evolved approximately in the mid-late Miocene (Serravallian–Tortonian), about 13-7 mya (million years ago). Presumably, the original Eligmodontia occurred in the region now inhabited by E. typus. Increasing aridity as a consequence of the beginning Quaternary glaciation combined with the uplift of the Patagonian Andes during the latter Pliocene (late Piacenzian to Gelasian, about 3–1.7 mya) split the population into a lowland and a montane lineage. The latter expanded southwards in the Gelasian, these populations becoming increasingly isolated and eventually became the E. morgani of our time. The same happened somewhat later, at the beginning of the Early Pleistocene (about 2–1.5 mya[10]) at the northern end of the genus' range, with the separating Altiplano population becoming the ancestors of E. hirtipes. Finally, in the Middle Pleistocene local uplifts in the Pampean region separated the ancestors of E. moreni and E. puerulus, and the lowlands population, isolated form its relatives since more than one million years, began also expanding into the uplands, yielding E. (t.) bolsonensis which currently well on its way to become another highly distinct species.[5]