Malpaisomys

Renaud, Sabrina & Michaux, Jacques, 2004, Parallel evolution in molar outline of murine rodents: the case of the extinct Malpaisomys insularis (Eastern Canary Islands), Zoological Journal of the Linnean Society 142 (4), pp. 555-572 : 568

DIFFERENTIATION AMONG MALPAISOMYS

POPULATIONS

After the colonization event, the ancestor of Malpaisomys may have evolved into the endemic form within an isolated gene pool. This evolution cannot be tracked in the fossil record because the oldest Pleistocene deposits in the eastern Canary Islands are of Malpaisomys rather than an intermediate form. Still, its late evolution during the Pleistocene until its extinction during historical times can be assessed. Preliminary results emerge from the four deposits examined. A morphological difference is observed between the Pleistocene and the Holocene populations from Fuerteventura ( Table 2). During this time interval, Mus colonized the islands and began to feature greatly in the deposits ( Michaux et al., 1996; Castillo et al., 2001). The invasion of an island by a generalist competitor has been observed to cause a size decrease of the resident species in several small mammals (Yom- Tov, Yom-Tov & Moller, 1999). The size decrease and shape difference among Malpaisomys populations may thus document a response to the arrival of the generalist house mouse. Still, the evolutionary rate estimated for this evolution ( Fig. 7 ) does not differ from observations of fossil lineages, suggesting that the new ecological interaction did not trigger an exceptionally rapid morphological evolution.

The population of the islet Lobos (4.6 km 2) also differentiated from the Holocene populations in Fuerteventura (1633 km 2; Fig. 5 ), although the presence of Mus suggests a similar age. This may document a smallscale insularity effect due to partial genetic isolation.

Figures

Figure 5. Morphological differentiation within Malpaisomys. Size of the first upper (A) and lower (B) molars. Shape of the first upper (C) and lower (D) molars, estimated by the scores on the first and second canonical axes of the analysis of Malpaisomys. The dotted ellipses indicate the 95% confidence interval for each sample.

Figure 7. Relationship between time interval and morphological evolution regarding size (A, C) and shape (B, D) of the first upper molars. Size distance is estimated as differences in A0, shape distance as the Euclidean distance calculated based on the Fourier coefficients of the first nine harmonics. (A, B) Log-log relationship between evolutionary rates and time interval. (C, D) Relationship between time interval and morphological distance. Lines correspond to a linear regression on the Occitanomys/Stephanomys lineage (black line), the Paraethomys lineage (light grey line), and modern taxa (dark grey line). Dotted lines P <0.05; solid line P <0.001. (A) pair including Malpaisomys; (Δ) corresponds to the distance between a pair of Malpaisomys samples, Costa Calma and CVA4. Locality or sample abbreviations per Table 1.