The name Anser scaldii was first used by Van Beneden (1872) in a brief text that read ‘Nous avons recu un humérus dans un parfait état de conservation, trouvé dans le crag, à Anvers’. The name was also used by Van Beneden (1873), but in both instances it is a nomen nudum. The name was made valid for the purposes of nomenclature by Lambrecht (1933: 368) when he entered Anser scaldii Van Beneden, 1872, with the following description and information: ‘Humerus typisch anserin, von der Größe von Tadorna casarca. Länge 129 mm. Material: Humerus im Mus. Bruxelles. Alter und Fundort: Obermiozän (Bolderian), Antwerpen. Etymologie: Artname nach der Schelde: Scaldia.’ At the same time he mistakenly gave the original combination as Anas scaldii Van Beneden 1872, which error was perpetuated by Gaillard (1939), Brodkorb (1964), Howard (1964), and Bochenski (1997), as noted by Mlíkovský (2002: 125). The statement by Lambrecht that this fossil is of similar length to humeri of Tadorna prompted Worthy et al. (2007) to suggest that Anser scaldii may have a bearing on the evolution of Tadornini in Europe. Accordingly, we re- examined the holotype in the Department of Paleontology, Royal Belgian Institute of Natural Sciences, Brussels, Belgium, to ascertain its relationships and its significance in Anseriform evolution.
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With an increasing number of reported cases of hybridization and introgression, interspecific gene flow between animals has recently become a widely accepted and broadly studied phenomenon. In this study, we examine patterns of hybridization and introgression in Ophthalmotilapia spp., a genus of cichlid fish from Lake Tanganyika, using mitochondrial and nuclear DNA from all four species in the genus and including specimens from over 800 km of shoreline. These four species have very different, partially overlapping distribution ranges, thus allowing us to study in detail patterns of gene flow between sympatric and allopatric populations of the different species. We show that a significant proportion of individuals of the lake-wide distributed O. nasuta carry mitochondrial and/or nuclear DNA typical of other Ophthalmotilapia species. Strikingly, all such individuals were found in populations living in sympatry with each of the other Ophthalmotilapia species, strongly suggesting that this pattern originated by repeated and independent episodes of genetic exchange in different parts of the lake, with unidirectional introgression occurring into O. nasuta. Our analysis rejects the hypotheses that unidirectional introgression is caused by natural selection favoring heterospecific DNA, by skewed abundances of Ophthalmotilapia species or by hybridization events occurring during a putative spatial expansion in O. nasuta. Instead, cytonuclear incompatibilities or asymmetric behavioral reproductive isolation seem to have driven repeated, unidirectional introgression of nuclear and mitochondrial DNA into O. nasuta in different parts of the lake.
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