Taxonomy of House Sparrows and Their Allies in the Mediterranean Basin

Approximately 3600 years ago there occurred a significant movement of northern European agricultural man into southern European and Mediterranean regions (establishment of the Lake Dwellers in northern Italy, see Brea 1946 et seq.). Seemingly also with man came his newly-acquired commensal, the House Sparrow (Passer domesticus), which had been living with man in northern Europe since its presumptive origins in the Near East, perhaps some 5500 years earlier. Indigenous sparrows in Italy, probably also commensal in 1600 BC and almost certainly Willow Sparrows (conventionally considered a species by most taxonomists as Passer hispaniolensis), shared a common ancestry with House Sparrows but were phenotypically distinct. Interbreeding of the two phenetic kinds ensued, followed by establishment of populations of phenetic heterogeneity throughout the Italian peninsula. Similar instances of commingling of gene pools occurred in other places in and around the Mediterranean, but on the Balearic Islands only House Sparrows became established and on Sardinia only Willow Sparrows. Later, only 300 to 400 years ago, there occurred persistent cold weather, the "Little Ice Age," sufficient to close Alpine passes for most of each year, with the result that the sparrows of the Italian peninsula were cut off from mainland European sparrows for many tens of generations. This short period of fairly stringent isolation presumably allowed the ongoing processes of selective reassortment of Italian sparrow genotypes to proceed toward relative stability more quickly than they would have otherwise, and upon re-establishment of Alpine contact gene flow between Italian sparrows and House Sparrows did not occur as freely as formerly. These events, or some such similar set, have given us a remarkable complex of sparrows around the Mediterranean Sea, and their variable geographic, phenetic, and genetic characteristics provide a continual challenge to understanding present relationships (e.g., Brehm 1842; Chigi 1914; Meise 1936; Macke 1965). One of the most intriguing aspects of the biology of these sparrows is that geographically disjunct populations may be phenetically much alike. This has suggested that they may also be genetically similar, but experimental verification of this hypothesis is lacking. It is known that the phenotypic feather coat of the sparrows has a highly polygenic basis (see below), and it is thus entirely possible that similar phenotypes can have different genotypes, and vice versa. Polygenic control of gross phenotype may well be the expected condition, but we need not merely assume it for Mediterranean sparrows; Macke (1965; see also table 7) has shown that the F1 of the cross, House Sparrow x Willow Sparrow, may include a sampling of nearly the entire range of intermediate phenotypes, hence of genotypes. The importance of this finding cannot be exaggerated. It means at one level that wherever there is a restricted range of phenotypes (= genotypes) the action of selection or restricted founder genefrequencies is strongly implied. But, if clines in phenotypes occur, then clines in genefrequencies occur, and these can only be responses to differential selection pressures. It means also, but conversely, that we must be prepared to find populations in which a wide range of phenotypes occurs. Consequent opportunities for "hybrid swarms" or for geographically-oriented spectra of phenotypes, or for intermediate conditions, have been only in part realized, owing variously to coincident gene frequencies in founder populations, selection of coincident characters, gene-flow, absence of gene-flow, etc. Current distribution of phenotypes and possible causal factors for such distributions are set forth below.