Single origin of human commensalism in the house sparrow

Abstract

The current, virtually worldwide distribution of the house sparrow (Passer domesticus) is a result of its commensal relationship with humans. It has been suggested that long before the advent of agriculture, an early glacial advance resulted in two disjunct ranges of ancestral house sparrows – one in the Middle East and another on the Indian subcontinent. Differentiation during this period of isolation resulted in two major groups of subspecies: the domesticus group and the indicus group. According to this hypothesis, commensalism with humans would have evolved independently in the two regions and at least twice. An alternative hypothesis is that morphological differences between the subspecies represent very recent differentiation, following expansions from a single source. To test between these hypotheses, we analysed genetic variation at the mitochondrial DNA control region and at three nuclear loci from several house sparrow populations in Europe, Asia and North Africa. No differentiation between the indicus and domesticus groups was found, supporting the single origin hypothesis. One of the subspecies in the indicus group, P. d. bactrianus, differs ecologically from other house sparrows in being migratory and in preferentially breeding in natural habitat. We suggest that bactrianus represents a relict population of the ancestral, noncommensal house sparrow. When agricultural societies developed in the Middle East about 10 000 years ago, a local house sparrow population of the bactrianus type adapted to the novel environment and eventually became a sedentary, human commensal. As agriculture and human civilizations expanded, house sparrows experienced a correlated and massive expansion in range and numbers. The pattern of genetic variation analysed here is consistent with this scenario.

Local adaptation within a hybrid species

“…Indeed, bite force has been shown to covary with several beak dimensions (Herrel et al, 2005). In addition, some Passer species, such as the Italian and house sparrows are human commensals (Summers -Smith, 1988;Saetre et al, 2012) and might have evolved adaptations in beak size and shape in response to the diet associated with agriculture, unlike other species less associated with humans (such as the Spanish sparrow, the other parental species of the Italian sparrow). In other bird species, beak shape has been shown to play an important role in processes of adaptive radiation (Schluter, 2000) and ecological character displacement (for example, Darwin's finches; Lack, 1947).…”

“…Population genetic inference indicates that the human-commensal house sparrow expanded massively in population size and distribution from a center of origin in the Middle East through the Palearctic region some 3000-7000 years ago, in parallel with the expansion of human agricultural societies (Saetre et al, 2012). In contrast, the Spanish sparrow has resided in the Mediterranean region for a longer time (Ericson et al, 1997).…”

Habitat urbanization and its effects on birds

“…Compared to urban adapters which are often early successional species from more natural habitats adjacent to cities, exploiters are well adapted to human-dominated landscapes, often sharing a long common history with humans (e.g. the House Sparrow, Ericson 1997, Saetre et al 2012.…”

Urbanized birds have superior establishment success in novel environments

“…Furthermore, these two measures of urbanization are strongly correlated with two other indices of urbanization in birds (Møller 2014b). These indices of urbanization all reflect the ongoing process of urbanization (Evans et al 2010) that started when the first commensal species such as the house sparrow Passer domesticus became associated with humans (Saetre et al 2012). It is important to remember that urbanization is still ongoing, as reflected by most urban species having colonized cities quite recently Møller (2014a).…”