How House Sparrows Became Our Closest Avian Companion
Despite being wild birds, house sparrows occur almost everywhere that people live, and are almost never found where people are absent. Did these birds evolve special adaptations that help them live in such close association with humans for so long?
by GrrlScientist for Forbes | @GrrlScientist
(Credit: hedera.baltica / CC BY-SA 2.0)
House sparrows are everywhere, it seems. Snacking on discarded food in the streets of the world’s cities and towns, popping up around seed spills on farmlands, and nesting in the eaves of our dwellings, these small brown, grey, and black songbirds can be found almost everywhere that people live. This small bird has such a strongly human-dependent ecology that it is known to go locally extinct when we abandon our settlements.
The house sparrow’s long, close association with humans has affected the species profoundly in other ways, too. For example, a study earlier this year by Tore Elgvin, an evolutionary ecologist at the University of Oslo’s Centre for Ecological and Evolutionary Synthesis, and his collaborators, found that the house sparrow hybridized with a close relative, the Spanish sparrow, and this gave rise to the Italian sparrow (ref). This hybridization probably occurred shortly after house sparrows arrived in Europe, as they tagged alongside colonizing prehistoric farmers.
Such companion species — often known as anthrodependent or human-commensal taxa — differ from domesticated animals because humans don’t play a direct role in selectively breeding them to conform to any particular “type”. Other ancient companion species to humans include a variety of rodents — especially the house mouse, and black and brown rats — which have spread far and wide around the planet, thanks to agriculture, permanent human settlements, and urbanization. As these companion species scavenged our rubbish and resources, they came to depend upon humans over time and developed an abiding association with human civilization, too.
The house sparrow is a well-studied scientific model for researching adaptation to urban environments (ref) and quantitative genetics (ref & ref). But considering how familiar the house sparrow is, it’s quite surprising how little we actually know about their evolutionary history and how they came to develop such a surprisingly persistent relationship with humans. Which raises the question: have house sparrows evolved specific adaptations for living alongside humans?
Meet the study sparrows
The house sparrow, sometimes referred to in the US as the “English sparrow”, Passer domesticus, is the most widespread member of the Passer sparrow group, naturally being found across Europe and the Middle East (blue; Figure 1). The hybrid Italian sparrow, Passer italiae, is a close relative that occurs on the Italian peninsula and also on some Mediterranean islands (yellow; Figure 1). Another close relative, the Spanish sparrow, Passer hispaniolensis (red; Figure 1), occurs across southern Europe, northern Africa, the Middle East and central Asia, where it co-occurs alongside yet another close relative, the Bactrianus sparrow, Passer domesticus bactrianus (dark blue; Figure 1).
(Credit: Mark Ravinet / doi:10.1098/rspb.2018.1246.)
The Bactrianus sparrow, which is confined to the Middle East and to the Central Asian steppes, is currently classified as a house sparrow subspecies (although some authorities think it’s a separate species because it is very different from house sparrows and rarely hybridizes with them). Amongst the Passer sparrow group, the Bactrianus sparrow is quite special because it is truly wild: unlike the house sparrow, the Bactrianus sparrow is migratory, it avoids people and their settlements, its skull morphology is distinct, and it also eats a very different diet.
The Bactrianus sparrow has been around for quite some time: it probably diverged prior to the evolution of human commensalism in house sparrows, and thus, can be considered a proxy for the ancestral house sparrow. Based on this assessment, comparing the DNA of the three European sparrows with the non-commensal Bactrianus sparrow may reveal genetic “signatures” of strong selection on the house sparrow genome that are not present in the Bactrianus sparrow. Thus, a comparative genome scan may shed light on how house sparrows adapted to associate so closely with humans.
(Credit: Mark Ravinet / doi:10.1098/rspb.2018.1246.)
Dr. Ravinet and his collaborators set out to investigate the genetic basis for how this ubiquitous little bird came to be a companion species to people — might this species’s DNA reveal any clues that could help answer this question? A reasonable method for asking a population genetics type of question like this is to compare the house sparrow’s DNA with that of its closest relatives to locate and examine any differences. Presumably, at least some of those genetic differences play a role in how the house sparrow evolved into one of our closest avian associates.
Dr. Ravinet and his collaborators sequenced the DNA of house (n = 46), Spanish (n = 43) and Italian (n = 31) sparrows from across Europe and the Middle East, and collaborated with scientists in Iran and Kazakhstan to obtain DNA from the Bactrianus sparrow (n = 19) to compare the genomic structure of each population and to look for evolutionary relationships between these sparrows — especially looking for evidence of genetic admixture across these taxa (Figure 1).
(doi:10.1098/rspb.2018.1246)
“Our findings suggest the house and Bactrianus sparrows probably split from each other about 11,000 years ago, around the time that humans were developing agriculture in the Middle East,” said Dr. Ravinet. “We also found evidence that the number of house sparrows greatly increased around 6000 years ago.”
This population increase ties in well with previous suggestions that the spread of house sparrows was closely associated with the spread of early agriculture.
What do these sparrows’ genes tell us about human evolution?
How did agriculture affect the house sparrow genome?
“We found several genes which seem to have been under strong recent natural selection in the house sparrow, but not in its wild close relative,” Dr. Ravinet said.
Dr. Ravinet and his collaborators discovered two of these genes located next to each other on chromosome number 8. One gene, COL11A, is involved in skull development whilst the other, AMY2A, plays a role in digestion of starches. Abnormal expression of a similar skull development gene in humans is associated with Marshall’s Syndrome, a genetic disorder characterized by increased skull thickness and abnormal facial structure. This is consistent with what we see in house sparrows, which have a more robust skull and craniofacial structure along with a longer beak than Bactrianus sparrows (ref). The shift in skull and beak morphology between these sparrow lineages is widely attributed to their dietary shift from natural seeds to agricultural food waste during the development of commensalism. (Similarly, introduction of human food resources appears to have altered beak morphology and — weirdly — weakened bite force in Darwin’s finches [ref].)
“The skull gene is interesting because we know that house and Bactrianus sparrow skulls differ in shape,” Dr. Ravinet elaborated. “But we were really excited to see the digestion gene there too.”
The other gene that experienced strong selection in Passer sparrows is AMY2A. AMY2A is part of a gene family linked to the transition to starch-based diets in both humans and dogs during the first agricultural revolution, which began during the Neolithic period, around 10,200 years BP (ref & ref): previous research found both dogs and humans increased copy numbers of their versions of this gene as their diets changed to include more agriculture-based grains.
Dr. Ravinet and his collaborators’ study neatly adds more detail to the emerging picture of how the Neolithic agricultural revolution presented a common selective pressure that resulted in similar adaptations in similar genes for three very different animals: humans, dogs and sparrows.
“We still have some way to go to investigate this further,” Dr. Ravinet said. “But it is exciting to think that the evolution of a species so familiar to us is tightly linked to a major event in the development of modern human civilization.”
Source:
Mark Ravinet, Tore Oldeide Elgvin, Cassandra Trier, Mansour Aliabadian, Andrey Gavrilov and Glenn-Peter Sætre (2018). Signatures of human-commensalism in the house sparrow genome, Proceedings of the Royal Society of London B: Biological Sciences, published online on 8 August 2018 before print | doi:10.1098/rspb.2018.1246
Also Cited:
Tore O. Elgvin, Cassandra N. Trier, Ole K. Tørresen, Ingerid J. Hagen, Sigbjørn Lien, Alexander J. Nederbragt, Mark Ravinet, Henrik Jensen and Glenn-Peter Sætre (2017). The genomic mosaicism of hybrid speciation, Science Advances, 3(6):e1602996 | doi:10.1126/sciadv.1602996
Lorna M. Shaw, Dan Chamberlain, and Matthew Evans (2008). The house sparrow, Passer domesticus in urban areas: reviewing a possible link between post-decline distribution and human socioeconomic status, Journal of Ornithology, 149(3):293–299 | doi:10.1007/s10336–008–0285-y
H. Jensen, B.E. Sæther, T. H. Ringsby, J. Tufto, S. C. Griffith, and H. Ellegren (2003). Sexual variation in heritability and genetic correlations of morphological traits in house sparrow (Passer domesticus), Journal of Evolutionary Biology, 16:1296–1307 | doi:10.1046/j.1420–9101.2003.00614.x
Shachar Ben Cohen and Roi Dor (2018). Phenotypic divergence despite low genetic differentiation in house sparrow populations, Scientific Reports, 8:394 | doi:10.1038/s41598–017–18718–8
Sepand Riyahi, Øyvind Hammer, Tayebeh Arbabi, Antonio Sánchez, Cees S Roselaar, Mansour Aliabadian and Glenn-Peter Sætre (2013). Beak and skull shapes of human commensal and non-commensal house sparrows Passer domesticus, BMC Evolutionary Biology, 13:200 | doi:10.1186/1471–2148–13–200
Originally published at Forbes on 21 August 2018.
