AboutZoos, Since 2008


Barn swal­lows may indeed have evolved along­side humans and their barns

pub­lished 10 Novem­ber 2018 | mod­i­fied 10 Novem­ber 2018

The evo­lu­tion of barn swal­lows, a bird ubiq­ui­tous to bridges and sheds around the world, might be even more closely tied to humans than pre­vi­ously thought, accord­ing to new study from the Uni­ver­sity of Col­orado Boulder.

The research, first pub­lished on 3 Sep­tem­ber in the jour­nal Mol­e­c­u­lar Ecol­ogy, offers pre­lim­i­nary insight sug­gest­ing that the barn swal­low and its sub­species evolved along­side — but inde­pen­dently from — humans. These new results make it one of the only known species, in addi­tion to micro­scopic organ­isms like bac­te­ria or viruses, to have devel­oped in such a way, upend­ing pre­vi­ous assump­tions that barn swal­lows evolved prior to human settlement.

Humans could be a really big part of the story. There’s very few stud­ies that can point to the exact influ­ence of humans, and so here, this coin­ci­dence of human expan­sion and per­ma­nent set­tle­ment and the expan­sion of a group that relies really, really heav­ily on humans is compelling.

Rebecca Safran, co-​author, Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy (EBIO), Uni­ver­sity of Col­orado, Boul­der, USA

barn swallow sub-speciesBarn swal­low sub­species are found through­out the north­ern hemi­sphere.
Barn Swal­low illus­tra­tions cour­tesy of Hilary Burn, and map cour­tesy of the Safran lab.

Barn swal­lows are found across the north­ern hemi­sphere and are char­ac­ter­ized by their mud-​cup nests that are built nearly exclu­sively on human-​made struc­tures. Despite their preva­lence, how­ever, not much is known about their evo­lu­tion­ary his­tory, the tim­ing of their expan­sion from north­ern Africa (where they orig­i­nated) or how the six sub­species evolved so phys­i­cally and behav­iorally dif­fer­ent yet remain almost genet­i­cally identical.

Pre­vi­ous stud­ies pub­lished in the Pro­ceed­ings of the Royal Soci­ety of Lon­don and Mol­e­c­u­lar Phy­lo­ge­net­ics and Evo­lu­tion looked into these ques­tions and found that the dif­fer­ent sub­species split early, well before human settlement.

This new study, how­ever, gave the topic a fresh look by exam­in­ing the whole genome of 168 barn swal­lows from the two sub-​species far­thest apart on an evo­lu­tion­ary scale: H. r. sav­i­g­nii in Egypt (a non-​migratory species that lives along the Nile) and H. r. ery­thro­gaster in North Amer­ica (a species found through­out North Amer­ica that migrates sea­son­ally to South America).

These data — which are on the order of 100,000 times big­ger than the pre­vi­ous dataset used — were then analysed with more sophis­ti­cated com­pu­ta­tional resources and meth­ods than pre­vi­ously avail­able. This allowed researchers to get a more com­plete pic­ture that places the tim­ing of barn swal­low dif­fer­en­ti­a­tion or spe­ci­a­tion (i.e., when the barn swal­low sub­species sep­a­rated) closer to that of when humans began to build struc­tures and settlements.

The pre­vi­ous stud­ies were play­ing with the idea of poten­tial impact on pop­u­la­tion sizes due to humans,” said Chris Smith, a grad­u­ate stu­dent in EBIO and the Inter­dis­ci­pli­nary Quan­ti­ta­tive Biol­ogy pro­gram, and the study’s lead author. “Our results sug­gest a much more sub­stan­tial link with humans.”

These new pre­lim­i­nary find­ings also sug­gest that this evo­lu­tion­ary link may have been forged through a ‘founder event’, which is when a small num­ber of indi­vid­u­als in a species take over a new envi­ron­ment and are able to expand their new pop­u­la­tion there thanks to an avail­abil­ity of resources and an absence of com­peti­tors. For barn swal­lows, this event may have occurred rapidly when they moved into a new, rel­a­tively empty envi­ron­ment: along­side humans.

Every­one is always won­der­ing how do you study spe­ci­a­tion? It’s been viewed as this long-​term, million-​year (process), but in barn swal­lows, we are not talk­ing about dif­fer­en­ti­a­tion within sev­eral thou­sands of years,” said Safran. “Things are really unfold­ing rather rapidly.”

Smith con­curred: “It’s inter­est­ing to study spe­ci­a­tion in the begin­ning steps.”

(Source: Uni­ver­sity of Col­orado Boul­der news release, 30.10.2018)

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