Climate‐driven build‐up of temporal isolation within a recently formed avian hybrid zone

Abstract

Divergence in the onset of reproduction can act as an important source of reproductive isolation (i.e., allochronic isolation) between co-occurring young species, but evidence for the evolutionary processes leading to such divergence is often indirect. While advancing spring seasons strongly affect the onset of reproduction in many taxa, it remains largely unexplored whether contemporary spring advancement directly affects allochronic isolation between young species. We examined how increasing spring temperatures affected onset of reproduction and thereby hybridization between pied and collared flycatchers (Ficedula spp.) across habitat types in a young secondary contact zone. We found that both species have advanced their timing of breeding in 14 years. However, selection on pied flycatchers to breed earlier was weaker, resulting in a slower response to advancing springs compared to collared flycatchers and thereby build-up of allochronic isolation between the species. We argue that a preadaptation to a broader niche use (diet) of pied flycatchers explains the slower response to raising spring temperature, but that reduced risk to hybridize may contribute to further divergence in the onset of breeding in the future. Our results show that minor differences in the response to environmental change of co-occurring closely related species can quickly cause allochronic isolation.

Migratory strategy explains differences in timing of female reproductive development in seasonally sympatric songbirds

“…In addition to the importance of understanding the mechanisms that mediate timing of reproductive development, allochrony can also play an important role in temporal reproductive isolation between populations (Feder et al, 1994;MacArthur & Levins, 1967;Sirkiä et al, 2018;Taylor & Friesen, 2017;Winker, 2010). Seasonal allochrony can result from factors that lead to divergent breeding phenology, such as flowering time (Devaux & Lande, 2008), adult emergence in insects (Burban et al, 2016;Santos, Paiva, Rocha, Kerdelhué, & Branco, 2013), spawning (Rosser, 2015) or animal migration (Bearhop et al, 2005;Fillatre, Etherton, & Heath, 2003;Fudickar et al, 2016;Winker, 2010).…”

“…To determine which populations will exhibit flexible timing, it is important to understand the mechanisms that mediate phenology. Additionally, young species or subspecies are experiencing secondary contact due to range shifts, leading to hybrid zones and higher resource competition (Chunco, 2014;Sirkiä et al, 2018). More studies need to examine young hybrid zones and whether newly co-occurring species continue to exhibit phenological differences despite ongoing shifts resulting from climate change (Sirkiä et al, 2018).…”

Difference in plasticity of resting metabolic rate – the proximate explanation to different niche breadth in sympatricFicedulaflycatchers

“…Collared flycatchers have only recently (i.e., during the early 1960s) settled on Öland, which makes this system ideal to study recent secondary contact after a period of allopatry (Qvarnström et al., ; Qvarnström, Rice, & Ellegren, ). Collared flycatchers have quickly displaced pied flycatchers from the preferred breeding sites on Öland (Vallin, Rice, Arntsen, Kulma, & Qvarnström, ), but pied flycatchers are better able to breed in less preferred poorer habitats where the levels of caterpillar biomass are lower (Sirkiä et al., ). As a consequence, when pied flycatchers are pushed from good habitats, they can both escape competition from collared flycatchers and reduce the risk of hybridization by breeding in poorer habitats (Rybinski et al., ).…”

Interspecific transfer of parasites following a range‐shift in Ficedula flycatchers

“…Previous studies have found evidence for asymmetrical transfer of parasites from invading, more competitive hosts, which are in the process of expanding their distribution ranges, to native hosts (Beadell et al, ; Reullier et al, ; Shea & Chesson, ). By contrast we have shown that collared flycatchers, that is, the more competitive species that are in the process of expanding their distribution range in response to a warming climate (Kulma et al, ; Sirkiä et al, ), appear to acquire new malaria lineages from the native pied flycatchers. In addition, collared flycatchers have maintained the high diversity of species‐specific malaria lineages in the newly colonized areas in Sweden and have not been able to escape the parasites in their core range.…”

“…The contact zone is dynamic, with collared flycatchers naturally encroaching northward (Huntley, Green, Collingham, & Willis, ), including the Swedish island of Öland, which was colonized about 60 years ago (Qvarnström, Wiley, Svedin, & Vallin, ). Both species commence breeding from early May to mid June, with more southerly and westerly populations tending to breeding earlier than northern‐most populations (Sanz, ; Sirkiä et al, ).…”