The maintenance of phenotypic divergence through sexual selection: An experimental study in barn swallowsHirundo rustica

Abstract

Previous studies have shown that sexual signals can rapidly diverge among closely related species. However, we lack experimental studies to demonstrate that differences in trait-associated reproductive performance maintain sexual trait differences between closely related populations, in support for a role of sexual selection in speciation. Populations of Northern Hemisphere distributed barn swallows Hirundo rustica are closely related, yet differ in two plumage-based traits: ventral color and length of the outermost tail feathers (streamers). Here we provide experimental evidence that manipulations of these traits result in different reproductive consequences in two subspecies of barn swallow: (H. r. erythrogaster in North America and H. r. transitiva in the East Mediterranean). Experimental results in Colorado, USA, demonstrate that males with (1) darkened ventral coloration and (2) shortened streamers gained paternity between two successive reproductive bouts. In contrast, exaggeration of both traits improved reproductive performance within H. r. transitiva in Israel: males with a combination treatment of darkened ventral coloration and elongated streamers gained paternity between two successive reproductive bouts. Collectively, these experimental results fill an important gap in our understanding for how divergent sexual selection maintains phenotype differentiation in closely related populations, an important aspect of the speciation process.

Beauty alone is insufficient: female mate choice in the barn swallow

“…This apparent match between the hypothetical scenario and actual geographic variation does not deny the importance of geographic variation in female preference for each secondary sexual characteristic (e.g., plumage coloration and tail length; Safran et al 2016). Because each ornament has its own information content, some of which seems to be population-dependent (e.g., Vitousek et al 2016; also see above), female choice for each ornament should itself depend on population.…”

“…The target, direction, and intensity of preference varies among populations at least for plumage ornaments (e.g., Safran et al 2016;Wilkins et al 2016; reviewed in Scordato and Safran 2014;Romano et al 2017a), affecting the geographic variation in male trait expression, although most studies have found positive female preference (i.e., toward exaggerated traits; Romano et al 2017a). Well-ornamented males provide less, rather than more, paternal care compared to less-ornamented males in barn swallows (e.g., de Lope and Møller 1993;Saino and Møller 1995;Maguire and Safran 2010;Hasegawa et al 2014;Hasegawa and Arai 2015a), which may be costly for females even if wellornamented males have fewer ectoparasites and thus lower risk of contagion (Møller 1994a).…”

“…Then, I present how consideration (or ignorance) of inconspicuous male traits affects our understanding of evolution of overall male phenotype with a simple hypothetical scenario. Interrelationships among male traits Several studies have demonstrated positive intercorrelations among male secondary sexual characteristics in barn swallows (e.g., Møller et al 1998a;Saino et al 2003;Hasegawa et al 2010a; but see Vortman et al 2011;Safran et al 2016 for no detectable relationships). Still, it is often unclear whether these patterns reflect evolutionary covariation without a well-designed experiment (e.g., Wagner et al 2012;van Noordwijk and de Jong 1986;Stearns 1992;Andersson et al 2002;Saino et al 2003), due to individual variation in resource availability (e.g., when the allocation between the traits varies less than the total investment, a positive correlation can be predicted even if there is a trade-off between two ornaments).…”

Consequences of divergence and introgression for speciation in Andean cloud forest birds

“…; Safran et al. ) and other characters important to avian speciation, especially vocalizations, are required to test the likelihood of these alternatives. Additionally, if secondary contact occurred but premating isolation was incomplete, hybrid incompatibilities arising due to “byproduct” genomic divergence could have precluded introgression, masking a history of secondary contact (Sobel et al.…”

Experimental manipulation of female tail length did not cause differential allocation by males in the barn swallow

“…In the control, the distant ends of the outermost tail feathers were trimmed without reducing their length (although trimming might slightly reduce tail length). We chose 11 mm because this length roughly corresponds to be aerodynamically costly part in female tails (10-12 mm: e.g., Buchanan & Evans, 2000;Rowe, Evans, & Buchanan, 2001;Cuervo & de Ayala, 2014), and recent experiments showed that a similar length (13 mm) affected mate behavior at least when manipulating male tail length (Safran et al, 2016;Vortman, Lotem, Dor, Lovette, & Safran, 2013). In addition, a 20-mm reduction (Cuervo et al, 1996) is impractical for the study population, in which the tails are shorter (78.91 ± 0.94 mm, n = 20) than European populations (84.5 ± 0.6 mm, n = 74: Cuervo et al, 1996).…”