The variety of extravagant plumage patterns certainly suggests a pivotal role for sexual selection.
“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick,” Charles Darwin wrote. He was trying to figure out why natural selection would produce such an elaborate and seemingly useless extravagance. Surely, the colorful tail of a peacock would lower its chances of survival. Pondering this conundrum led Darwin to develop his theory of sexual selection, in which males compete for access to females. Male peacocks use their beautiful plumage patterns to attract potential mates and show off their “good genes”. A bird that can produce such extravagant feathers while fending off parasites and predators should definitely become the father of your offspring.
Peacock is actually a common name that refers to several species in the genera Pavo and Afropavo, which have been classified in the family Phasianidae (the gallopheasants). A quick glance at other members of this bird family reveals an additional wealth of colorful feathers in species such as Bulwer’s Pheasant (Lophura bulweri) or Golden Pheasant (Chrysolophus pictus). It is not surprising that several ornithologists have argued that sexual selection has been the driving force behind the diversification of this bird group. A recent study in the journal Zoologica Scripta put this idea to the test.
Before we can infer the evolutionary importance of sexual selection, we need a proper phylogeny. However, previous molecular studies of the gallopheasants could not confidently resolve the evolutionary relationships between and within genera. This rampant phylogenetic conflict was attributed to the rapid succession of several speciation events. In this study, Peter Hosner and his colleagues used ultraconserved elements, nuclear introns and mitochondrial DNA sequences to unravel this phylogenetic knot. Their analyses resulted in a well-resolved evolutionary tree with few conflicting branches (see figure below). The issues in previous analyses were probably due to a limited number of genetic loci. Sometimes just adding more data can work.
Next, the researchers determined the strength of sexual selection by quantifying the degree of sexual dimorphism in different species. Sexual dimorphism refers the differences in appearance between males and females of the same species. Larger differences, such as more colorful males, suggest stronger sexual selection. If the evolution of gallopheasants was driven by sexual selection, then the gain of sexual dimorphism on certain branches should have resulted in accelerated diversification.
In contrast to the expectations, there was no clear phylogenetic signal that the strength of sexual selection accelerated the speed of evolution. Moreover, other factors, such as female morphology and ecological differences, also contributed to the diversification patterns across the phylogeny. For example, divergence in environmental niche can explain the evolution of Golden Pheasant (Chrysolophus pictus) and Lady Amherst’s Pheasant (C. amherstiae). The authors concluded that their findings add “to the growing body of literature suggesting that multiple factors work in concert and that focusing on sexual selection alone as a driver of diversification may lead to erroneously narrow conclusions.”
Indeed, although sexual selection has played an important role in certain gallopheasant species (as exemplified by their extravagant plumage), we should not automatically discard other processes. Evolution involves the complex interplay of numerous selective pressures, spiced up with some chance events.
Hosner, P. A., Owens, H. L., Braun, E. L., & Kimball, R. T. (2020). Phylogeny and diversification of the gallopheasants (Aves: Galliformes): Testing roles of sexual selection and environmental niche divergence. Zoologica Scripta, 49(5), 549-562.
Featured image: Golden Pheasant (Chrysolophus pictus) © Eric Kilby | Wikimedia Commons