By Catalina Palacios
Linked paper: Shallow genetic divergence and distinct phenotypic differences between two Andean hummingbirds: Speciation with gene flow? by C. Palacios, S. García-R, J.L. Parra, A.M. Cuervo, F.G. Stiles, J.E. McCormack, and C.D. Cadena, The Auk: Ornithological Advances.
I am lucky that one of the species of hummingbird I study, the Blue-throated Starfrontlet (Coeligena helianthea), occurs on my university’s campus in the mountains of Bogotá, Colombia. However, after years studying this species, I only observed it for the first time the very same week that our first paper on its genetic divergence from its sister species, the Golden-bellied Starfrontlet (C. bonapartei), was accepted at The Auk: Ornithological Advances. I knew that C. helianthea was beautiful, because I had seen tons of pictures of it, but finding a female feeding on a fuchsia plant on my way to the sports center where I go swimming was a wonderful surprise. While I watched her humming from one flower to the next, my first thought was, wow, she’s almost as beautiful as her genes are! My second thought was, now I must finish my PhD! I have been investigating the evolution of these Andean hummingbirds for several years now, but the story behind our paper began before I started my PhD.
In 2009, Juan Luis Parra and others were surprised to find that C. helianthea and C. bonapartei, two highly distinct species from Colombia and Venezuela, did not form separate groups in an analysis of their evolutionary relationships based on DNA (mitochondrial genes and nuclear introns). This observation prompted questions about what was up with these hummingbirds and their genes. It’s quite uncommon for species that one can easily tell apart based on their plumage to be seemingly undifferentiated genetically. Juan Luis and my PhD advisor Daniel Cadena thus decided to work on this system in more detail and recruited Silvana García, who did the first detailed analyses of the species for her master’s work. Then came me! I have been long in love with genes and genomes, and I had decided to work with birds for my PhD because I was fascinated by hummingbirds, so when Daniel introduced me to the Coeligena system I immediately fell for it.
We thought about evolutionary mechanisms that could explain the lack of genetic divergence despite distinct plumages. One possibility was hybridization, but aside from a few old, seemingly intermediate specimens housed at the British Museum (whose genomes I would love to study in the future!), there is no evidence that C. helianthea and C. bonapartei interbreed, even though both species are found in the same areas. The remaining options were divergence with gene flow (the species became different while still having genetic interchange, with selection driving divergence) and incomplete lineage sorting (both species have retained and share alleles present in their common ancestor). We also thought about mechanisms that could account for the differences in plumage coloration, figuring that these hummingbirds might be an example of Gloger’s rule, whereby darker colors are adaptive in more humid environments. Juan, Silvana, Daniel, and I joined forces with Andrés Cuervo, who collected specimens in Venezuela and Northern Colombia that completed our genetic sampling and helped generate molecular data, and with Gary Stiles, who contributed field data allowing us to test for morphological differences between species. We also brought John McCormack on board following my visit to his lab in Los Angeles in 2014 to learn about techniques for sequencing Ultra Conserved Elements (UCEs) from the nuclear genome.
Looking back, it’s amazing to realize how much time may pass between conceiving an idea, collecting and analyzing relevant data, and publishing a paper. It’s also amazing that after so much time I still find our results so exciting. I will not spoil the paper for you, but allow me to say that we found C. helianthea and C. bonapartei lack genetic divergence in mitochondrial markers, UCEs, and a candidate gene controlling pigmentation! We found no support for Gloger’s rule, and although we could not rule out incomplete lineage sorting, our data are consistent with divergence-with-gene flow, suggesting that recent speciation in these hummingbirds may have been driven by some form of selection. Which evolutionary processes were involved in their speciation? Were both recent divergence and gene flow involved? How did the ancestor of our Coeligena species differentiate? What mechanisms and traits were involved in divergence? I like to imagine the answers to these questions humming, dancing in front of me like the rose and aquamarine colors of the C. helianthea that I saw, answers hiding somewhere in the genes related to coloration. After all, you may think, two sister species cannot be so distinct in coloration and show no genetic divergence at all. They do not, indeed… but I will tell you about their genomes later.
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