At the molecular level, there's more than one way to fly higher

Parallel adaptation is coming into its own lately, as we’re increasingly able to examine the molecular changes underlying similar adaptations in distantly related species. A fundamental prediction of evolutionary theory is that species coping with the same environment should converge on similar solutions — a canonical example is the evolution of wings in the ancestors of modern birds and in bats. A corrollary is that the more distant the relationship between converging species, the more likely they are to use different means to get to the same place. Birds and bats are both tetrapods, and though their wings are different in a number of ways, they are formed from some of the same bones. Flying insects, however, had to “find” an entirely different developmental basis for their wings.
Recently on this very blog we covered a study that showed how deep the commonalities of convergent evolution can be, identifying 47 genes shared between pine and spruce that underlie adaptation to winter cold in both lodgepole pine and interior spruce, despite the fact that these species shared a common ancestor about 140 million years ago. Another paper recently published in the same journal, Science, takes an even more fine-grained look at a different convergent adaptation, and finds that, at the most basic level, more differences than commonalities.
High altitude is a uniquely stressful environment that arises (hah) all over the globe. Life atop high mountains requires coping with extreme temperatures and a thinner atmosphere that provides less protection from ultraviolet radiation and fewer molecules of oxygen per lungful. There’s a large literature already on adaptations that help wring more oxygen from high-altitude air, and the new study zeroes in on one particular class, changes to the oxygen-binding power of hemoglobin. Hemoglobin is the molecule carried in red blood cells, which binds oxygen in the lungs and releases it in lower-oxygen tissues throughout the body; how “tightly” it binds to oxygen molecules needs to be fine-tuned to the avaialbilty of oxygen in the lungs, and tighter binding can help to gather more oxygen in thinner air.
The new study’s authors, led by Chandrasekhar Natarajan and Jay Storz at the University of Nebraska, Lincoln, identified 28 pairs of bird species which were each others’ closest relative, and in which one member of the pair lived at much higher altitude than the other. The authors then sequenced the DNA code for the proteinst that compose each species’s hemoglobin, and looked for points, across all 56 bird species, at which the amino acid sequences making up high-altitude hemoglobin differed in a consistent way from the low-altitude sequences. They found many points of difference between individual species-pairs, including a number that showed up in multiple pairs, consistent with convergent evolution driven by parallel molecular changes. Isolating samples of hemoglobin from each species, they tested its oxygen-binding capacity in vitro, and confirmed that the higher-altitude species have hemoglobin that binds oxygen more strongly.
In a second round of experimental validation, the authors synthesized hemoglobin molecules containing each of the individual molecular changes to identify which of the replicated changes actually increased oxygen-binding — this turned out to be a much smaller subset, just four of changes seen in multiple species pairs. “Clearly,” the authors note, “evolutionary increases in Hb-O2 affinity can be produced by amino acid substitutions at numerous sites.” One reason for this is that swapping one amino acid for another isn’t always as simple as swapping a blue bead on a necklace for a red bead. Proteins form complex three-dimensional structures, and changes at one point in the structure may interact with multiple other points, so a change that increases oxygen binding in one hemoglobin sequence might not have the same effect on a different sequence.

To drive this point home, the authors synthesized hemoglobin molecules based on reconstructions of ancestral states at different points in the history of modern birds. They made versions of these “resurrected” proteins with high- and low-altitude versions of one of the four replicated changes, and compared their oxygen binding. The particular change had occurred in both a high-altitude hummingbird, the Andean hillstar (Oreotrochilus estella), and in a high-altitude bird called the black-throated flowerpiercer (Diglossa brunneiventris), which last shared a common ancestor with hummingbirds about 60 million years ago. Swapping that change into the reconstructed ancestor of the hillstar and its low-altitude sister species improved the oxygen-binding of that ancestral hemoglobin, and it had a similar effect in the reconstructed hemoglobin of the common ancestor of all hummingbirds, and the common ancestor of all flowerpiercers. But it had virtually no effect in the reconstructed hemoglobin of the ancestor that hummingbirds share with flowerpiercers, or in a reconstructed hemoglobin from the ancestor of all modern birds. That means this particular change only improves oxygen binding in concert with other (unknown) changes that happened somewhere along the history of hummingbirds and flowerpiercers after the two diverged.
This is pretty much exactly what we expect from evolutionary theory, and it’s quite consistent with work like the pine-spruce convergence study, which tested for convergence at the level of whole genes. It’s almost inevitable that complex biological systems will provide multiple routes to any particular higher-order change. What will take much longer to understand is how evolution “chooses” among these many paths over millions of years of history.
References
Natarajan, C, FG Hoffmann, RE Weber, A Fago, CC Witt and JF Storz. 2016. Predictable convergence in hemoglobin function has unpredictable molecular underpinnings. Science 354:336-9. doi: 10.1126/science.aaf9070
Rosenblum, EB, CE Parent, and EE Bradt. 2014. The molecular basis of phenotypic convergence. Annual Reveiew of Ecology, Evolution, and Systematics 45:203-26. doi: 10.1146/annurev-ecolsys-120213-091851
Yeaman, S, KA Hodgins, KE Lotterhos, H Suren, S Nadeau, JC Degner, KA Nurkowski et al. 2016. Convergent local adaptation to climate in distantly related conifers. Science 353:1431-3. doi: 10.1126/science.aaf7812