A new study involving citizen scientists and 95% of the world’s living bird species has illustrated how beaks evolved, beginning with rapid changes in the Cretaceous, followed by gradual fine-tuning.
The work, by Christopher Cooney at the University of Sheffield and colleagues in the UK and US, was reported in Nature.
Biologists have a decent understanding of how evolution occurs on a small scale. Generally, species descended from a common ancestor will evolve as they fill different ecological niches.
Initially, this happens at quickly, so species can capitalise on resources. But eventually the rate starts to slow as the niche gradually fills up with new species. Among one population over a relatively short time-period, this process is known as microevolution.
Cooney and colleagues wanted to investigate this on a wider scale: how fast do traits evolve across an entire class, spanning different populations, all the way around the globe?
To study this broad time-adaptation relationship, the team decided to focus on the beak, a structure common to all the world’s bird species.
“The avian bill is closely associated with species’ dietary and foraging niches and represents a highly adaptable ecological trait known to have a key role in classic avian adaptive radiations,” the researchers write.
Using samples from museum collections, the team took 3-D beak scans from 2,000 species, representing around 95% of birds alive today and spanning the full range of beak diversity.
These were loaded into a website called MarkMyBird, which asked volunteers to help “landmark” the beaks by placing dots at significant points, such as the tip, or the back left corner, or by tracing the midline.
A leaderboard encouraged a healthy dose of competition among the citizen scientists, and the crowdsourced data painted a valuable picture of the morphological variety of the world’s bird beaks.
Comparing this with existing avian evolution knowledge, the researchers found signs of a fast-track evolution in the early stages of pre-history, as species began to dominate their niches.
“It is likely that the rise of modern birds from the late Cretaceous onwards occurred in a rapidly changing world, coinciding with extensive ecological opportunity,” the researchers write.
This eventually gave way to a more individual, independent diversification of beak shape, which was influenced by environmental changes or shifts, and the density of each population’s niche.
The results also show the process isn’t straightforward: along the way, rare but major discontinuities have resulted in unusual or mutated beaks in some species. Despite this, the researchers write “the major axes of within-group bill-shape evolution are remarkably consistent across birds”.
This points to the global “mega-evolution” of avian species, which later gave way to “smaller scale fine-tuning” as different species emerged.
“Perhaps the most powerful message of the current study is that large-scale evolution is contingent on both history and catastrophe,” writes Bhart-Anjan Bhullar, a geophysicist at Yale University in the US, in an accompanying News & Views article.
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