October 28, 2020

The evolution of Darwin's finches is the exception, not the rule | Science

The evolution of Darwin's finches is the exception, not the rule | Science



Guillermo Navalón is a paleontologist who studies mesozoic fossils. In the deposit Las Hoyas, in Cuenca, remains of the first birds that existed have been found, corresponding to this period. To know how these birds fit into the primitive ecosystem, and thus understand the relationship between modern birds and dinosaurs, he and his companions must first find out what they ate. There are few clues, but, initially, they thought that the shape of the peak could be one of the best.

His plan was based on "an idea that has been had since Darwin", according to Navalón: that the beaks of the birds are exactly adapted to each mode of feeding, thanks to natural selection. "That axiom is floating in the collective imagination of paleontologists, and we swallow it completely. We were convinced that we could develop a tool to infer the ecology of each bird by the shape of its beak, "he says. They were wrong.

It's not that Charles Darwin was not right. The finches that the father of evolution observed in the Galapagos Islands do have optimized peaks for their way of feeding. For example, the birds with the thickest beaks eat seeds or beetles that are difficult to crack. In later research, the biologists at Princeton University Rosemary and Peter Grant have shown that, for such specialized birds, even a deviation of one millimeter from the optimal length or depth of the beak can lead to death by starvation during the months of scarcity.

The error of the scientists was to assume that this degree of anatomical adaptation to the diet was generalizable to the whole group of birds. "Darwin's finches are the exception rather than the rule," says Navalón, who is in the last year of his doctorate at the University of Bristol (United Kingdom) and the Autonomous University of Madrid. He and colleagues from Spain, the United Kingdom and the United States have analyzed the peak of 176 non-extinct avian species of very varied groups. Your study appears published in the last issue of the scientific journal Evolution.

The team has found that, although there is a general correlation between the peak profile and the ecology of the birds – what they eat and how they eat it – the link is weak. So much so, that the diet justifies 12% of the anatomical variation observed among the peaks of the birds that live today. "As your hands they are integrated in the wings, they use their beaks for practically everything, "says Navalón. It is the tool with which they groom, with which they build nests, with which they sing and sometimes with which they fight. For some species, the beak has very specific additional functions, such as toucans, which use it to regulate their body temperature.

Far from being a "book" example of adaptation, the evolution of the peak is a messy process that usually resolves in some medium term

Natural selection favors the characteristics that improve survival and reproduction. Although the feeding is undoubtedly important, for some groups of birds other selective pressures are even more powerful, so much so that they push the evolution of the beak towards forms that are not at all efficient to eat. The Atlantic Puffin (Fratercula arctic), for example, feeds almost exclusively on fish. However, its short, deep beak is not at all like the sharp structures that characterize other piscivorous species. That's because they also use it "as a flag" to recognize other individuals, explains Navalón, and also to dig burrows.

Far from being a "book" example of adaptation, scientists have discovered that the evolution of the peak is a messy process, in which many selective pressures that usually resolve in some average way compete. In fact, the most abundant beak form among the birds, of all those that studied, is that of a straight and thin beak, of medium length, useful for deworming, but also to pick up objects from the ground and to eat a variety of foods . It's the Swiss knife of the peaks.

Modular heads

"This is a good quality study," says Arkhat Abzhanov, a biologist at Imperial College London who is not involved in the research. Abzhanov observes that birds can adapt to a specialized diet by means other than the evolution of the beak. Parrots, for example, have developed unique muscles in the head that increase the strength of their bites, and the crows display an intelligence that allows them very sophisticated feeding behaviors.

Abzhanov's main criticism is that the analysis is based on a two-dimensional projection of the profile of each peak. "We know from morphometric studies that it is important to look at the 3D structure to understand the evolution of the avian skull," says the biologist. His team, which studies the evolution of the skull in several animals, recently demonstrated that the most diverse and specialized groups of birds, such as the finches of the Galapagos Islands, have extremely modular heads, where each piece develops independently. This facilitates evolution by natural selection of individual elements, such as the peak.

His finding agrees with a previous investigation of the companions of Navalón. They found that the development of peaks in raptors is tied for structural and genetic reasons to the shape of the entire skull, so evolution can not optimize its shape without compromising some other aspect of anatomy. Perhaps that is why the success of its fighters depends more often on the adaptations of its claws than those of its beak. The latter almost always use it to pluck meat from already dead animals.

In light of the findings, scientists suspect that these restrictions of cranioencephalic development may be common to the evolution of most birds. But they are scientists; Now they must confirm or deny these suspicions. As for the Mesozoic birds of Cuenca, it will be necessary to find another way to find out what they ate. "We will have to look at new anatomy structures that have fewer functions than the peak," suggests Navalón.

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