NorthATURAL SELECTION, as proposed in Charles Darwin’s masterpiece, “On the Origin of Species,” explains how organisms evolve and adapt to their circumstances. However, paradoxically, it is a bit confusing on the real subject of its title, that is, how the parent species are derived from new daughter species. Darwin recognized that the various ecological niches encourage speciation (although he did not use those terms not yet invented). But he did not ask how fledgling daughter species were prevented from mixing again before they had been properly separated.
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Somehow, it is necessary to erect barriers against miscegenation. They can be geographical: fish in different lakes, for example. Or they can be ecological, such as a change in food preferences that causes insects that previously belonged to the same species to feed on different types of trees. But sometimes species form in circumstances where such barriers are not apparent. That is puzzling.
An established exception to the rule that speciation requires separation is hybridization. This is a common source of new plant species, but it is rarer for animals. However, it is believed to account for several groups of brightly colored animals, including the Great Lakes cichlids of Africa, the Heliconid butterflies of South and Central America, and the southern capuchins, a family of songbirds that also live in the tropics of the New World. . Now, Sheela Turbek of the University of Colorado in Boulder has gone one step further by showing that a process called sexual selection plays a role in what is happening.
Sexual selection, a phenomenon first scientifically described by Darwin, occurs when one sex (usually female) chooses another (usually male) on the basis of a distinctive genetic trait. The classic example is the tail of a peacock. But sexual selection could also serve to separate species in what is more or less a one-step process, due to the sudden appearance of such a characteristic. And that, as described in an article in Science, is what Ms. Turbek believes has happened in the case of Iberá and the tawny-bellied sowers.
These two birds live, among other places, in the San Nicolás Reserve, in Argentina. They feed on the same grasses, reproduce at the same time, and have breeding grounds that can be as close as 50 meters from one occupied by the other species. They are also similar enough to be successfully crossed in captivity. But, as far as is known, they never do in nature.
Genetic sequencing shows how similar these species are. Only 12 of their genes differ, less than 0.1% of their genomes. Interestingly, one of the 12 is part of one of the sex chromosomes, hinting at a role for sexual selection. Three others play a role in the coloration of the male plumage, which is also suggestive. Because, although the species are of similar sizes and shapes, adult tawny-bellied males, as their name implies, have orange-brown chests, while Iberá males have black throats and sand-colored bodies. Their songs are also different. They use the same frequency range, but different syllables.
First, Turbeck and his colleagues established that, in fact, females of both species regularly selected the right mates. Observation of the pairs suggested this, and genetic testing of the offspring (necessary because more than half of the females in the study mated with additional males in addition to their long-term partners) found no exceptions either. Once this was done, they looked at the males’ abilities to distinguish conspecific rivals from heterospecific neutrals. During the 2019 breeding season, decoy males were mounted in the territories of 40 male fawn-bellied seeders and 36 male Iberá seeders. They then monitored the responses of residents when a woman was around (see image).
They used four decoys. One had the plumage and song of a male Iberá seedling. One similarly resembled a tawny-bellied male seedbed. The other two had, in one combination or another, the appearance of one species and the song of the other.
If a resident male viewed a decoy as competition, he would act aggressively, flying and pecking at it. And the men could certainly tell the difference. They were more aggressive with lures that resembled their own species, moderately with lures that combined characteristics of both, and less with those that resembled the other species. Since these were not actual birds, it must have been the coloration and song that were the cues.
The differences between the songs of the species can be cultural and have occurred after the separation. But the differences in plumage are clearly genetic. Furthermore, they are so small that they could have occurred only once as a result of accidental hybridization, since similar genetic differences are found in various combinations in southern Capuchin crops. Furthermore, what happened once presumably could have happened often. So it seems plausible that Ms Turbeck discovered the essence of the mechanism by which this group has become so diverse. If so, then you have helped write another page in that still unpublished volume, “The Real Origin of Species.” ■
This article appeared in the Science and Technology section of the print edition under the title “Choice and Determinism.”