Darwin’s finches that live on the Galapagos Islands continue to evolve at breakneck speed. An international team of biologists led by researchers from McGill University who studied them for 17 years says that two new species are on the way to form.
These evolutionary biologists sought to understand how new species emerge and to clarify the role that natural selection plays in this process. To do this, they looked at four species of finches of the genus Geospize which live on the island of Santa Cruz, in the Galapagos. For 17 years in a row, from 2003 to 2020, they captured 3,400 individuals of these four species which they ringed, measured and from which they took a drop of blood in order to carry out genomic analyses.
“Sometimes we recaptured individuals that we had already examined in previous years. This informed us about the individuals that were found most frequently in the population. [qu’on étudiait], and therefore who contributed the most. Furthermore, by calculating the difference between the last year of capture and the first year of capture, we were able to estimate the “apparent survival” or “apparent lifespan” of these birds,” indicates Marc-Olivier Beausoleil, first author of the article which describes the results of this study in the journal Evolution.
The longer the individuals had been present in the population studied, the more we could deduce that they were well adapted to their environment – “otherwise we would not have seen them again” – and that “their “adaptive value” was high”. “Organisms that have a high adaptive value “perform” better than others, live longer, have higher reproductive success, and therefore have greater offspring,” specifies Mr. Beausoleil, who is completing a doctorate under Rowan’s supervision. Barrett at McGill University.
The biologists then investigated whether apparent survival had a link to the size of the finches’ beaks. They found that there was indeed a very clear association between the two. Thus, the small-billed gophers, notably those of the species Geospiza fuliginosaas well as the large-billed gophers, those of the species Geospiza magnirostris, thrived very well in their environment, where they found resources adapted to the morphology of their beaks, which ensured them long survival. However, the researchers noticed that birds with intermediate-sized beaks “performed” less well and therefore survived less long.
In addition, their mathematical model revealed to them that each of the four species they studied is on the verge of reaching an “adaptive peak”, that is to say an optimal adaptation to its living environment which allows it to survive longer and thus have more offspring. The population of medium-billed gourds (Geospiza fortis) appeared to them divided into two subpopulations: one characterized by a smaller beak and the other by a larger beak. And each of these two subpopulations is strongly approaching an adaptive peak, as evidenced by their apparent long lifespans.
“The species of medium-billed beak is in the process of diverging, separating into two distinct species. We are witnessing the formation of two new species in real time. We always thought that evolution was very slow. But, in fact, depending on the characters that are in play, it can be quite quick. In the space of a few years, we can see a significant effect of natural selection,” explains Mr. Beausoleil enthusiastically.
The researcher emphasizes that it would in fact be easier for birds to evolve towards a new beak size than towards a new beak shape (more tapered or more flat). “There would be more genetic constraints [à surmonter] to evolve into a different beak shape that would allow the bird to reach its adaptive peak. It would take a stronger selection pressure in the environment and a greater number of years to change the shape of the beak,” specifies Mr. Beausoleil, whose genetic analyzes confirm that the shape of the beak is largely determined by the genetic.
“However, we cannot say with certainty that two distinct species will form, because it also depends on environmental conditions which can vary greatly, particularly during periods of El Niño and La Niña,” he emphasizes.
The medium-billed beak species is diverging into two distinct species
“Moreover, if the government of Ecuador decides to build a hotel on this island, the speciation process could even be reversed,” he says. The team of Professor Andrew Hendry, from McGill University, observed in 2016 that the populations of different species of geospize living near the town of Puerto Ayora, which has experienced dazzling demographic growth, seem to be in the process of “merging together”. “. “It seems like we are going backwards, losing species near cities,” he notes.
“It is important to better understand the evolutionary mechanisms leading to the formation of new species in order to be able to identify the ecological factors that most influence natural selection under particular conditions. This is essential to predict the effects that climate change and the modifications imposed on the environment will have on the evolution of organisms,” he emphasizes.