Long after they inspired Darwin to discover natural selection, the birds of the Galapagos continue to teach us about evolution. New analysis suggests that the finches that were Darwin's prime ornithological study have reached an evolutionary equilibrium, even while other bird species continue to diversify. The finding is important both for what it says about the history of evolutionary theory, and potential practical uses in future.
When a volcanic island appears, species move in to colonize it. At first, new species have ample ecological niches to fill, and new arrivals have a good chance of flourishing, and may well split into several species to make best use of the range of resources available.
However, biologists have proposed that as diversity increases it becomes harder for later arrivals. If they are to survive they must overcome an existing species already filling a niche. Likewise, existing inhabitants become less likely to diversify.
Logical as this may be, it “has never been tested in detail,” says Rampal Etienne of the University of Groningen, Netherlands. We have had, Etienne says, a “lack of data and the right analytical tools.” To resolve the second part of the problem, Etienne developed a publicly available mathematical model, Dynamic Assembly of Islands by Speciation, Immigration and Extinction (DAISIE), and where better to seek the data than the place where evolutionary theory began?
Using phylogenetic trees of Galapagos birds based on recently collected genomic data, DAISIE estimated the changes occurring to different types of birds on the islands. Etienne published the results in Ecology Letters.
“The analysis shows that for the finches, diversity does indeed have a negative effect,” says Etienne. “There is no more room for new species, unless one of the existing species becomes extinct, so the islands are saturated regarding finch-type species.” The data represents averages and does not predict an immediate extinction to balance the new finch reported last year on Daphne Major.
Adaptable though the finches have proved to be, niches remain for other birds. Four species of mockingbird are still diversifying, Etienne concludes. “We found that the rates of both evolution and extinction are very high for Darwin's finches,” says Etienne. “That is probably why these birds have reached an equilibrium.” It is probably no coincidence that it was the rapidly evolving finches in which Darwin saw the mark of natural selection. Meanwhile, the slower-evolving mockingbirds have yet to take up every opportunity. Given another million years without changes to the islands, the number of mockingbird species would be expected to increase, while new finches would simply replace old ones.
The knowledge that DAISIE provides is important, Etienne argues, because it could teach about future speciation, which could be important for working out how to prioritize conservation efforts. “We are not just conserving existing species, but also future diversity,” Etienne says.