Darwin’s finches – whose discoverer and namesake Charles Darwin founded evolutionary theory – are among the best-known animals in the world. They live on the Galapagos Islands, an archipelago in the Pacific Ocean, about 1.000 kilometers off the coast of Ecuador. The islands’ location makes them particularly interesting for biologists, since such remote islands are considered excellent natural laboratories for studying evolution. As relatively secluded ecosystems, they form valuable model systems to study biodiversity in greater detail. A team headed by the Portuguese evolutionary biologist Dr. Luis Valente, who is currently conducting research at the University of Potsdam, has tested a model using the example of Darwin's finches that offers surprising insight into the mechanisms of biodiversity development.
On the Galapagos Islands, the birthplace of Darwin's theory of evolution, species evolve at varying rates. While many bird families are still producing new species, Darwin’s finches seem to have reached a certain type of equilibrium. “Our results show that the famous Darwin’s finches, at least in terms of the number of species, have already reached an evolutionary endpoint,” says Valente, “albeit a productive one: New species are still being added but only when others die out. The reason for this may be their ‘rapid evolution’ with fast speciation and extinction rates, which continue up until today.”
The number of other bird species – and, thus, diversity – on the Galapagos Islands as a whole is still increasing. This suggests that classic island theory does not apply here. The prevailing theory about evolution on islands since the 1960s predicts a dynamic balance between immigration and extinction of species.
“Until now, it has been impossible to empirically verify this hypothesis, because the statistical tools suitable for examining millions of years of biodiversity development were not available,” Valente says. Together with Prof. Rampal Etienne from the University of Groningen and Dr. Albert Phillimore from Edinburgh University, Valente developed a mathematical model that allowed the computer program DAISIE (“the Dynamic Assembly of Islands by Speciation, Immigration and Extinction”) to reconstruct island biogeography. DAISIE has enabled the researchers to analyze the evolution of bird species on the archipelago. The researchers published the results of their study – the first-ever analysis of evolutionary dynamics over very long periods – in the high-impact journal Ecological Letters. The authors have made the DAISIE model available to other researchers as a software package in the free and widely used R environment.
"DAISIE processes molecular data we have collected on the species on the islands, and provides us insights regarding colonization and extinction rates as well as differentiation,” says Valente. The researchers collected information on particular genes of representative specimens of many bird species on islands. Related mainland species serve as a comparative foil. The corresponding data are fed into the DAISIE database. Once the data is as complete as possible, the program simulates thousands of potential scenarios of how the species on the islands have developed over millions of years – compared to the mainland populations – and identifies the most likely one.
“DAISIE offers a useful, innovative approach,” says Prof. Ralph Tiedemann, with whom Valente has been researching as a Humboldt Research Fellow since the end of 2014. “The program is able to analyze speciation mechanisms and identify patterns – for various species and islands.”
Dozens of archipelagos will be examined
Even if the results on the Galapagos Islands are spectacular, they are still only the first step for Valente. His next steps will take him from Potsdam to around the globe. DAISIE has long been able to “check out” remote archipelagos around the world. “Together with Ralph Tiedemann, we now want to examine very different types of archipelagos and their particularities: large and small ones, those with high elevations and very flat ones, very remote ones and those nearer continents,” Valente says enthusiastically. With the help of DAISIE, they will be studying about 20 archipelagos and their bird species. The “eligibility criteria” are very strict: The archipelagos must be sufficiently far apart from other landmasses – and have always been. This, in fact, only applies to volcanic islands. The requisite data on the species living there should either be available or at least accessible. This is a real challenge, Valente explains. “Much is available on existing databases,” the researcher explains. “Other times, finding data is very difficult.”
Take, for example, the island of São Tomé. About 240 kilometers off the African coast in the Gulf of Guinea, it is a part of the country of São Tomé and Príncipe. About 50 species of birds live on the island, but DNA sequences are not yet available. Valente is now in contact with an ornithologist who has long studied the birds on the island – and who also has the sample material the Potsdam biologist covets. “It is absolutely essential to be well networked,” says Valente. “We talk to many ornithologists and have to do a lot of negotiating, because not everybody is willing to collaborate and make his or her results available.” Valente also finds information in museums. He evaluates old samples, some of which are centuries old. Because of their old age, these samples are often very degraded, and as a result routine laboratory work often turns into a “forensic investigation”. Student assistants carry out the laboratory tests. “It is great for our students to be able to research at an early stage,” adds Tiedemann. “They work on material from some of the most extraordinary places in the world. In summer 2014, one student, for example, sequenced the DNA of birds on the Canary Islands, off the west coast of Africa.”
Valente’s current workplace is mostly at his computer. He has to enter the results of sample analyses into the database and then model the scenarios with DAISIE. This is not to say that he does not miss working in nature. Valente worked in the botanical gardens of London and Madrid and wrote his doctoral thesis on the evolution of plants in South Africa. This always involved fieldwork. He soon realized, though, that research, especially as an evolutionary biologist, also implied mathematical modeling. “We need to reconstruct connections that may have existed millions of years ago, for which we have no field data,” says Valente. “This is only possible with theoretical models.” In fact, the modeling of scenarios has become indispensable to evolutionary biology, adds Tiedemann. “Many data can only be used to describe connections in terms of probabilities.”
If the registration of samples progresses well, DAISIE will soon be able to calculate models for the first ten archipelagos. This will show whether the results for the Galapagos Islands were a fluke – or a peculiarity that can be applied to other species elsewhere. “This study could help answer a fundamental question of evolutionary biology: Does biodiversity tend towards equilibrium or does it not?” Tiedemann says, “We can model the general development of an ecosystem, one influenced by humans, for example. This provides an important source of information for environmental protection. Preserving biodiversity is the key to successful nature conservation.”
Prof. Ralph Tiedemann studied biology, computer science and Icelandic at the universities of Kiel and Reykjavík. After working as a visiting scholar at the Free University of Brussels (ULB), he received his habilitation in zoology with genetic studies on speciation as well as avian and mammalian population structures. Since 2002, he has been Professor of Evolutionary Biology/Systematic Zoology at the Institute of Biochemistry and Biology of the University of Potsdam.
Institut für Biochemie und Biologie
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As an evolutionary biologist, Dr. Luis Valente studied biology and ecology in London. Valente is currently a Guest Researcher at the Institute of Biochemistry and Biology of the University of Potsdam. He received the 2015 Brandenburg Postdoc Award for his research on diversification on islands.
This research is linked to the research initiative NEXUS: Earth Surface Dynamics, which clusters approaches from various scientific disciplines in the Berlin-Brandenburg area within the overarching theme of Earth surface dynamics. The University of Potsdam, along with its partnering institutions the Helmholtz-Centre Potsdam - German Research Centre for Geosciences (GFZ), the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) and partners from the Potsdam Institute for Climate Impact Research (PIK), the Museum für Naturkunde - Leibniz Institute for Evolution and Biodiversity Science (MfN) and the Technische Universität Berlin (TUB) therefore combines the outstanding expertise from geo-, bio-, climate and data sciences.
Text: Matthias Zimmermann
Online gestellt: Matthias Zimmermann
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