The conversation
How virtual worlds can recreate the geographic history of life
Forest near Sarayaku, Ecuador. skifatenum, CC BYThe Amazon and the adjacent Andean slopes in South America host an amazing wealth of plants and animals. These species have been sources of food, shelter and medicine since the arrival of humans and a target of scientific curiosity since the days of the first European naturalistic explorers. What processes produce these species rich hotspots and why does biodiversity gradually decrease towards higher latitudes and drier climates? Scientists have proposed many competing explanations, but there is no easy way to test them. As biogeographers, those of us who study the geography of life on the planet, we don’t have the option of carrying out experiments in the real world. It would be impractical and unethical to undertake massive introductions or exterminations of species and then wait centuries or millennia for results. Instead, as reported in our 2018 study published in the journal Science, we assembled an interdisciplinary team of biogeographers and climate modelers to create a virtual world – a place to do virtual experiments. The world we recreated was a time-lapse simulation of life on the continent of South America, from 800,000 years ago to the present, through the wind-saw climates of the last eight glacial cycles. If the patterns of biodiversity produced in this simulated world produced reasonably realistic patterns of diversity, we could be sure that the ecological and evolutionary processes embedded in the simulation were correct. What we found was a surprise beyond our greatest expectations. The South American species diversity maps that emerged from our simulations looked remarkably similar to maps of birds, mammals and living plants. In addition, the simulations confirmed corridors of intermittent migration between the Andes and the Atlantic Forest in southeastern Brazil. These regions are currently isolated from each other by drier climates, but scientists have long suspected the existence of connections, based on the presence of closely related living species in both regions. Virtual life in a virtual world Each simulation started with a single imaginary species, sown somewhere on a detailed topographic map of South America. In 500-year time intervals, totaling 1,600 steps in all, the climate was updated with a model state-of-the-art paleo-climatic created by our colleagues Neil Edwards and Phil Holden of The Open University in the United Kingdom. over a thousand simulations, each with a different combination of configurations for only four variables: – How long should a population be isolated to become a new species – How quickly can species evolve to survive in response to climate change – A how far a species can move through inadequate habitat – how closely related species compete with each other. Why was the strong correspondence between our simulated species richness maps and real-world maps for birds, mammals and plants so surprising? Because our simulations covered only a small part of the time in South America’s long history. Eight hundred thousand years may seem like a profound time, but South America separated from Africa 130 million years ago and the Andes began their ascent 25 years ago. millions of years. It is now known that a growing list of South American plant and animal groups has diversified towards the end of the quaternary period – roughly in the last 800,000 years – but most species on the continent are much older. We were also surprised that our simulated maps were so similar to the actual patterns of species richness, because our maps were not guided by any specific pattern of diversity. They were built strictly on fundamental processes, understood from basic research in ecology and evolutionary biology. For example, we model evolutionary adaptation to climatic extremes using principles and equations from population genetics. From the cradle to the museum to the grave The species alive today are survivors. They are the upper ends of evolutionary trees with many dead branches below, which represent extinctions in the past. Evolutionary biologists are now able to infer, in many cases, where the ancestors of living species may have lived. The regions where species have proliferated in the past have come to be called the “cradles” of speciation. For example, the Andean slopes have long been considered a hot spot for speciation. The first diagram of an evolutionary tree by Charles Darwin, from his first notebook on the transmutation of species (1837). His notes make it clear that he understood that extinctions are an essential element of evolution: ‘Thus, genres would be formed in relation to ancient types with various extinct forms.’ The regions where the species persisted for particularly long periods are called “museums”. Any region, such as the Amazon, where many ancient species persist, can be considered a biogeographic museum. In contrast, calculating where the dead branches on the evolutionary tree should be placed on the map – the “tombs” – is virtually impossible by studying the geography of living survivors. Through our simulations, we monitor and map the entire “life trajectory” of each virtual species, from the cradle to the grave, in space and time. As the climate changes step by step in a simulation, the geographic distribution of a species (its location on the map) can be fragmented by inadequate weather. If a fragment remains isolated long enough, a new species is declared. The fragmentation time and the location of such a fragment during this period of isolation defines the “cradle segment” of its life trajectory. When and if a virtual species becomes extinct, we record the time and plot the location of the decline towards extinction on the map, which represents the “serious segment” of the species’ life trajectory. The time and place that each species persists between the stage of the cradle and the stage of the tomb defines the “museum segment” of its life trajectory. Our simulations produced maps of cradles, museums and, for the first time, tombs. The maps confirm that the eastern slopes of the Andes and the western Amazon are cradles of speciation. Extinction tombs coincided with cribs in some regions, such as the Amazon, and were moved from cribs in others, such as the Andes. The eastern slope of the tropical Andes proved to be not only a cradle, but also a rich biodiversity museum. We also monitored when speciation and extinction peaked and decreased over the course of the simulations, and we found that glacial cycles drove both processes. Extinction peaks tend to follow speciation peaks in periods of rapid warming at the end of cold glacial periods. Climate dynamics and topography drive the patterns Our study leads us to believe that the richness patterns for living species, regardless of the age of the species, have their origins in the same underlying processes that we model in the simulation. The interaction between the turbulent climates of the last 800,000 years and the dramatic landscapes of South America has led to speciation in some younger groups of plants and animals, but has changed the location of young and old species together, indiscriminately. Human activities are forcing changes in the global climate at an unprecedented pace, much faster than the climate dynamics in our model. We know that species are already in motion, their intervals changing at alarming rates on land and in the seas, with profound effects on human life and livelihoods. Although our simulations were not designed to predict the future, they vividly reveal the dynamic power of climate change to shape life on Earth. This article was republished from The Conversation, a non-profit news site dedicated to sharing ideas from academic experts. Read more: Why we need a ‘trip to the moon’ to catalog the Earth’s biodiversity; will we soon see another wave of bird extinction in the Americas? Simulating evolution: how close do computer models get to reality? Robert K. Colwell received funding from the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil, for this project. Thiago F. Rangel was supported by the National Council for Scientific and Technological Development (CNPq) and by the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil. This project is also supported by the INCT in Ecology, Evolution and Conservation of Biodiversity, funded by MCTIC / CNPq and FAPEG.