“How extremely stupid not to have thought of that!” was Thomas Henry Huxley’s reflection on reading Darwin’s Origin of Species. What might elicit such a reaction from a contemporary biologist? Today the question is answered by Douglas J. Futuyma, Distinguished Professor in the Department of Ecology and Evolution, Stony Brook University. A member of NCSE’s Advisory Council, Futuyma is the author of the successful textbook Evolution (third edition 2013).
My fascination with the diversity of life, which impelled me to become a biologist, went into high gear when I first visited the tropics as a student in one of the first courses offered by the Organization for Tropical Studies (OTS) in Costa Rica. This tiny country has more species of birds than have been recorded in the United States, and the relatively small plot of rain forest at the La Selva Biological Station has more than 270 species of trees. The “latitudinal diversity gradient,” a term for the greater diversity of most major groups of terrestrial, freshwater, and marine plants and animals in tropical than in higher-latitude ecosystems, has been a subject of debate among ecologists and evolutionary biologists for at least 60 years. Why are there more species in the tropics?
Some early authors suggested historical explanations, such as greater extinction of high-latitude species by Pleistocene glacial episodes during the last two million years. During the 1960s, however, ecologists formulated “equilibrium theories” of diversity that proposed to account for variation in diversity by ongoing ecological processes. It was suggested, for example, that tropical environments are more stable than temperate-zone environments, enabling species to become highly specialized on different resources such as food types, and so to reduce competition that might otherwise diminish diversity. It was also suggested that species had originated at higher rates in tropical regions in the recent past, when dry periods forced populations of birds and plants into isolated wet areas where they evolved into distinct species.
Having absorbed these ideas as a graduate student in the 1960s, I duly lectured about them when I had the good fortune of teaching many later OTS courses. But during one such course in the 1980s, I reflected that not only were species more diverse in Costa Rica, but also so were higher taxa. Costa Rica has whole families of birds, such as motmots and parrots, and countless genera of reptiles, such as Anolis lizards and Bothrops vipers, that are restricted to tropical regions. These taxa are much older than the species thought to have formed in the Pleistocene; the richness of the tropical biota must therefore have a very long history—which, I found, was indeed affirmed by paleontologists. This suggested to me that the higher latitudes might be inhabited only by rather few lineages that had managed to overcome some kind of genetic constraint, and adapt to cold conditions.
But I had little idea of how to test the notion that most lineages had an ancient history of origination and diversification in the tropics, nor the hypothesis that genetic constraints prevent most lineages from easily adapting to high-latitude conditions, so I turned my attention to other, perhaps easier problems. But other researchers were more imaginative. Both evolutionary ecologists (for example, Robert Ricklefs and Dolph Schluter, 1993) and systematists (for example, Michael Donoghue and John Wiens, 2004) started to develop similar ideas into the concept of “phylogenetic niche conservatism,” and provided evidence that related taxa may retain very similar “climate niches” over long periods of time. Among young evolutionary ecologists, Paul Fine and Richard Ree (2006), recognizing that most of the Earth had a tropical climate for much of its history, calculated that the tree diversity in tropical, temperate, and boreal biomes is correlated with the area of each climatic zone, integrated over the last 50 million years or so. Moreover, some evolutionary biologists used phylogenetic methods and data (which, to be sure, were far less well-developed in the 1980s) to show that major taxa, such as tree frogs, had a long history of tropical diversification, and that only a few lineages have penetrated and diversified in the temperate zone (Wiens and others 2006). It’s when I look back at that body of work that I am tempted to say, with Huxley, “How stupid not to have thought of that!”
These historical, evolutionary considerations have greatly changed thinking in community ecology. There is now plenty of evidence that we need to acknowledge the imprint of deep evolutionary and environmental history on today’s patterns of diversity. Species may often adapt rapidly to certain factors, but adaptation of some organisms to some environmental challenges can be very slow. This is also becoming evident from analyses of limits on genetic variation that may prevent populations from adapting rapidly (Blows and Hoffmann 2005; Futuyma 2010). Why is this important? Because human alterations of the environment, especially climate, are threatening biodiversity with mass extinction. For vertebrate species to adapt fully to the rates of climate change projected during the next century, their “climate niches”—the range of temperature and rainfall conditions within which a species lives—would have to evolve more than 10,000 times faster than has been typical in the past (Quintero and Wiens 2013). Tropical species—which account for most biodiversity and which already live near their critical temperature maximum—are probably at the greatest risk of extinction. Evolutionary adaptation is unlikely to prevent the demise of much of life’s exquisite diversity. We must change our ways.