Scientists have for the first time measured how fast large-scale evolution can occur in mammals, showing it takes 24 million generations for a mouse-sized animal to evolve to the size of an elephant.

Research just published in the Proceedings of the National Academy of Sciences describes increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.

An international team of 20 researchers, including University of Georgia Odum School of Ecology dean John Gittleman and assistant research scientist Patrick Stephens, discovered that rates of size decrease are much faster than growth rates. It takes only 100,000 generations for very large decreases, leading to dwarfism, to occur.

Team leader Alistair Evans, research fellow in the School of Biological Sciences at Monash University in Australia, said the study was unique because most previous work had focused on microevolution, the small changes that occur within a species.

Gittleman agreed. “This is the first study to try to quantify evolutionary rates at such a large time scale and across so many groups of species,” he said. “This is important because it can begin to tell us a lot about how species adapt. Some have the capacity to evolve more quickly than others. That might bode well for those species in the future, as major environmental changes occur.”

The study concentrated on large-scale changes in body size.

“We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change – a massive change, but also a very long time,” Evans said. “A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations.”

The paper looked at 28 different groups of mammals, including elephants, primates, and whales, from various continents and ocean basins over the past 70 million years. Size change was tracked in generations rather than years to allow meaningful comparison between species with differing life spans.

Erich Fitzgerald, Senior Curator of Vertebrate Palaeontology at Museum Victoria and a co-author, said changes in whale size occurred at twice the rate of land mammals.

“This is probably because it’s easier to be big in the water – it helps support your weight,” Fitzgerald said.

Evans said he was surprised to find that decreases in body size occurred more than ten times faster than the increases.

“The huge difference in rates for getting smaller and getting bigger is really astounding – we certainly never expected it could happen so fast!” Evans said.

Many miniature animals, such as the pygmy mammoth, dwarf hippo and ‘hobbit’ hominids lived on islands, helping to explain the size reduction.

“On small islands, where resources are scant, getting smaller as fast as possible is a real advantage,” said Gittleman, because smaller organisms don’t need as much food and are able to reproduce faster.

The research furthers understanding of conditions that allow certain mammals to thrive and grow bigger and circumstances that slow the pace of increase and potentially contribute to extinction. The findings build upon earlier work by the group that explored the evolution of the maximum body size of mammals.

“Now that we have developed a means to measure the rates of evolution, we can look at related ecological questions more closely,” said Gittleman. “For instance, we can think about differing evolutionary rates in response to environmental change and disease outbreaks.”

Stephens said these questions are important for understanding the continuing evolution of species. “We can ask how many generations organisms will have to adapt to climate change, for instance,” said Stephens. “Looking at evolutionary rates may tell us which ones are likely to survive.”

Funding for the study was provided by the Australian Research Council, Monash University, the National Science Foundation, the European Union, and the Harold Mitchell Foundation.

Besides Evans, Gittleman, Stephens, and Fitzgerald, the paper’s authors were David Jones, Alison G. Boyer, University of Tennessee; James Hemphill Brown, University of New Mexico; Daniel P. Costa, University of California, Santa Cruz; S. K. Morgan Ernest, Utah State University; Mikael Fortelius, University of Helsinki; Marcus J. Hamilton, University of New Mexico; Larisa E. Harding, Umeå University; Kari Lintulaakso, University of Helsinki; S. Kathleen Lyons, Smithsonian Institution; Jordan G. Okie, University of New Mexico; Juha J. Saarinen, University of Helsinki; Richard M. Sibly, University of Reading; Felisa A. Smith, University of New Mexico; Jessica M. Theodor, University of Calgary; and Mark D. Uhen, George Mason University.