In the first study to measure how fast large-scale evolution can occur in mammals, scientists have calculated that it takes 24 million generations for a mouse-sized animal to evolve to the size of an elephant.
Published in the Proceedings of the National Academy of Sciences (PNAS), the study by an international team of biologists and paleontologists describes increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.
Daniel Costa, professor of ecology and evolutionary biology at UC Santa Cruz, was the marine mammal expert on the team. The study found that changes in whale size occurred at twice the rate of changes in land mammals.
"As a group, marine mammals are bigger than terrestrial mammals. It appears that life in the ocean requires large body size, so there must be a higher selection pressure to attain large body size," Costa said. "Also, they don't need as much skeletal material because they are supported in the water, and this probably makes it easier to grow larger, and to grow large fast."
The researchers, led by Alistair Evans of Monash University in Australia, 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. Evans, an evolutionary biologist and Australian Research Fellow, said the study was unique because most previous work had focused on microevolution, the small changes that occur within a species.
"Instead we 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.
The researchers found that body size can decrease much faster than it can increase. It takes only 100,000 generations for very large decreases, leading to dwarfism, to occur. The evolution of pygmy elephants, for example, took ten times fewer generations than the equivalent sheep-to-elephant size change. 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. "When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands," Evans said.
The study highlights the comparative difficulty of major changes in body size, especially increasing in size. The speed of these changes has implications for understanding the extensive changes in fauna that occur after mass extinctions or during "adaptive radiations," when a single ancestral species gives rise to an array of new species with different traits.
