Mouse to Elephant? Just Wait 24 Million Generations
ScienceDaily (Jan. 30, 2012) — 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.
Dr. Alistair Evans with the skulls of a mouse and an elephant. (Credit: Image courtesy of Monash University)
Research 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
Led by Dr Alistair Evans of Monash University's School of Biological Sciences a team of 20 biologists and palaeontologists 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.
Dr 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," Dr 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.
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The maximum rate of mammal evolution
Alistair R. Evans a,1, et al
Author Affiliations
aSchool of Biological Sciences, Monash University, VIC 3800, Australia.
Abstract
How fast can a mammal evolve from the size of a mouse to the size of an elephant? Achieving such a large transformation calls for major biological reorganization. Thus, the speed at which this occurs has important implications for extensive faunal changes, including adaptive radiations and recovery from mass extinctions. To quantify the pace of large-scale evolution we developed a metric, clade maximum rate, which represents the maximum evolutionary rate of a trait within a clade. We applied this metric to body mass evolution in mammals over the last 70 million years, during which multiple large evolutionary transitions occurred in oceans and on continents and islands. Our computations suggest that it took a minimum of 1.6, 5.1, and 10 million generations for terrestrial mammal mass to increase 100-, and 1,000-, and 5,000-fold, respectively. Values for whales were down to half the length (i.e., 1.1, 3, and 5 million generations), perhaps due to the reduced mechanical constraints of living in an aquatic environment. When differences in generation time are considered, we find an exponential increase in maximum mammal body mass during the 35 million years following the Cretaceous–Paleogene (K–Pg) extinction event. Our results also indicate a basic asymmetry in macroevolution: very large decreases (such as extreme insular dwarfism) can happen at more than 10 times the rate of increases. Our findings allow more rigorous comparisons of microevolutionary and macroevolutionary patterns and processes.
haldanes, biological time, scaling, pedomorphosis
Footnotes
1To whom correspondence may be addressed. E-mail: arevans@fastmail.fm or jhbrown@unm.edu.
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The authors declare no conflict of interest.
This article contains supporting information online atwww.pnas.org/lookup/suppl/doi:10.1073/pnas.1120774109/-/DCSupplemental.
