Islands spark accelerated evolution

September 12, 2006
Giant tortoises at the Darwin Station on Isla Santa Cruz in the Galápagos Islands
Giant tortoises at the Darwin Station on Isla Santa Cruz in the Galápagos Islands. (Photo: Catriona MacCallum)

The notion of islands as natural test beds of evolution is nearly as old as the theory itself. The restricted scale, isolation, and sharp boundaries of islands create unique selective pressures, often to dramatic effect. Following what’s known as the “island rule,” small animals evolve into outsize versions of their continental counterparts while large animals shrink.

Giant tortoises and iguanas still inhabit the Galápagos and a few other remote islands today, but only fossils remain of the dwarf hippopotami, elephants, and deer that once lived on islands in Indonesia, the Mediterranean, and the Pacific Ocean. The fossil record suggests that these size changes occur rapidly after species become isolated on islands, but this long standing assumption has never been empirically examined in a systematic manner. Now, in a new study published in PLoS Biology, Virginie Millien confirms that island species undergo accelerated evolutionary changes over relatively short time frames, between decades and several thousand years.

Millien collected data from text, figures, and tables in an extensive survey of the published literature. From these datasets, she calculated a total of 826 evolutionary rates for 170 populations representing 88 species. Rates of evolutionary change, she found, decreased over time for both island and mainland species, with a slower rate of decrease for island species. The differences in evolutionary rates between island and mainland pairs also decreased over time, becoming statistically insignificant for intervals over 45,000 years. Overall, island species evolved faster than mainland species—a phenomenon that was most pronounced for intervals between 21 years through 20,000 years.

The finding that mammals evolve faster on islands, Millien argues, comports with the island evolution theory prediction that mammals respond to their new island homes with rapid morphological and size adaptations. The brisk pace of these changes may explain why the fossil record harbors few examples of intermediate forms between the mainland ancestor and island descendant. Millien’s results are also consistent with the hypothesis that evolution rates for island species slow down after the initial period of accelerated change, approaching rates on the mainland.

If island species can evolve quickly, Millien argues, it stands to reason that mainland species retain a similar capacity. As habitat destruction continues to pose the number one threat to biodiversity, many mainland habitats are beginning to resemble islands, with isolated pockets of wildlife separated by degraded or developed lands.

Thus, island species may serve as a model for understanding how mainland species will adapt to the rapidly changing environmental conditions brought on by habitat destruction and global warming. It appears that some mainland species are already responding like island species: a 1989 study followed the island rule in linking fragmented habitat to body size changes in 25 European mammals over the past 200 years. How long animals can continue to evolve in the face of these changes, however, remains to be seen.

Citation: Millien V (2006) Morphological evolution is accelerated among island mammals. PLoS Biol 4(10): e321. DOI: 10.1371/journal.pbio.0040321. dx.doi.org/10.1371/journal.pbio.0040321

Source: Public Library Of Science

Explore further: Study shows treeshrews break evolutionary 'rules'

Related Stories

The secret life of island animals

December 11, 2017

Islands are places of myth and wonder that have fascinated people for centuries. They're also places to find odd versions of some familiar animals.

How a 'flipped' gene helped butterflies evolve mimicry

November 7, 2017

Female swallowtail butterflies do something a lot of butterflies do to survive: they mimic wing patterns, shapes and colors of other species that are toxic to predators. Some - but not all - swallowtail species have evolved ...

Recommended for you

Cells lacking nuclei struggle to move in 3-D environments

January 20, 2018

University of North Carolina Lineberger Comprehensive Cancer Center researchers have revealed new details of how the physical properties of the nucleus influence how cells can move around different environments - such as ...

Information engine operates with nearly perfect efficiency

January 19, 2018

Physicists have experimentally demonstrated an information engine—a device that converts information into work—with an efficiency that exceeds the conventional second law of thermodynamics. Instead, the engine's efficiency ...

Team takes a deep look at memristors

January 19, 2018

In the race to build a computer that mimics the massive computational power of the human brain, researchers are increasingly turning to memristors, which can vary their electrical resistance based on the memory of past activity. ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.