Nature's Laboratories: How Islands Shape Evolution

The idea of islands as laboratories where natural selection expresses its stranger side is not difficult to justify. Many species on islands are unique, and uniquely different from their mainland counter-parts. Dwarf elephants, giant carnivorous birds, the world’s largest stick insect (and largest insect to boot) – all of these are (or were) the inhabitants of islands. The biotas of islands often consist of multiple endemic species , and whilst the term island conjures up images of tropical paradise, for biological purposes an island can be taken to mean anything from an isolated mountain-top, to a patch of forest in otherwise monotonous grassland.

Figure 1: Elephant bird (Aepyornis maximus)

The capability of mainland species’ to reach islands is of importance in how well the biotas in these restricted areas end up reflecting the composition of mainland biotas.  As such, two broad divisions can be made: harmonic and disharmonic biotas. Harmonic biotas are those in which the assemblages of organisms present are similar to that found on the mainland in that whilst fewer species occur, proportionately the ecological groupings or taxa are the same. Disharmonic biotas result due to a difference in dispersal ability of organisms, as well as factors such as competition. Essentially, those organisms which are better at reaching islands (such as flying animals or wind-borne seeds) may be disproportionately represented. Other taxa from the mainland may never make it to the island at all, and, naturally the more isolated the island the fewer species are expected to occur there. The larger an island’s size, the more species may reasonably be expected to survive there. These are all common sense rules of island biogeography .

Once a population becomes established in its new island home, the different selective pressures it faces may result in some very strange changes in evolutionary development. Some of these evolutionary trends are common to island organisms. Reduced dispersal ability is one such development. Now it may seem counter-intuitive that organisms on islands should lack the ability to leave, but loss of long distance mobility has advantages. In insects, specimens which restrict their movements to a home range area are more likely to avoid death at sea on an oceanic island, thus they will survive and breed. Flightless birds sacrifice mobility for lower energy investment. They no longer have to grow strong flight muscles, as predators are often missing from island areas. Coupled with the fact that resources may be scarce, it makes sense to avoid unnecessary commitment of energy to flight.

Flightlessness can also be rooted in another island phenomenon: Gigantism. On islands, organisms may have the opportunity to evolve to much larger sizes. The island of Madagascar for example once housed Elephant birds - 400 kilograms and three terrifying meters tall. The development of such an imposing physique may be related to ecological release (the concept that in the absence of competition, a species may develop in a way that allows broader habitat and niche use). Competition between members of the same species becomes more important than competing with other species, and therefore being larger may mean access to better territory and food resources. Many examples of gigantism on islands exist. On the island of Minorca, around 3 million years ago, a massive rabbit relative Nuralagus rex outstripped its mainland relatives in size by twelve times. Along with a Minorcan giant tortoise, this lumbering rabbit dominated the landscape in the absence of predators or competitors.

Figure 2: Nurolagus rex as compared to modern rabbit

Naturally, however, a different rule must apply to large animals reaching islands. Well, actually, it’s the same rule, the so called ‘island rule’ which states larger species will evolve to become smaller on islands and smaller species will develop to become larger. Dwarfism is also prevalent on islands, and if we return to the example of Madagascar where the world’s largest bird became extinct, we find an entire genus of dwarf chameleons still wandering the leaf litter of the forest floors. Dwarfism may result from limited resource availability or dwarf organisms may simply be better adapted to obtain nutrients and efficiently utilise them than their larger counterparts.

Figure 3: Crotalus ruber 

A fascinating example of how resources and the pattern of colonisation can affect species size can be found on the island of Angel de la Guarda in the Gulf of California. Two species of rattlesnakes occur on the mainland near the island, the large species Crotalus ruber and its half-sized relative Crotalus mitchelli. On the island itself however, Crotalus ruber specimens are half the size of C. mitchelli, an exact reversal. The reason for this is thought to be that C. mitchelli dispersed to the island first where it became larger than its mainland counterpart, due to lack of competitors. When C. ruber arrived, the ‘large snake’ niche was already taken, thus limited resources caused a trend towards smaller specimens, until ultimately the snakes on the island filled the same niches as their mainland relatives, simply in reversed order.

The existence of species in isolated patches such as islands allows scientists a unique opportunity to study evolution and evolutionary processes on a manageable scale. It’s no co-incidence that both Darwin and Wallace came to their revolutionary ideas by studying island fauna. Islands exaggerate evolutionary processes in a way that allows for deeper understanding of the workings of processes like Natural Selection in a contained, natural laboratory.

Works Cited:

BROWN, J.H. & LOMOLINO, M.V. 1998. Biogeography. 2^nd Edition. Sinauer Associates Inc., Massachussets.

KAY, A.E. 1994. A Natural History of the Hawaiian Islands: Selected Readings II. University of Hawaii Press, Honolulu.

QUINTANA, J., KOHLER, M. & MOYA-SOLA, S. 2011. Journal of Vertebrate Paleontology 31(2):231-240. Available online at: http://www.bioone.org/doi/abs/10.1080/02724634.2011.550367 (Accessed 01/03/2013).

TYSON, P. 2008. Gigantism and Dwarfism on Islands. http://www.pbs.org/wgbh/nova/evolution/gigantism-and-dwarfism-islands.html (Accessed 01/03/2013).