What happens when animals evolve in isolation?

A Bit About Island Life

Steve Finn/Getty Images                              ­ Komodo dragons, like Raja, shown at the London Zoo, are examples of the gigantic proportions that island sp­ecies can grow to in comparison to their mainland cousins.
Steve Finn/Getty Images ­ Komodo dragons, like Raja, shown at the London Zoo, are examples of the gigantic proportions that island sp­ecies can grow to in comparison to their mainland cousins.
Steve Finn/Getty Images

In mid-September 1995, Hurricane Marilyn struck the Caribbean. The storm killed at least 13 people and caused more than $2 billion in damages. Many people lost their homes to the force of the hurricane. But not everyone who was affected by Marilyn was human. A group of 15 green iguanas is believed to have drifted from the island of Guadeloupe to Anguilla after clinging to storm debris for three weeks. When the green iguanas set foot on Anguilla, they became potential founding members of a new species.

If the process of speciation does take place, it could take a while -- if the gradualists are correct. Or it could happen rather quickly, as the punctuated equilibrium adherents believe. Of course, the iguanas may not live long enough to thrive in their new environment, if they are unable to adapt to the differences of life in Anguilla quickly enough.

But the setting of the iguana's new environment speaks volumes about evolutionary isolation. On the island, the iguanas are physically isolated and could evolve to become reproductively isolated from the other members of their species. Islands represent the ultimate in isolation. As such, island living has produced some radical swings in phenotypes -- outward characteristics -- among species. Both gigantism and dwarfism have been discovered as a result of speciation.

Perhaps no recent example of radical development toward the smallish has been as documented as the case of Homo floresiensis. On Flores, an island in Indonesia, a small skull was discovered in 2005. It was small, about the size of a chimp's skull, and that's probably what the skull's discoverers would have believed it to be -- had it not also been surrounded by several primitive tools. These tools suggested that the skull belonged to a human, or at least a human relative. But for a skull that small, it would have been a very tiny human. Testing of the skull found that the person it belonged to had lived 18,000 years ago on Flores. Calculating from the size of the skull, the researchers who found it believed that the person stood just more than 3 feet tall. News of the find rocked the scientific world, and even captured the imagination of some of the general public.

Whether or not the skull represents a new type of human is being challenged. Researchers at Penn State University believe that the skull found represents a Homo sapiens who suffered from a developmental disease -- microcephaly, just like the Amish group in Pennsylvania. But the notion that a group of small humans, distinct from Homo sapiens evolved on an island is an example of what is called the island rule -- a generalization about speciation where animals on islands often grow much smaller or larger than their mainland cousins.

Biologists are still trying to explain why this happens. The most logical explanation is that the environmental pressures on an island are much different than those on the mainland. The smaller sizes of islands offer fewer food sources. This would seem to explain dwarfism found in many species. But what of those that grow to gigantic proportions, like the Komodo dragon? It's possible that these species have developed to be larger than their cousins because islands offer fewer predators and less competition for the food sources available.

The prevailing theory concerning the island rule is that a variety of local environmental factors cause animals in island isolation to evolve to be large or small. It's possible that there is no rule of thumb that accounts for all island evolution across the board. Perhaps Anguilla's newest arrivals -- the green iguanas -- will shed light on the island rule, as well as speciation.

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