User:Sportsphreak73/Evolution of Island Mammals

Evolution of Island Mammals

Mammal species that have been isolated in an island environment have been shown to have different rates of evolutionary change, and other different phenotypic changes like body size when compared to their mainland species equivalent. One of the most important determinant factors in why and how a species adapts or evolves is the organism’s environment. Environmental pressures often determine how an organism is preyed upon or how they hunt down their own prey. Almost every aspect of an organism’s life history is affected by their environment. Mammals that are isolated in an island environment to some extent, separated from members of their own species still on the mainland, can evolve at a faster rate due to the dramatic changes that may occur in their environment. This phenomenon has been described at the “island effect” or “Foster’s rule”. Another description known as the “island rule” is also defined as “the tendency for a graded series of changes in the size of island vertebrate species in relation to mainland congeners, such that small‐bodied species tend to get larger, and vice versa”—definition from a standard textbook on Island Biogeography. An evolutionary biologist, Van Valen in 1973 first introduced us to this based on a study by a mammologist J. Bristol Foster in 1964. One well known example of these differences is known as island gigantism, this is when the species isolated on an island is much larger than the same species located on the mainland. These ideas related to the “island effect” are still a topic being explored and debated by current evolutionary biologists.

History

Maleo at Bronx Zoo

Peromyscus (Deer Mouse)

There are multiple historical examples of island mammals exhibiting biological differences from their mainland counterparts. In the 1800s, Alfred Russel Wallace hypothesized that the environment could help to shape the diversity of live by favoring certain traits over others. Wallace went on to coauthor a paper with Charles Darwin a theory that animals evolve by adapting to their environment. Wallace's work was centered on what is now Indonesia's island of Sulawesi in search for a bird called the maleo. While versions of this bird had been found off the island, the maleo found on Sulawesi was very specific species located only on the island. Another early example of the "island rule" was data collected for a nature article published in 1964, by mammologist J. Bristol Foster. Foster studied two species of deer mice on the Queen Charlotte Islands, approximately 40 miles off the coast of British Columbia, Canada. The deer mice on the island averaged three times the weight of the deer mice on the mainland. The local scientists had two different ideas on why these island mice were so much larger. It was hypothesized that the mice were left over from an ancient time when the deer mice were a much larger species. Foster had hypothesized that the mice living in the separated island environment evolved differently than the mainland deer mice. Foster’s hypothesis was supported by his 1964 study printed in the Nature journal. Foster studied many mammals on these same islands. The rodents consistently showed a larger size. Other mammals like rabbits and carnivores showed the opposite trend and were smaller than their mainland similar species, but all showed significant phenotypic differences from their mainland equivalent species. A similar biological phenomenon is allopatric speciation. Allopatric speciation can occur when certain members of the same species experience adaptations, not experienced by the other members, because they  become isolated in a separate geographical location. This can occur due to changes in mountain ranges, water levels, and with human interruption to normal geographical boundaries experienced by certain species.

Urocyon littoralis standing (Island fox)

More Recent Evidence

Amsterdam Island Cattle

Limited populations that live exclusively in island environments are often susceptible to extinction due to their lack of genetic diversity, climate change, and habitat loss. In 2015, a study was completed on the California Channel Islands concerning the dwarf foxes that habitat these island environments. They studied the mitochondrial DNA of these smaller island gray foxes in comparison to the mitochondrial DNA of the mainland gray foxes. Mitochondrial DNA is a valuable tool for understanding the evolutionary relationships in wild mammals. Known changes in the size of the Channel Islands, and their distance from the mainland California shoreline caused changes in species extinction and colonization rates. Currently, there are only ten species of land mammals present on these islands. Their collected data suggested that the island fox underwent rapid evolution, due to their differing behavior and morphology. This rapid evolution was attributed to probable interactions with humans, climate change, and pressures from life on an island. The fox presented traits of tameness and smaller size, both understood to be evolutionary pressures of their island life. Evidence showed that Native Americans transplanted and buried island foxes so the smaller size and tameness phenotypes again evolved with the influences of human interactions on the islands not experienced on the mainland. In 2017, there was another study published in Nature concerning the dwarfism of feral cattle on the Amsterdam Island in the southern Indian Ocean. The cattle were introduced in 1871, but eventually eliminated to protect the natural species of the endangered albatross, and the natural flora that the cattle were destroying by years of predator free grazing. The T test completed by the investigators showed a clear size difference in the feral cattle located on the island. The data showed mass differences in the male and female cattle that averaged between 49-81% reduction in mass. In approximately 117 years of the feral cattle inhabiting Amsterdam Island, the initial dwarfism did appear to occur very rapidly in evolutionary terms.

The Continued Debate

In 2018, a review was completed of almost 150 different studies concerning the “island rule”. The reviewers found that over half of the studies were completed more recently, occurring after 2005. They also noted that the rule did not apply to many different species. Size differences in mammals were much more prevalent than in other species like lizards. The reviewers also believed there were multiple examples of studies that displayed “HARKing” Hypothesizing after the results are known. These reviewers concluded that there is not enough evidence to support the “Island rule”. A study completed through Tel aviv university analyzed the skulls and teeth of members of the order Carnivore in populations separated from the mainland. They found that there was little evidence for the “island rule” when a population was examined versus when one species was tested. Data has been collected from all over the world, and over multiple decades. While multiple scientists disagree with the reviewer's description of the “island rule”, they believe their data supports the validity of the "island rule".