Why didn't the Recovery Program for Rodent Population Work?

By: Rebecca Maguire and Abbie Grise

My colleague and I were hired to research a species of squirrels on the West Indie Islands. We are experts in mammalian reproductive strategies and have discovered that although the population of the Sciurus Kittsisaltus, or more commonly known as the Kitts Jumping Squirrel, is quite small and threatened, the population of the rodents on Nevis are thriving and healthy. We tried to use the rodents from Nevis to reproduce with the rodents from St.Kitts but they weren't successful in reproducing. We have deduced that although these rodents are very similar in appearance, they are different species. The chart below shows the differences we found in the two rodents. The scientific name for the newfound species that lives on Nevis is Sciurus Nevisitardum and the common name is the Nevis Tree Squirrel. We have done extensive research and have concluded that there are a few reasons for the creation of a new species on Nevis.

Allopatric Speciation, Genetic Drift and Directional Selection

Allopatric speciation is when a new species are formed due to geographic isolation. When the geographic isolation occurs the two populations evolve independently. While we were researching the area around the West Indies we discovered there was a possible land bridge connecting the Nevis and St.Kitts islands at one point millions of years ago. This land bridge was covered when glaciers melted and caused sea levels to rise. The species of rodent living on the large island was split due to the land bridge being covered. Over many generations each population of rodent evolved independently, eventually creating two distinct species.

As a result of allopatric speciation genetic drift also occurred. There was a larger population of rodents on St.Kitts and their genetic composition stayed relatively the same, while the genetic composition of the smaller population of rodents on Nevis changed drastically over time. This is a random occurrence but is more likely to occur with smaller populations, like the one on Nevis, because a certain gene can be lost easier when there are less animals to reproduce with. In the case of the rodents, the gene that contributed to long limb length and speed was lost on the island of Nevis because there were less rodents with it therefor there was a lesser chance of the gene being carried to future generations.

There is also another possible explantation to the evolution of the new species which is knows as directional selection, a mode of natural selection. One of the main predators of the rodent that is on St.Kitts died off on the island Nevis. Because of this the rodents on Nevis don’t have to travel as fast to get away from the predator. Over time the gene for speed and long limb length wasn’t needed by the Nevis rodents and a shift in phenotype occurred. The limbs of the Nevis rodent became shorter over time, as well as their speed and jumping height and a new species was formed. This could be how they evolved and became much slower then the St.Kitts rodents.