Theory regarding mammal evolution questioned

November 14th, 2007 - 8:37 am ICT by admin  
The earlier theory, know as the ‘Island rule’, suggested that in island environments, small mammals such as rodents tend to evolve to be larger, and large mammals such as elephants tend to evolve to be smaller. The key determining factor in this concept was thought out to be the original size of the species.

But the new research findings suggest that the tendency to either evolve larger or smaller on islands varies from one group of species to another, irrespective of original size.

According to the research team, a number of external factors, including the physical environment of the particular island, the availability of prey, the presence of predators and the presence of competing species, play a role in determining the size evolution of island mammals.

Other factors also helped scientists question the ‘island rule’ theory.

According to Dr Shai Meiri from the NERC Centre for Population Biology at Imperial College London, “If the island rule was correct, then most large mammals living on islands would be smaller than their continental relatives, and most small island mammals would be larger those living on continents. Our large dataset of mammal body sizes shows that this isn’t the case. There is evidence that most mammal groups show no tendency to consistently either grow larger or smaller, in contradiction to the island rule.”

“When we examined size change in light of the evolutionary relationship between different species, there was no connection between an evolution towards large size and greater degree of dwarfism on islands, or between evolution towards small size and island gigantism.,” said Meiri.

Though the earlier theory has been put into question, researchers say that there does appear to be a weak correlation between the size of a mammal and how its size then evolves in an island habitat. But it still reflects only the specific tendencies of some groups towards gigantism or dwarfism, and not the general course of evolution.

“The course of size evolution is dependent on a complex interplay of many other factors that have led to the evolution of fascinating miniature and giant species of mammals on islands,” said Meiri. (ANI)

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