Foster’s Rule for NEET

Foster’s rule, also commonly known as the island rule or island effect, is a biological rule in evolutionary biology that describes the physical changes occurring in species with regards to their size depending upon the resources available in the environment. The rule emphasizes on a phenomenon that the height of the organism of a species might increase or decrease over a period of time depending on the availability of resources.

The rule also states that the height of the organisms of a species may increase, meaning they might grow bigger in size when they are isolated on an island with abundant resources, coupled with the absence of predator species.

Simultaneously, organisms of a species become smaller in size over the period of time, when they are isolated on an island with competition for resources or when the resources are almost over coupled with predator species living in the same habitat.

Bristol Foster’s Theory

The Foster rule was first proposed by Bristol Foster in 1964. He published this theory in his paper titled ‘Evolution Of Mammals On Islands’. Foster theorized island rule after comparing 116 mainland varieties to their island species. He suggested that some organisms turned into insular dwarfs or smaller versions of themselves, while some species turned into insular giants or bigger versions of themselves depending on two main factors:

• Availability of resources

• Absence of predation pressure

This hypothesis also further indicated that various species grow differently under varying conditions of the environment. This theory was stressed upon for quite long as it suggested that small prey grow bigger when there’s plenty of food around and no competition for it due to the absence of predators while predators who had fierce competition for food in their habitat became smaller in size with fewer resource requisites.

Insular Dwarfism

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Insular dwarfism is a type of phyletic dwarfism that refers to the decrease in the average size of organisms of a species. It is also known as island dwarfism and can occur in the mainland. Insular dwarfism is a type of phyletic dwarfism that refers to the decrease in the average size of organisms of a species. It is also known as island dwarfism and can occur in the mainland. Basically, insular dwarfism explains the process of the shift of growth of large animals to reduced body sizes. This shift gradually occurs when their population's range is limited to a small environment, primarily islands or caves or isolated valleys and mountains.

• Insular dwarfism is natural and is distinct from the intentional creation of dwarf breeds. This phenomenon has taken place many times throughout evolutionary history.

• Also, it does not specifically occur on islands, but in an isolated ecosystem that is outlying from external resources like caves, desert oases, isolated mountains or valleys, etc. 

• The primary cause of insular dwarfism is the limited range of the population to a rather smaller region. 

• It is often seen in mainland animals who populate islands or isolated places. This is because in isolated places, the food supply is limited and the requirement of this food supply is sufficed by small animals hence they survive for long in such regions. 

• Lives in these regions flourish when the food supply reloads.

• Insular dwarfs also have fewer generation times and shorter gestation periods.

• The availability of food (prey) and their size contributes to being a major factor for carnivores.

Insular Gigantism

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Insular gigantism refers to the process of drastic increase in the sizes of organisms of a species in an isolated island as compared to its mainland counterparts. Foster’s rule postulates that when mainland animals populate islands, they launch into developing bigger versions of themselves.

Due to the evolution of species over a period of time, island endemics and other animals have become extinct, which has also been observed in plants (insular plants).

Huge carnivores animals fail to survive on islands due to oceanic dispersal. In the absence of these, huge predators, small carnivorous animals, reptiles and birds thrive and have an adequate supply of food. Thus, carnivores like reptiles, birds and other species grow larger. 

Reduced predatory pressure on small animals allows them to become huge and also results in a lesser competition to them. In addition to this, increased sizes of these organisms decrease vulnerability during food deprivation scenarios as they are able to survive and travel for longer intervals without food.