Foster's rule

Garganornis ballmanni (reconstruction by Stefano Maugeri)

Foster's rule, also known as the island rule, is a principle in evolutionary biology that describes how the size of animals changes on islands. It was named after the ecologist J. Bristol Foster, who first formulated the rule in 1964. According to Foster's rule, small species tend to become larger on islands, while large species tend to become smaller.

Overview

Foster's rule suggests that the isolated conditions of islands can lead to significant changes in the size of animal populations over time. This phenomenon is attributed to several factors, including reduced predation pressures, limited resources, and the absence of competitors that are present in mainland environments. For small species, an increase in size can confer advantages such as increased territorial dominance and greater storage of body fat, which can be beneficial in environments where food resources are unpredictable. Conversely, large species may experience selective pressure to decrease in size due to constraints on available resources and space.

Examples

One of the most famous examples of Foster's rule is the case of the dwarf elephants that once lived on several Mediterranean islands during the Pleistocene epoch. These elephants evolved from larger mainland ancestors but became significantly smaller after being isolated on islands. Similarly, the Komodo dragon, the largest living species of lizard, is thought to have evolved its large size due to the absence of other large predators on the islands it inhabits.

Conversely, instances of small species becoming larger include the giant tortoises of the Galápagos Islands and the Seychelles, which have no close relatives of similar size on the mainland. The isolation of these islands has allowed these tortoises to grow much larger than their mainland counterparts.

Mechanisms

The mechanisms behind Foster's rule are complex and involve a combination of genetic drift, natural selection, and possibly founder effects. In small, isolated populations, genetic drift can lead to significant evolutionary changes over relatively short periods. Natural selection may favor individuals that are better adapted to the specific conditions of island life, such as those capable of exploiting a wider range of resources or those that are more efficient at reproduction in isolated environments. The founder effect, where a small group of individuals colonizes a new area, can also lead to rapid evolutionary changes as the genetic diversity of the population is initially limited.

Criticism and Limitations

While Foster's rule has been supported by numerous observations and studies, it is not without its critics. Some researchers argue that the rule is not universally applicable and that there are many exceptions. The variability in island sizes, climates, and the time species have been isolated can all influence the extent to which Foster's rule applies. Additionally, human activity, including the introduction of non-native species and habitat destruction, can significantly impact the evolutionary trajectories of island species, sometimes obscuring or reversing the trends predicted by Foster's rule.

Conclusion

Foster's rule provides a fascinating insight into the dynamics of evolution on islands. It highlights the adaptability of species when faced with new environments and the complex interplay of evolutionary forces that drive changes in species size. Despite its limitations and the exceptions to the rule, Foster's rule remains a key concept in the study of island biogeography and evolutionary biology.