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In evolution, sometimes species get big, and sometimes they get small. The factors behind changing body sizes are extremely varied, and many trends in size evolution are poorly understood. This episode, we discuss Evolutionary Gigantism and Dwarfism.
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Evolution Big and Small
The giant ground sloths of the Ice Age; the diminutive earliest birds; the pygmy hippos and gorilla-sized lemurs of Madagascar; the whales. All of these, and many more examples, are the result of evolution producing populations significantly smaller or larger than their ancestors. This process is known as evolutionary (or phyletic) gigantism and dwarfism – not to be confused with medical gigantism and dwarfism.
The conditions that favor gigantism and dwarfism can vary quite a lot from case to case. Sauropods’ great size has been attributed to their efficient breathing and feeding styles, among other things; the tiny size of the earliest birds might have been a requirement for them to take to the skies; and the famous giant insects of the Carboniferous were likely able to grow to such impressive sizes because of high oxygen levels, although the link between oxygen and body size does not apply to many other animals, including later insects. Each example of giant or tiny species is unique, which can make it quite tricky to study these evolutionary phenomenon.
Trendy Trends in Body Size Evolution
Perhaps the most famous trend in body size evolution is the “Island Rule” (aka Foster’s Rule), which states that when animal populations are isolated on islands, large species tend to grow smaller and small species tend to grow bigger. This is undoubtedly a thing, as evidenced by the many examples of giant island birds and miniature island mammals. However, some studies have found that island gigantism and island dwarfism might not be as common or predictable as once thought, in which case this might not be so much a “rule” as a “thing that happens sometimes.”
Other famous trends in size evolution include Cope’s Rule, which states that animal lineages tend to grow larger over time, as is clearly seen in elephants, many dinosaurs, whales, and more. With this “rule,” too, there is debate about just how consistent this trend actually is.
Another “rule” that seems to hold up quite well is Bergmann’s Rule, which posits that animal species tend to grow larger in colder environments. This rule holds up quite well among mammals and birds, where many species reliably vary in size from low latitudes (warmer) to high latitudes (colder). A similar concept might be seen in the case of “deep sea gigantism,” where ocean-dwelling species often grow larger in the cold depths of the sea.
Why some animals are shrinking
Gigantism and Its Implications for the History of Life (technical, open access)
Plants obey (and disobey) the island rule (technical, open access)
Global patterns in plant height (technical, open access)
The island rule: An assessment of biases and research trends (technical, open access)
Bergmann’s Principle and Deep-Water Gigantism in Marine Crustaceans (technical, paywall)
If you enjoyed this topic and want more like it, check out these related episodes:
- Episode 4 – Island Evolution
- Episode 128 – the Deep Sea
- Episode 41 – The Evolution of Whales
- Episode 101 – Sauropods
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