Evolving Giants and Miniatures
Animals of gigantic size fascinate us. So do animals that are incredibly tiny. But how do they get that way? Why do some animals evolve into giants, while others downsize?
Scientists have several hypotheses, all with evidence but also exceptions
Island evolution. Suppose a population gets trapped on an island, with finite real estate and other resources. Those who miniaturize (also called dwarfing) need less “stuff” to grow more offspring, and to reach reproductive maturity sooner; thus, natural selection. Examples include Shetland ponies, mini mammoths, even dwarf dinosaurs. And there’s the famous maybe-human “hobbit” of Flores, Indonesia. Other resource-limited environments–such as the interior of a pitcher plant–can select for the tiniest frog.
But…islands also evolve the giant rat, and the Galapagos tortoise. Hm, back to the drawing board.
Temperature. In colder climates, according to Bergmann’s rule, larger animals do better, because they have larger volume-to-surface ratio. Since heat loss is proportional to surface area, larger animals lose less heat per calorie than smaller ones. That explains why the ice age saw large mammals such as the woolly mammoth and rhino. Once the ice thawed, smaller forages outgrazed and outreproduced them. Similarly, polar indigenous people tend to be heavier than those of the tropics.
But some birds (chickadees) are smaller in colder climates. For that matter, how do teensy robins and cardinals ever make it through our Ohio winter, while I huddle in three sweaters–never mind.
Predation. To avoid predation, it helps to get large. This is especially true of marine fish. The open ocean, as we all know from Jaws, has no place to hide. Fish and other marine denizens, such as squid and whales, evolve to tremendous sizes.
But–the alternative is to be small and make a gazillion offspring, some of whom always evade capture. The ocean also contains the largest populations of medium small fish such as menhaden, critical to support the larger ones; along with teensy plankton. And the teensiest microbes of all, such as Prochlorococcus. Barely big enough to contain working ribosomes, yet one of nature’s most important producers of oxygen.
A British research team concludes, “The course of size evolution is dependent on a complex interplay of many other factors.”
What do you think?
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Ultraphyte 
Does Bergmann’s Rule apply to all phyla, or just warm blooded ones? Tropical climes seem to have the largest representatives of arthropods. If you are cold blooded, small size would seem the best way to warm up when the sun rises in the morning.
The examples you gave show that while there may be general tendencies, there is plenty of variation to create counter examples.
Bergmann’s rule applies only to warm-blooded animals; although, there is some ambiguity as to what warm-blooded means, with respect to large animals. For example, a large animal might thermoregulate through heat released by gut microbial fermentation.
For a given animal, there may be many factors that outweigh thermoregulation. For instance, resource scarcity might have greater selective pressure than temperature. That might explain why chickadee size works the other way.
I think Bernd Heinrich wrote something about how golden-crowned kinglets get through the winter in Winter World. That and his Thermal Warriors (about insect thermoregulation) are great reading if you want to geek out on temperature regulation (they’re written for laymen, fortunately).
As for size, the general rule in the literature for island gigantism and island dwarfing is that if it’s bigger than a rabbit, it dwarfs, while if it’s smaller than a rabbit, it gets bigger. Obviously, someone forgot to send copies of Island Biology to the Caribbean, where all these tiny herps live. That’s ultimately the problem: animals can’t read, especially when they’re evolving, and those tiny frogs, geckos, and blind snakes don’t know that they were supposed to be getting bigger.
As for human size, I live in a state that had both the tallest (Mojave) and shortest (Yuki) Indian tribes in North America (heights by average). Note that this doesn’t follow Bergmann’s rule, since the Yuki lived in the northern Sierra and the Mojave lived in the Sonoran Desert (yes, the Mojave lived mostly in the Sonoran desert). Thing is, California had probably the highest density of separate tribes in the US, so one has to question whether there was any biology involved (unlikely) or whether it was a statistical artifact of having a big chunk of the sample population (more likely, in my opinion).
Those tiny geckos and snakes must be evolving with a very limited resource of some sort. That would be my hypothesis.
About the people, that reminds me that in Africa the tallest (Tutsi) and shortest people (pygmy) live in the central region, quite close together. That partly reflects how long people have evolved in Africa; a long time for generating diversity. To a lesser extent, California may represent the same, a place where a lot of native Americans diverged for a long time.