Well, they’re not giant, but, by way of Majikthise, I found this story about an isolated population of freshwater crabs that has been dwelling in Roman ruins for 3,000 years.
During this period of isolation, the crabs have increased in size by at least fifty percent and possibly more than 100% (based on molts) compared to the wild Greek popoulations. What’s interesting is that the population is thought to be around 1,000 individuals, so there are some neat studies that can be done looking at the effects of genetic bottlenecks (
we need a crab genome!). One thing puzzled me though:
While in nature the crab grows to a length of five centimetres (two inches), it is more robust in the ruins, growing to more than eight centimetres. “Once we found a moult (shed exoskeleton) measuring 12 centimetres!” Scalici said during a tour of the site.
“Gigantism is one animal response to isolation, and it is a phenomenon that requires a long time,” he noted.
I always thought that dwarfism was the usual response to isolated populations in small geographic areas. Seems there’s a lot of interesting work to be done in this system.
Insular dwarfism and gigantism both happen–think giant tortoises, Komodo monsters & dodo birds (giant flightless pigeons).
From what I’ve read it’s that bigger animals such as elephants, horses and people get smaller, while small animals get bigger (especially rats).
Obviously, it depends on the environment. I expect the major factors are the availability of food and the prescence/lack of preditors. In the article it says:
It also sounds like there is no food shortage.
Well, the crabs didn’t evolve into puppies, so obviously this just proves that evolution is false!