Weird things happen when organisms get cut off into small areas. Look at islands. Homo floresiensis, the Flores Island “hobbit,” was a very small human that probably evolved on an island. On the other hand, you’ve also got wetas, which are the biggest crickets you’ll ever see, which also evolved on an island.
It seems that big things get small and small things get big on islands. I guess the perfect size for any animal on an island is a rabbit.
Ponds and small lakes are also islands. They’re wet instead of dry, but from the point of view of evolution, you have a lot of the same factors that come into play: small populations, sometimes few competitors or predators, and new niches to exploit.
This paper looked at evolutionary body size changes in nine-spined stickleback. These hardy fish are sometimes the only fish in small ponds and lakes in their range in northern Europe, but in larger lakes, rivers, and the ocean, they are one among many fishes – often bigger fishes, with large appetites.
Although the title refers to “giantism,” giant is a relative thing. The largest fish they found had a length of about 7 cm; “palm of the hand” kind of size. But when you consider that many other populations had lengths of about 4 cm, you have to admit that almost twice the size probably counts as giant for that species.
Those largest fishes were found in the isolated ponds, as suspected. The pond fish sorted out into two groups. The smallest sticklebacks were in ponds that also contained three-spined sticklebacks or trout.
When they reared eggs from fish collected from these various ponds in the lab, they found the “giants” were on a completely different growth trajectory almost from the get go. So it did not seem to be the case that the large fish were large just because they were living longer because there were few predators around. Turned out the big ones also lived longer, giving those populations a double whammy to reach their large size. These two points suggest that these size differences are genetically controlled, and thus heritable, and are not due to increased food availability, say, or other environmental effects.
The authors rightly refer to the all of their lakes and rivers as “natural experiments.” Still, a natural experiment is still not as clean as a lab experiment, and perhaps there could be some manipulations to test some of the hypotheses coming from their data. And although this paper supports the idea that islands are hotbeds for evolution of body size, it doesn’t seem to help move forward in predicting if a species in isolation is going to get bigger or smaller.
Herczeg, G., Gonda, A., & Merilä, J. (2009). EVOLUTION OF GIGANTISM IN NINE-SPINED STICKLEBACKS Evolution DOI: 10.1111/j.1558-5646.2009.00781.x