Almost 200 years ago, in 1831, Charles Darwin embarked on his now famous journey aboard the S.S. Beagle. This was the journey on which he visited the Galapágos Islands and formalized his theory of evolution via natural selection. (The idea of natural selection was actually hypothesized earlier by Jean-Baptiste Lamarck.)
Everything about the fauna on the islands, Darwin noticed, was simultaneously familiar and exotic. Animals typically diminutive in mainland environments were much larger on islands, and animals typically massive in mainland environments, much smaller. This made Darwin wonder if there were something about islands that gave way to the unique physical traits of the species he was observing, like the Galapágos’ famed giant tortoises.
Evolutionary biologists have since theorized that islands’ status as inversions of mainland ecosystems can be explained by their comparative lack of resources. Megafauna like elephants can’t grow as large on islands because they don’t have access to the same nutrient sources offered by mainland ecosystems. Similarly, small mainland species — rodents, for instance, or tortoises — grow much larger on islands because their growth isn’t constrained by mainland ecosystems’ larger predators.
Further research has examined how evolution itself differs between island and mainland ecosystems. In a 2006 study titled “Morphological Evolution Is Accelerated Among Island Mammals,” biologist Virginie Millien analyzed the differences in rates of evolution between island and non-island species. She analyzed a total of 88 species, and noted that while rates of evolution among all of the species (island and mainland) slowed over time, the rate at which it slowed for island species was slower than the rate at which it slowed for mainland species. Put another way, island species evolve faster than mainland species.