In 1972, a pet shop owner on the island of Oahu, in Hawaii, ordered a few dozen Jackson's chameleons from Kenya to sell. When the lizards arrived, rattled by their long journey, the shop owner left them in his backyard to recover - allowing them to escape, and thus inadvertently causing the colonisation of Oahu by invasive chameleons.
Jackson's chameleons (Trioceros jacksonii xantholophus) have flourished in Hawaii, where there are fewer predators for them to worry about. And this singular point of introduction - while bad for the island's ecology - has allowed researchers to watch natural selection occurring with a level of detail they don't normally get.
One unusual feature they've noticed is the chameleons' colours changing. Male Jackson's chameleons are normally a bright green, but they change colour to become lemon yellow when trying to attract females, or ward off other male rivals.
According to a paper in Science Advances, this shade of yellow has become brighter in Hawaii.
"Jackson's chameleons are native to East Africa, where they are preyed upon by a wide range of mostly bird and snake predators," says lead author Professor Martin Whiting, a researcher at Macquarie University's school of natural sciences.
"In Hawaii, however, there are virtually no bird or snake predators to worry about."
Without this selection pressure, the chameleons have no detriment to getting brighter.
But becoming brighter does give them a couple of advantages in sexual selection (finding, and reproducing, with a mate).
"If you're a brighter, higher quality individual, you might be doing better in contests and you might also be more attractive to females," says Whiting.
Bright colours are often a boon for sexual selection in the animal world, especially if you're an animal that can change colour.
"We've seen that in guppies, for example, and there are various birds and fishes where being brightly coloured has a reproductive advantage," says Whiting.
The researchers figured this brightness out by designing experiments in both Kenya and Hawaii with wild Jackson's chameleons.
Individual chameleons were placed in a perch and exposed to another male chameleon, a female chameleon, a model snake, and a model bird (the latter two both predators).
The researchers then used a spectrophotometer - an instrument that measures the intensity of light - to record the chameleons' colour changes as they reacted to their stimuli.
While the chameleons all turned the same hue of yellow, the Hawaiian chameleons were much brighter in each case - that is, their colour was much more intense.
"It's the brightness that's really been selected for," says Whiting.
The Hawaiian chameleons were also less good at hiding or blending into their backgrounds when threatened with fake predators.
They noted that the chameleons' colour made them stand out even more in Hawaiian vegetation, compared to Kenyan vegetation.
Whiting says that, given this change only took 50-60 generations, it's likely an example of rapid evolution.
"There are relatively few examples of animals in the wild significantly changing their colour signals in response to natural selection being relaxed, and this is the first case of an animal with dynamic colour change," he adds.
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