Which animals are evolving fastest?

Evolution occurs over millennia, but it can also happen in just a few generations. For example, Darwin's finches in the Galapagos rapidly evolved specialized beaks as their food options changed, green anoles (Anolis carolinensis) evolved larger toe pads that enabled them to climb higher to escape predators, and peppered moths (Biston betularia) became darker colored as the Industrial Revolution polluted cities with black soot.

So which vertebrate animals are evolving the fastest? It's a close call.

"I don't know if any particular organisms are always evolving fast," Michael Benton, a vertebrate paleontologist at the University of Bristol in the U.K., told Live Science.

There are two main perspectives, he said. "One would be that actually there is something innate, something fundamental in certain organisms that means that they are fast-evolving and that others are slow-evolving," he said. The other is that every organism is capable of rapidly evolving, but that it's contingent upon environmental change — the catalyst of evolution.

The medal for "fastest evolver" is therefore highly controversial. Some scientists have awarded that title to tuataras (Sphenodon punctatus), lizard-like Animals found only in modern-day New Zealand. These reptiles are the only remaining survivors of the order Rhynchocephalia, which was more diverse during the Mesozoic era (251.9 million to 66 million years ago) than Squamata, the order of modern-day lizards and snakes, is today, Benton explained.

Related: Could evolution ever bring back the dinosaurs?

A 2008 study in the journal Trends in Genetics analyzed ancient and modern tuatara DNA and found that these so-called living fossils actually have the fastest-recorded rate of molecular evolution in a vertebrate animal. This means that although the species' phenotype, or physical appearance, has changed little over millions of years, its DNA has been rapidly evolving. Similarly, scientists found that Adélie penguins (Pygoscelis adeliae) have genetically evolved two to seven times faster than earlier estimates, even though their physical appearance has changed little.