10 Strange Times When Species Evolved Backward

Evolution is often thought to be a slow but unstoppable march of every living species toward its supreme form. Think about the iconic image of an ape gradually becoming man. This is popularly known as “The March of Progress,” and at no stage does the man revert to a more ape-like state. For a long time, it was thought this could not happen. Many scientists believed in “Dollo’s law,” which says evolution is irreversible.

In reality, evolution is not always straightforward nor slow. Many species have regained traits after having lost them for generations. Some have even evolved traits for just a few years before evolving back to how they were before. Here are ten fascinating times these strange phenomena have happened.

Related: 10 Animals That Use Biomimicry to Outsmart Predators

10 Dark-Eyed Juncos

Los Angeles is certainly home to more famous residents than the dark-eyed junco. Still, these small sparrows were making headlines of their own in December 2025. That was because scientists had discovered that between 2020 and 2025, these birds had evolved a different beak, then lost it again. The juncos born in 2021 and 2022 had longer, slimmer beaks than those born earlier, but those born after 2023 had short, stocky beaks again.

The researchers were amazed by the change and proposed explanations. One is that COVID lockdowns led to less human food waste lying around, so the birds had to return to their traditional diet of bugs and seeds. Those with longer beaks would have been more effective at feeding, making them more likely to eat, survive, and breed. But when the pandemic ended, the juncos returned to their pre-2020 ways.^[1]

9 Peppered Moths

Dark-eyed juncos are not the first animals to have evolved one way, ostensibly due to human activity, and then gone back again. The classic example cited by biologists is that of England’s peppered moth. This traditionally white insect relies on blending in with tree bark to avoid predators. But between the late 1800s and the 1950s, almost the entire moth population turned black. Then, from the 1970s onward, they turned white again.

The leading theory is that the black variety took over because industrial air pollution was causing trees to darken with soot, making white moths easy to spot. When environmental conditions improved, the lighter-colored moths returned. The appearance of the first black moth was once assumed to be a random mutation, but later research showed that natural selection drove the dramatic shift. Similar changes were observed in many other moth species during the same period.^[2]

8 Three-Spined Sticklebacks

These spiky fish are another classic counterexample to Dollo’s law and have helped scientists learn a lot about how traits develop among species. In fact, so many different forms of them have evolved around the world that naturalists once believed they were observing dozens of separate species. In reality, they were all variations of the same fish adapting to different environments.

Marine sticklebacks are covered in plates and spines to protect against predators. Freshwater sticklebacks, which migrated from the sea thousands of years ago, typically evolve to have less protection. But scientists have shown that they can regain their armor when conditions change. For example, when Lake Washington was cleaned up in the 1960s, fully plated sticklebacks went from 6% of the population to nearly 50% by 2008. Clearer water increased predation risk, and the fish adapted accordingly.^[3]

7 Ichthyosaurs

The examples above show that traits can disappear and return fairly quickly, but that is not always the case. For ichthyosaurs, giant ocean-dwelling reptiles that lived during the Jurassic and Cretaceous periods, it took around 70 million years.

Scientists believe some early ichthyosaurs were hypercarnivores that ate large prey. For tens of millions of years, however, they shifted to feeding on smaller fish and invertebrates. One of the last known species, Kyhytysuka sachicarum, evolved the ability to eat large animals again. Fossil evidence from Colombia revealed a skull with multiple tooth shapes suited for crushing and piercing.

Sadly, returning to a meat-heavy diet did not save these reptiles. Ichthyosaurs ultimately died out more than 100 million years ago.^[4]

6 Chain Ferns

Some fern varieties have been around since long before the dinosaurs, which is to say they have had plenty of time to experiment with evolution. Ferns are among the oldest plant lineages on Earth, so if evolution were a one-way march toward ever more complex forms, they should provide clear evidence of it.

Instead, scientists have found that ferns often evolve back and forth between specialized and less-specialized forms. Many plants reproduce via flowers, seeds, or fruits, separating reproduction from photosynthesis. Ferns reproduce using spores, but one fern family—chain ferns—is able to produce separate leaves for reproduction and photosynthesis.

Despite having evolved this more complex trait, chain ferns sometimes revert to using the same leaf type for both functions, demonstrating that evolutionary paths are not always linear.^[5]

5 Liolaemus Lizards

The ability to evolve back and forth between different reproductive strategies is not exclusive to plants. Animals can do it too, as researchers discovered while studying Liolaemus lizards that roam the Andes Mountains in South America. Like many lizards, this species originally reproduced by laying eggs.

As the Andes rose and the climate cooled, incubating eggs became difficult, and many species evolved to give birth to live young instead. More recently, however, some populations moved to lower, warmer elevations. There, egg-laying once again became viable.

Amazingly, scientists found evidence that some Liolaemus species had reverted to laying eggs. While researchers have theories about why this reversal occurred, how the lizards physiologically regained the ability remains an open question.^[6]

4 Skinks

In Southeast Asia, another group of lizards has also been found to re-evolve a long-lost trait in response to environmental change. Skinks in the region are thought to have lost their legs around 62 million years ago, allowing them to burrow efficiently through loose, dry soil.

When the climate shifted roughly 40 million years ago and monsoon rains made the ground wetter and denser, burrowing without limbs became more difficult. Some skinks subsequently re-evolved legs, making them faster hunters and more capable diggers.

Today, skinks exist along a spectrum, from legless, snake-like forms to species with short or fully developed limbs, each adapted to different ecological niches.^[7]

3 Walking Sticks

Stick insects, also known as walking sticks, are famous for their camouflage, even though several species can fly. There was, however, a roughly 50-million-year period during which they were entirely wingless.

The wings their ancestors possessed disappeared, likely because maintaining them carried costs that outweighed the benefits. Researchers later discovered that walking sticks eventually regained their wings after tens of millions of years.

Genetic studies showed that the instructions for building wings were still present during the wingless period. The trait had simply been switched off, only to be reactivated when environmental conditions favored flight again.^[8]

2 Guenther’s Marsupial Frog

Guenther’s marsupial frog stands out among the world’s roughly 7,000 frog species for several reasons. It is exceptionally rare and may even be extinct, with its last confirmed sighting reported in 1996. It also possesses a full set of upper and lower teeth, unlike most frogs, which typically have only upper teeth.

Lower teeth disappeared from frog ancestors around 230 million years ago. Guenther’s marsupial frog lost them too, but uniquely regained them about 20 million years ago, after roughly 210 million years without them.

One theory suggests the frog regained its teeth because it eats larger prey, including other frogs, making a full set of teeth useful for preventing meals from escaping.^[9]

1 The Aldabra Rail

The Aldabra rail is a flightless bird found on islands in the Indian Ocean that, unlike the famous dodo, managed to come back from extinction. Fossil evidence shows that it lived on the Aldabra Atoll in the Seychelles as far back as 136,000 years ago.

The atoll was later completely submerged, wiping out nearly all life on the islands, including the rail. When the land resurfaced, the rail’s flying ancestor, the white-throated rail, recolonized the area.

Over roughly 20,000 years, the birds once again lost the ability to fly, evolving back into the same flightless Aldabra rail that had existed before. It is one of the clearest known examples of iterative evolution.^[10]