Ensatina

A ring species occurs when a single ancestral population spreads out gradually around a geographic barrier, such as an unsuitable habitat or mountain range, forming a continuous chain of populations that each interbreed successfully with their immediate neighbours along the way. Over enough distance and time, populations at the far ends of this ring can become different enough, through the accumulation of small genetic and physical changes along the chain, that when the ring finally closes and those two end populations meet again in one place, they no longer interbreed successfully, effectively behaving like separate species despite being connected the whole way around by populations that can interbreed. The ensatina, distributed in a ring around California's Central Valley, is one of the most thoroughly studied real-world examples of this phenomenon, illustrating how new species can gradually arise through accumulated small changes rather than a single sharp split.

Like all members of the family Plethodontidae, the largest salamander family and one lacking lungs entirely, the ensatina absorbs the oxygen it needs directly through its skin and the moist lining of its mouth and throat, a process called cutaneous respiration that depends heavily on the skin remaining consistently damp to allow gases to diffuse efficiently across it. This is why lungless salamanders are almost always found in cool, moist microhabitats such as leaf litter, under logs, or in damp soil, and why they are particularly vulnerable to drying out if exposed to open, dry air for any significant length of time.

Certain populations, particularly in parts of California, display bold orange blotches on a darker background that closely resemble the warning colouration of the Pacific newt, a genuinely toxic salamander relative found in overlapping areas. This resemblance is considered a strong candidate example of Batesian mimicry, a strategy in which a harmless or only mildly defended species evolves to resemble a more dangerous or distasteful one, gaining some protective benefit from predators that have learned to avoid the toxic model without the mimic needing to produce the same potent toxins itself.

Beyond the toxin-mimicking colouration shown by some populations, it has a genuinely functional physical defence in its tail, which can detach relatively easily when seized by a predator, similar to the strategy used by many lizards. Once detached, the tail continues to twitch and wiggle vigorously on its own for a period of time, drawing and holding a predator's attention on the still-moving tail piece while the rest of the salamander, now missing its tail, makes its escape. The tail can regrow over subsequent weeks and months, though it typically does not perfectly match the original in appearance.