One salamander is a very unusual life form:
View the animal up close, and another oddity becomes apparent: its cells are up to 300 times larger than those of a lizard, bird or mammal. You can see, with a simple magnifying glass, individual blood cells zipping through the capillaries in its transparent gills.
The Neuse River waterdog, Necturus lewisi, and other salamanders represent a long-standing conundrum scientists are only now starting to understand. The animal’s strange traits stem from a hidden burden: Each of its cells is bloated with 38 times more DNA than a human cell. The waterdog has the largest genome of any four-footed beast on Earth. The only comparable animals of any kind are lungfish, which also have sluggardly tendencies.
Douglas Fox, “Junk DNA Deforms Salamander Bodies” at Scientific American (February 1, 2022)
They have, we are told, 10 billion to 120 billion base pairs because, it is thought, parasitic DNA has multiplied out of control. They live a slow, dull life and can live about 100 years. They can regenerate not only limbs but brain parts.
And the challenge for evolution?
As for salamanders, one has to wonder why their burden hasn’t dragged them down to extinction. Their very perseverance suggests that our idea of evolution, particularly “survival of the fittest,” has a serious moralistic bias: Work hard, young species, hone your body and brain for high performance, and someday you will succeed. But salamanders owe their success to lying around. They have found a way to cheat the system…
The salamanders would be on death’s door if they were human. “Everything about having a large genome is costly,” Wake told me in 2020. Yet salamanders have survived for 200 million years. “So there must be some benefit,” he said. The hunt for those benefits has led to some heretical surprises, potentially turning our understanding of evolution on its head…
These bloated beasts have demonstrated, time and again, that when it comes to survival of the fittest, our notion of “fitness” is biased toward strength and agility. Genomic parasites have slowed the waterdog’s development, swelled its cells and distorted its anatomy. This odd circumstance has pushed the animal onto a bizarre evolutionary side track that redefines fitness in such a way that hearts and complex brains are reduced to an afterthought. Yet somehow the animal’s lineage persists, even as fires, floods and asteroids obliterate other species—furry, feathered and scaled—that seem more fit.
Douglas Fox, “Junk DNA Deforms Salamander Bodies” at Scientific American (February 1, 2022)
This isn’t schoolbook natural selection, that’s for sure.