Once lost, is a trait gone forever?
Dollo’s law, lungs and beaks
Draft. The argument is set up but the conclusions are yours, and this is the one where I would most want your voice on the final section.
There is an old idea in evolutionary biology, usually attached to the Belgian palaeontologist Louis Dollo, that evolution does not run backwards. Once a complex structure is lost, the thinking goes, it does not come back.
The reasoning is intuitive. Building something complicated takes a lot of genetic and developmental machinery, and if that machinery stops being used it stops being maintained, and once it has decayed the road back is closed.
The trouble is that the principle keeps meeting exceptions. And tadpoles turn out to be an unusually good place to test it, because they hand you the same experiment twice, in two completely different systems.
System one: lungs
Lungs are ancient, they are complicated, and some tadpoles have lost them outright.
That loss happened recently enough, and often enough independently, that you can still see the circumstances that caused it, which is exactly what you want if you are asking whether the loss is reversible.
FILL IN: what the comparative work says about regain. How strong is the evidence, and how confident are you? This is the paragraph a search committee member will actually remember, so it is worth writing slowly.
System two: beaks
Most tadpoles scrape their food. They have a hardened beak and rows of tiny keratinised teeth, and they use them to rasp algae off rock and leaf. It is fussy, specialised equipment, and it is old.
The narrow-mouthed frogs, family Microhylidae, mostly do not have it. Somewhere in the ancestry of that family the whole apparatus was dropped, and their larvae became filter feeders that hang in the water column and strain particles out of it instead. So far, so ordinary: a structure stopped paying for itself and was lost.
Except that some microhylid tadpoles have keratinised mouthparts. If the loss really happened in the common ancestor, then those lineages did not keep the structure. They rebuilt it.
FILL IN: which lineages, what the phylogenetic evidence looks like, and how carefully you want to hedge it while the manuscript is still in progress.
So which losses are reversible?
This is the actual question, and it is more interesting than a yes or a no.
FILL IN: your argument. The framing I would suggest is that “can it come back” is the wrong question and “what makes some losses easier to reverse than others” is the right one, but say it your way. Whatever goes here is the intellectual centre of your research programme, so it is worth more drafts than anything else on this site.
Phillips, JR, PH Dias, and MC Womack (2025). Lungless tadpoles breathe fresh air into hypotheses for tetrapod lung loss and trait regains. Evolution 79(12), 2776-2790.
Phillips, JR, PH Dias, S Harrington, and D Paluh (in preparation). The loss and regain of larval tooth structures in anuran tadpoles.