Can Traits Evolve More Than Once In the Same Species?

This is an interesting and complex question.

A fellow named Dollo says no, and in 1893 he proposed a law- Dollo’s Law, in fact- that stated that once lost, genetic traits could not be regained via evolution. We’ll discuss how well Dollo’s Law has held up a bit more further down.

A trait can absolutely appear more than once in a species. In fact, certain traits go in and out of style all the time in some species based on changing environments.

Let’s discuss transient polymorphism. Now, if you don’t know what polymorphism is, it’s when multiple forms exist within a single species. This can be based on having different sets of genes within the same species, like jaguars with melanistic coats.

Both coat types have their disadvantages and advantages, so neither ever completely outcompetes the other (hence, the trait is never fully “fixed.”)

A much more obvious genetic polymorphism is chromosomal sex in mammals. Obviously two (well, more than two!) different forms of the same organisms have to exist concurrently in our own species.

Polyphenism is when the different forms arise from environmental triggers rather than from different genetic material. Examples of this are spadefoot toad tadpoles. If their pond or puddle environment starts drying up, some tadpoles will grow larger and switch from an herbivorous to meat-eating diet in order to cannibalize their siblings and survive.

Ah, the majesty of nature.

But let’s focus more on genetic polymorphism, specifically transient polymorphism. Unlike the example of melanistic jaguars, species with transient polymorphism do not show both traits stabilized in the population at the same time. Instead, they switch from one trait to another the environment changes.

An extremely textbook example is that of the peppered moths. Everybody who’s taken basic high school-level biology is groaning now, but here we go.

The peppered moth hides from avian predators by blending in with spotty tree bark. During the industrial revolution of the 1800s, these trees were blackened by soot. This caused a melanistic variant of the peppered moth to become more successful than the original, because it blended in better with the tree trunks.

So the melanistic variant became MUCH more common in the population, gradually overtaking the peppered variety. However, today the peppered variety is actually making a strong comeback due to reduced air pollution and cleaner trees.

Ok, so there it is: traits can easily go in and out of style in a species (and then in again). However, depending on who you ask, this is not considered strictlyevolving the same traits more than once because the traits are never actually eliminated from the population.

And that is a trickier question, because then you have to decide the meaning of ‘elimination.’ Rarely does evolution clean up its stray ends. If you look at humans, we still have webbed fingers and tails as embryos before development hastily clears them away. Some individuals are in fact born with elongated spines and webbed fingers.

In the genes of a chicken there still remain lingering bits of genes that can be induced to produce teeth. However, the embryos with this teeth-forming mutation are nonviable and can’t survive long enough to hatch. This is an example of a trait that would be nearly impossible to bring back, then, because there is simply no way the chickens could survive long enough to pass on the working teeth-genes.

So within a species there are many traits that, once lost, are exceptionally hard to regain. The reverse is true as well, with traits gained being hard to lose. Long tails in widowbirds is actually a good example of this, as with most sexually selected traits- female preference for longer tails is so coded that it may really not matter if the shorter-tailed males have higher survival rates. The females still won’t want to mate with them!

This could really lead to the extinction of the species, which seems counter-intuitive, but we have to remember that evolution actually works in favor of the individual, not the species*. The longer-tailed widowbirds aren’t going to stop being successful just because it’s a bad long-term strategy. Those genes work immediately, so they spread faster.

(*This is distinct from the fact that evolution works on populations and not individuals. Confusing, I know.)

So can a species (or rather, a continuing genetic line, since species are rather brief and ill-defined) only evolve a specific trait once? Was Dollo correct?

It looks like the answer might be a firm no.

A study in 2011 pointed out the reappearance of teeth in the lower jaw of frogs in the genus Gastrotheca. (Most frogs forgo lower jaw teeth because it makes it easier to shoot out their tongue.) Any way you swung it, the phylogeny of these frogs couldn’t be rearranged to not have toothless ancestors. Teeth disappeared… and then reappeared.

Another example of a middle finger to Dollo’s law is the re-appearance of wings in phasmid insects. Again, evidence shows that all of the direct ancestors to this clade were wingless, and yet wings reappeared- possibly on multiple different occasions.

And there are quite a few examples of this ‘re-evolution’, with more popping up all the time.

In the end the reappearance or re-evolution of a trait depends largely on how well that trait’s developmental pathway was conserved. We have a lot of little gems tucked away in our genetic material that we no longer express, as again evolution tends to scratch things out rather than erase them. With the right mutation in the right environment, it might be possible for many traits to unearth themselves again and again.

A final note: regardless of everything I just spoke about above, it is also possible for the same phenotype (visible trait) to evolve more than once in a lineage using completely different genes and underlying mechanisms. Convergent evolution isn’t just limited to distantly related species.

References and Further Reading

Andersson, M. (1982). Female choice selects for extreme tail length in a widowbird. Nature299(5886), 818-820.

Collin, R., & Miglietta, M. P. (2008). Reversing opinions on Dollo’s Law. Trends in ecology & evolution23(11), 602-609.

Cook, L. M. (2003). The rise and fall of the carbonaria form of the peppered moth. The Quarterly review of biology78(4), 399-417.

Harris, M. P., Hasso, S. M., Ferguson, M. W., & Fallon, J. F. (2006). The development of archosaurian first-generation teeth in a chicken mutant. Current Biology16(4), 371-377.

Storz, B. L. (2004). Reassessment of the environmental mechanisms controlling developmental polyphenism in spadefoot toad tadpoles. Oecologia,141(3), 402-410.

Whiting, M. F., Bradler, S., & Maxwell, T. (2003). Loss and recovery of wings in stick insects. Nature421(6920), 264-267.



About Koryos

Writer, ethology enthusiast, axolotl herder. Might possibly just be a Lasiurus cinereus that types with its thumbs.
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