Now for one that just blew me away!
From Science Daily:
A study published online July 23 in the Proceedings of the National Academy of Sciences finds that in the case of insects that developed resistance to a powerful plant toxin, the same adaptations have occurred independently, in separate species in different places and times.
The paper examines 18 insect species across four orders — beetles, butterflies and moths, flies, and true bugs — that all feed on plants containing powerful toxins called cardenolides.
Common to milkweeds and foxglove, cardenolides are lethal to nearly all insects and function effectively as a defense against pests. Cardenolides work by binding to a cell’s sodium pump, one of the most fundamental systems found in all animal cells. The sodium pump works when an essential enzyme (Na,K-ATPase) carries important elements, sodium and potassium, across the cell membrane. Cardenolides bind to the enzyme and disable it, thereby shutting down cells, which results in severe damage.
Among the 18 insects surveyed, the researchers found a few methods that the insects use to resist cardenolides. In monarch butterflies and a species of leaf beetle, for example, resistance is due to a specific mutation — called N122H — of the Na,K-ATPase gene. The mutation reduces cardenolide binding to the sodium pump enzyme.
“Already knowing how monarchs deal with the toxin, we wanted to see if it was the same molecular solution used by beetles, flies and true bugs that are also resistant to cardenolides,” said Anurag Agrawal, a Cornell professor of ecology and evolutionary biology and a co-author on the paper. Susanne Dobler, a professor of molecular evolution at Hamburg University, is the paper’s lead author.
By examining molecular changes in the sodium pump gene, the researchers found the mutation N122H in all four orders of insects studied. Furthermore, they discovered a second mutation in the same gene that also conferred resistance in 11 of the 18 species.
“This is truly a remarkable level of evolutionary repeatability and suggests that evolving resistance to the plant toxin had very few effective options,” said Agrawal.
The researchers tested the effectiveness of these gene changes by inserting the single Na,K-ATPase mutations into cell cultures and then dosing those cultures with cardenolides. They found the mutations gave the cells resistance, and when cells were given the two mutations that repeatedly evolved together, they had twice the resistance as cells with a single mutation, implying a synergistic effect.
The standard gene for the sodium pump is essentially the same in all insects, and even mammals carry the gene in a relatively unmodified form. The sodium pump thus originated from a common ancestor hundreds of millions of years ago and is central to the functioning of most animals. Out of that background, insects from different orders over the last 300 million years specialized on plants with cardenolides and evolved resistance independently, and in numerous cases, through exactly the same gene change.
Convergent evolution isn’t anything new to science.
However, convergent evolution using the same mutation is!
This is pretty damn amazing. Here, you have animals that are very different in terms of their phylogeny and taxonomy, but they have exactly the same mutation that allows them to deal with these particular toxins.
I don’t know if such a thing has been discovered before, but it is pretty astounding.
Mutations are random. All of us have some mutations. Most are neutral. Some are advantageous. Some are deleterious.
The chances of organisms evolving a similar adaptation using the same mutation would appear to be quite low.
But here we have an example of that very unlikely phenomenon happening.
Now, I’m sure that the creationists will have a lot of fun with this one, but the truth is I don’t know of a single other case in which very organisms from very divergent ancestries evolve convergent adaptations through the same genetic mechanism.
It’s a weird case.
The authors think that the reason why this mutation has been implicated in all these different species is that evolving this particular adaptation just has such limited options.
It’s not likely applicable to other situations.
For example, the chances that all the hairless dog breeds in the world that have a dominant hairless allele that stems from the same mutation all developed that mutation independently are unbelievably low. Unlike these insects, they all derive from a common ancestor that lived in Mexico 4,000 years ago.
It’s a giant leap to use this particularly unusual discovery in insects and apply it to other organisms.
And I hope people can refrain from doing so.
But they won’t.
Another key issue here is that we’re talking about insects which even in temperate climates can have many generations in one year. Thus they have had much more subjective time to come up with this very unusual example of convergent evolution.
All mutations are not equal. Some are much more probable/likely/ “easy” than others. Begin with the “hard” end of the spectrum. The biosphere rests on one enzyme, rubisco, which basically transfers the energy captured through chlorophyll and its variants into a carbohydrate. Rubisco is a ridiculously slow enzyme, and operates at ~1/1000th the speed of most enzymes. A major fraction of the plant protein in the biosphere is tied up in this one enzyme. It is surprising that over ~2.5 billion since photosynthesis appeared on the planet, evolution hasn’t been able to come up with a better enzyme to do the job (genetic engineers are trying, but I don’t think they’ve had much success). Evolution has fine-tuned the enzyme, eg., by modifications that make it efficient in an oxygenated, as opposed to anoxic, environment. But none of these evolutionary paths has been able to circumvent the fundamental bottleneck for energy synthesis in the biosphere.
I’d guess that the mutation which reduces cardenolide binding to the sodium pump enzyme is a ‘probable’ mutation, requiring only a minor tweak to evolve.
I grow grapes, which you really can’t do without spraying to control various fungi. The damn fungi rapidly mutate to adapt to develop resistance to new fungicides. New releases confront resistance in less than a decade. Vintners still use chemicals to which fungi have evolved resistance . . . they just make sure to keep shifting between different groups of fungicide that attack different fugal processes. Most of the newer, chemically complex, fungicides should not be used more than twice in a single growing season because the fungi develop resistance. As I understand it, what we fight is both drifting in of fungal strains that are resistant, and in situ evolution of resistance. And then there’s the huge trouble the medico’s are having with rapid evolution of drug tolerance in the AIDS virus.
There seem to be differences in speeds of evolutions way up on the taxonomic ladder. Fungi, in general, are clever little chemical acrobats, and are prone to competing with chemical warfare (penecillin comes out of a chemical used by fungi to get rid of bacterial competitors). Insects are remarkable in their ability to circumvent plants chemical defences. The mechanisms of chemical ‘dodging’ seem to vary somewhat between taxonomic groups . . . but save that for some other time.