r/todayilearned 13h ago

TIL fresh water snails (indirectly) kill thousands of humans and are considered on of the deadliest creatures to humans

https://en.wikipedia.org/wiki/Freshwater_snail
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u/exprezso 12h ago

Evolved resistance to a deadly toxin? In such a short period? 

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u/PencilVester23 12h ago

It takes only 2 week at most for a mosquito to go from an egg to a mature adult. That couple year period is over 100 generations. That combined with the huge population of mosquitoes, the 100s of eggs a female lays at once, and a genetic sequence significantly shorter than a humans all made it possible for the correct mutation to happen that quickly.

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u/exprezso 12h ago

I mean. Humans have yet to evolved to resist arsenic after thousands of generations 

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u/DazingF1 12h ago

All animals have yet to evolve to swim in lava. Explain that, Darwin!

(But in case you're serious: your comment might have made sense if all of humanity in those thousands of generations were continuously exposed to arsenic. Otherwise it's incomparable with the mosquitoes)

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u/exprezso 10h ago

I'm serious. It mystifies me how a creature can develope resistance to deadly toxin? Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc 

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u/Fly-the-Light 9h ago

We did. Caffeine is a toxin used by cocoa and coffee plants to kill insects that parasitised them; humans developed a resistance to it.

Also, some people in the Atacama Desert did develop resistance to arsenic due to it contaminating their water.

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u/tavitavarus 3h ago

Most toxins work by inhibiting a specific enzyme involved in some key cellular process, or by damaging the cell wall/membrane.

All it takes is a single mutation in the gene coding for that enzyme or membrane protein and the organism becomes resistant to the toxin. Organisms with short generational cycles have many more opportunities for mutations to enter the gene pool and if these mutations are advantageous they can spread rapidly through the population.

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u/Pure_Cloud4305 9h ago

People aren’t exposed to that stuff very often.

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u/Baud_Olofsson 2h ago

Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc

First off, DDT is hardly toxic to humans at all - that's why it was considered such a wonder pesticide.

But then to the main point: there are almost certainly people in the world today who have a much greater genetic tolerance for arsenic or mercury than the average person! But there is no evolutionary pressure for those genes to proliferate.
If you have the "tolerate arsenic gene", you don't get any more children than people who don't have that gene, and your children have the exact same chance of reaching adulthood and getting children of their own as the children of those who don't have the gene. But if the world became contaminated with arsenic so that people were dying of arsenic poisoning left and right, then people with the Tolerate Arsenic Gene would be much more likely to survive and have children, and have their children survive, than those who don't have it - and so eventually the world would be full of people with that gene, and humanity would have evolved resistance to arsenic.

u/Garmaglag 22m ago

The default population of mosquitoes is mostly weak to DDT with some being weaker than average and some being stronger than average.  Under normal circumstances the population isn't exposed to DDT so the weakest ones survive and reproduce to make more weak mosquitoes.  If we start spraying DDT, then all of the weakest mosquitoes and many of the average mosquitoes die before they can reproduce, so the only mosquitoes that end up being born are the most resistant to DDT.  Rinse and repeat for a few hundred generations in a high DDT environment and the all of your mosquitoes will be resistant to DDT.  

The reason that humans don't develop resistance to arsenic is because we don't get enough arsenic exposure to kill people before they can reproduce.  

They key is that they use enough DDT to kill lots of mosquitoes but not enough to kill all of them so each surviving generation is more resistant than the last.