[imakesnowflakes]

>Evolution doesn't have an intent. Diseases can't "evolve themselves."

I know. But when you remove mosquitoes from the picture, aren't you putting selection pressure on the diseases to be airborne?

>Evolution happens through random mutations* and selection of the fittest...

Yes. Say a virus of a disease x that normally spreads via mosquitos, gain a mutation to be airborne. But since there are an abundance of mosquitoes, an airborne strain does not have an advantage over mosquitoes borne strain. So it dies off (because of competition)

But when you eradicate the mosquitoes, or reduce their number significantly, suddenly the airborne strain has a tremendous advantage over the mosquito borne strain. Hence it can grow in numbers and eventually completely replace the mosquito borne strain...

Isn't this even remotely possible?

[saalweachter]

> I know. But when you remove mosquitoes from the picture, aren't you putting selection pressure on the diseases to be airborne?

Organisms also need the opportunity to evolve; removing their only vector is analogous to trying to apply selective pressure to pigs to evolve to fly by throwing them off a cliff. Yes, any pig who could fly would survive and have a huge advantage over all the now-dead non-flying pigs. But it just ain't gonna happen.

[imakesnowflakes]

That would be true if you are going to eradicate mosquitoes overnight...

Also, I am just putting forward one possibility other than the effect on the food chain...

[slapshot]

> aren't you putting selection pressure on the diseases to be airborne?

> an airborne strain does not have an advantage over mosquitoes borne strain

There are hundreds of millions of people today who don't currently have malaria, but would be at risk to get malaria if it went airborne. Airborne malaria wouldn't be competing with mosquitoborne malaria. It'd be competing with running out of people to kill.

There seems to be a startup lesson here: your competition isn't legacy players. It's non-consumption.

[imakesnowflakes]

Sorry. I have no idea what you are talking about.

Are you a biologist/ecologist or someone knowledgeable in the field?

[jessaustin]

Instead of asking for credentials, just think a little harder. If "airborne malaria" could possibly be a thing, it would already be a thing. Plasmodium isn't a work crew responsible for killing a given number of people every year, after which they relax for a bit, only getting really creative when they fall behind their quota. It is a reproducing species that will reproduce and thrive as much as possible. There is no intelligence guiding mutation. If flying were an option, Plasmodium would be flying.

Your question is like asking, "if we rounded up all the lions in Africa and put them in pens, would they evolve wings to escape?" Yes it's that silly.

[imakesnowflakes]

Sorry. I have to ask this to you also. Are you a biologists/ecologists or some one knowledgable in the field?

[icambron]

I can tell you both the parent and grandparent posters are knowledgeable enough to help answer your question. I think you're misunderstanding how evolution and selection pressure work at a conceptual level, and you don't need an evolutionary biologist to give expert input on it any more than you need an astronomer to explain why the moon has phases.

Let me take one more crack at this: think of natural selection as evaluating `if` statements: "if this organism has the ability to spread through the air, then it is more likely to survive an reproduce". That means mutations enabling that are going to propagate. Doesn't make any mutation more or less likely; it's just a question of whether it survives and reproduces or not. Importantly, it does not say "if this is a big improvement over the status quo, keep it". The forces at play here don't know what "improvement" means and they don't know what the status quo is (though see below about competition).

The point the parent and GP were making is that the advantage in being airborne exists whether or not malaria is being killed off. Right now, before any mosquito-killing-off initiatives, a plasmodium would do very well for itself and its offspring by escaping the confines of a mosquito and infecting zillions of people through the air. Its chances of reproduction in that scenario are presumably high, because there are so many people to infect. It doesn't become more likely to make that mutation and survive the results if its vector is being killed off. It doesn't know it's being killed off.

So how does selection pressure fit in? Imagine a beetle. If you change something about its environment, say, by introducing a new predator, then traits which previously provided the beetle no advantage (say, tasting bad to that predator) suddenly provide that advantage. Then the `if` statements are decidedly different now! It's not that the tasting-bad mutation is more likely to happen, it's just more likely to impact survival. So you expect more of that mutation to survive, and soon you get a whole ton of beetles that taste terrible to our new predator. But note how this doesn't help malaria go airborne because there's no "suddenly provide an advantage" part. It was always an advantage. That it might now impact whether the species survives or isn't part of the `if` statement.

One possible way in which this can be confusing is what the GP was specifically addressing: often the value of a mutation is a function of how it affects the organism's ability to compete with the rest of the species. If there's only so much food around, then being slightly better or worse at eating it affects an organism's survival because it needs to be better at eating than its brethren or it will starve. So in that case the current state of affairs gets baked into the `if` statement. The thing to note is that this logic doesn't apply to malaria going airborne; there isn't a competition over humans to infect.

[imakesnowflakes]

Thanks for the explanation.

But you haven't said anything that I don't know already.

>The point the parent and GP were making is that the advantage in being airborne exists whether or not mosquitoes is being killed off.

When there is an abundance of mosquitoes, an airborne strain does not have a sufficient advantage over the mosquito borne strain.

Now this is an assumption I am making. And this is where you people are hung on.

You are saying that an air borne strain has an advantage even now. But what if an air borne strain is limited by distances it can travel before it dies of for want of a host? A mosquito borne strain can travel arbitrary distance and spread over a vast area..

So when there are mosquitoes, it is more or less an even match.

When you take mosquitoes out, suddenly the airborne strain gains a huge advantage over the mosquito borne strain. Right? Because there are less number of mosquitoes, the spread of mosquito borne strain is reduced. This gives the air borne strain more chance to propagate to the next generation.

If you are thinking, how does it give more probability for the air borne strain to spread? Please consider this scenario.

Let there be two indviduals A and B, A infected with an Airborne strain and B infected with a mosquito borne strain. Let there be a healthy indvidual C that is, say, 10 meters away from A and B.

Case 1: Current situtation, with an abundance of mosquitoes.

A mosquito bites B, taking in the virus and takes off. At the same time, a virus of airborne strain start from A. Now, the mosquito proceeds to bite C right away. Now the virus enters C's body. After a while, C's immune system starts a response to fight this off. A day passes. C's immune system is still fighting the infection.

It is at this time at which the air borne strain enters C's body. But there is a fight going on in there with C's immune system in full alert and is killing of off the likes of this viruses. The small amount of air borne virus that manage to enter C's body gets slaughtered before it gets a chance to grow there. C might or might not get infected with mosquito borne strain...

But The airborne strain does not propagate to the next generation.

Case 2: Mosquitoes are being killed off.

Now there is a reduced number of mosquitoes. So when the airborne strain enters C's body, the mosquito borne strain is still not there yet. Immune system has not yet started fighting viruses of this type. So it gets a head start, and ends up successfully infecting C.

The airborne strain propagate to the next generation.

[jonnathanson]

(I'm a layperson, too, for the record.)

"I know. But when you remove mosquitoes from the picture, aren't you putting selection pressure on the diseases to be airborne?"

Not necessarily that specific pressure. Except in carefully controlled laboratory situations, we can't specify the selection pressure being applied. There are too many potential pressures at work, and the mutation outcomes are too stochastic. At best we can force pressure in general. The outcome of that pressure might be entirely different from what we expect it to be.

Let's say we eradicate mosquitos. What other vectors of transmission does a virus like Zika have? What other hosts? It's possible the virus finds a new insect-borne transmission pathway: say, ticks instead of mosquitos. It's possible the virus 'focuses' (to use the term very very loosely) on other hosts, and effectively ceases to be a human concern. I'd wager that either of these outcomes is the more likely adaptation case than a leap to airborne transmission.

Evolving an entirely new means of infectious transmission seems to be a much rarer adaptation than adapting through other means (increased infectious potential; severity of infection; adaptation to new host types; etc.). It's popular in TV and movies to speak about a virus "going airborne," but in actual record, that's usually not what happens. Evolution doesn't have any agency or self-direction; it usually arrives at the 'laziest' and least costly alternative in response to imposed pressures. In this scenario, evolving airborne survivability and transmissibility is probably more costly than adapting to whatever enzyme prevents fleas and ticks from being carriers.

Source: http://evolution.berkeley.edu/evolibrary/news/141003_ebola

[inimino]

There seems to be a mixup here between "pressure" as in "this organism is under pressure" (implying an unfavorable environment) and selection pressure, which acts on genes, not species, and is really more of a filter.

It's not as if an organism can "release" the "pressure" by evolving in a new direction. In your example, if we eradicate mosquitos, one transmission vector becoming less viable doesn't make other vectors more likely to arise, as if by some conservation of total population.

> least costly alternative in response to imposed pressures

Evolution is even lazier, alternatives don't arise in response to imposed pressures at all, so in this scenario the lazy thing is extinction.

[imakesnowflakes]

So it seems that it is unlikely, but not entirely impossible.

Also, I am just putting forward a dangerous possibility other than the effect on the food chain..