Source
r/askscience•
RobotRollCall

Hudson River PCB immunity story

This National Geographic story showed up in another subreddit this morning. I was confused by an aspect of it, but unfortunately the comments on the submission are a catastrophe, so I'm asking here instead.

In the story as written — I looked for but did not find the referenced paper — it's said that a particular type of fish is sometimes found in nature to have a variation that affects the expression of a particular protein. This protein offers a resistance to the negative effects of a particular toxic substance.

Fish of this kind that live in a river with a high concentration of that particular substance have been found to have this particular variation. Thus, they're surviving in an environment that would be toxic to fish that lacked the variation.

The story as written says the fish population has "developed a gene that renders them immune," and calls it "lightning-fast evolution."

My question is this: How reasonable is it, really, to call this "evolution" at all? A particular variation occurs naturally in the population. That variation renders individuals immune to a certain type of toxicity. That toxicity appears, all the members of the population who aren't immune to it die off, the ones who are immune survive.

Isn't this nothing more than a culling?

As I said, I haven't found the referenced paper, and I'm not educated in the way biologists use words like "developed" or even "evolution" in a technical context. So maybe I'm just even more ignorant now than I usually am, and I'm more than prepared to be told such.

Thanks in advance.

Posted Feb 19, 2011, 1:50 PM UTC

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gippTheoretical Chemistry | Computational Chemistry•

That's exactly what natural selection is, though. Just taken to a greater extreme. You have a point of random genetic variation -- in this case, a simple on/off mutation (it would appear). Selection pressures are applied -- in this case, PCBs. That pressure selects for those at the favored end of the variation -- in this case, the "off" gene for that particular protein.

I think it just feels counterintuitive because you have two somewhat unusual circumstances at once - you have a completely dichotomous variation, just on or off, no in between, and you have very extreme selection pressure. It is absolutely natural selection (or at least as "natural" as you choose to call the addition of PCBs).

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RobotRollCallOP•

Sure, no argument. It's "natural" selection, for a sufficiently liberal definition of "natural."

But my confusion arises over the use of the word "developed," here, I guess. These fish didn't develop a gene in response to environmental pressure. That gene was present in the population all along. It's just that the fish without the gene were culled from the population, leaving only the ones that already had the gene.

The news story — again, not the actual paper, which I didn't find when I looked for it — reads to me like there's a cause-and-effect relationship being implied. PCB levels in the river went up, and in response the fish developed an immunity. I guess my question is whether that's a valid way of describing what's been observed. I feel like it isn't, but again, I'm very ignorant of this subject.

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clessaGenetics | Medical Biology•

News reporters almost never fail to get a science-related article wrong in its most basic principles, either through a misunderstanding on the part of the reporter, or in a misguided attempt to make the article more digestible to the average reader.

You are correct in saying that the fish didn't "develop" a gene. A less misleading summary may be that PCB levels in the river went up, and in response the fish population changed its composition to have its surviving members be only those with the resistant gene, reflecting the extreme environmental selection pressures.

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RobotRollCallOP•

Thanks for that. That's how I would have described it if asked, but since I really have no idea what I'm talking about when it comes to this stuff, I wasn't at all confident.

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gippTheoretical Chemistry | Computational Chemistry•

Oh, ok, I think I understand your question a little better. You're correct in finding that misleading. It's one of the most common misconceptions about how evolution works among the general population, and pop-sci journalism doesn't help any. No, for an immunity to be directly created by a toxin would, AFAIK, require either direct "Design" or some very unlikely chemical happenstance. There may be some evolutionary or cell response mechanism I'm unaware of, though, so some evo biologist may come in and correct me.

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mamaBiskothuCellular Biology | Immunology | Biochemistry•

I didn't read the article above, but this boils down to a question about evolution that is still kinda unanswered: Do species have lots of polymorphisms from which they keep selecting traits that are better suited to natural constraints or do these polymorphisms that actually get selected arise only after the constraints are placed.

From what little I know we don't know which is the general case.

As much as everyone accepts evolution, how it works is still largely unknown.

But this article doesn't sound all that more impressive than many other cases of animals adapting really fast to tackle problems in their surroundings.

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whimbrelCognitive Neuroscience | fMRI Research•

It's a science express paper, here:

if you have university access, which I assume you do. For non-subscribers, theddman kindly posted an easyshare link earlier in the comments.

Reading the paper, the authors state:

We identified a six-base deletion in AHR2 as the basis of resistance and suggest that the HR population has undergone rapid evolution probably due to contaminant exposure. The mechanistic basis of resistance in a vertebrate population provides evidence of evolutionary change due to selective pressure at a single locus.

Later, they point out that this was not the development of something new, but rather selection pressures on an existing variant:

The presence of AHR2-1 at low frequencies in tomcod from these two cleaner nearby estuaries within 150 km of the mouth of the HR suggests that AHR2-1 was present as a standing variant at low frequency in the HR prior to anthropogenic disturbances. The presence of AHR2-2 at low frequencies in the HR suggests that its frequency was altered recently through selection.

In short, I think this is an emphasis on the "selection" part of "evolution through natural selection". The population composition changed in response to an environmental stressor and the population is fairly geographically contained. However, this genetic variant probably has disadvantages in a non-toxic environment, discussed at the end of the paper, which is why it's less prevalent in clean-water fish. So now you start to see the splitting of populations -- the PCB fish all have this variant, the non-PCB fish predominantly have the other variant. If enough of these differences accumulate, then these PBC-codfish will no longer be viable mates with the non-PCB codfish and, voila, a new species has emerged. Macroevolution in action.

I agree, though, this seems expected. I'm not sure why it's worthy of a science publication for advancing the field of evolutionary biology, but it does seem like a cool finding for anyone wanting to know how to reduce PCB toxicity...

edit: clarification in penultimate paragraph

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theddmanMechanistic enzymology | Biological NMR•

http://www.easy-share.com/1913919452/science.pdf

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[deleted]•

Warning: I'm not biologist.

If we go with Wikipedia definition: "organic evolution is the change over time in the proportion of individual organisms differing in one or more inherited traits", this is good example of evolution. Changes in variation are one part of evolution. Culling is part of evolution as I understand it.

Of course, saying "developed a gene that renders them immune", tries to make it more than what it is.

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RobotRollCallOP•

I guess this comes back to my personal opinion — unscientific and highly biased — that that definition of "evolution" is an entirely useless one. Populations change with time, you say? The way populations change is a function of the environment in which they live, you say? If a trait kills you before you can reproduce, that trait won't be passed on to any offspring, you say? Well knock me over with a feather.

I'm sure it was a surprising idea a hundred and fifty years ago, but it strikes me as boringly obvious. I'm personally more interested in the origins of biodiversity, which as I understand it — and I almost certainly don't understand it correctly — is a much less obvious question.

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gippTheoretical Chemistry | Computational Chemistry•

Ah, but that's the tricky thing about evolution, really. The basic principles by which it operates are, indeed, all blindingly obvious in retrospect. It was really the scale of the changes, the universality and extent of changes, that really took people by surprise.

To your second point, it sounds like you're interested in the specific mechanisms of genetic change and phenotypic development. Beyond mutation, things like chromosomal crossover, transposable genetic elements, RNA interference, and epigenetics. It's covered in most any evo bio course. If you're really interested in looking into it, those topics I listed are probably a good place to start.

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[deleted]•

I think biologist may need the all compassing term, because there are different causes for evolution, natural selection and genetic drift for example.

Maybe we should talk about natural selection instead?

For me (from computer science background) genetic evolution falls into category of heuristic optimization algorithms, subcategory evolutionary algorithms. They use reproduction, mutation, recombination, and selection. It seems that in your fish case, only selection was driving evolution. There was no mutation (gene appears infrequently in other populations) and recombination did not bring the benefits.

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32koala•

...it strikes me as boringly obvious.

Then why do some people not believe it?

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RobotRollCallOP•

Can we not turn this into yet another circus of intolerant mockery? Is that a thing we can do just this once? I normally wouldn't ask, but in this particular instance I did ask the question, and if it counts for anything, that's my preference.

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32koala•

I know that, based on the demographics of Reddit, my post probably appeared to be an insult to... to someone or some people. But it wasn't; I think this si an important question to ask: What is it, specifically, about the way we explain evolution that makes it hard for people (even highly analytical people like yourself) to understand?

You mentioned biodiversity. (Warning: undergraduate-level explanation). Biodiversity was central to Darwin's theory, but he had no frick'n clue what caused it (much like Newton and gravity). Today, we know a lot more. It starts with the genome. Point mutations in unicellular organisms (or the eggs & sperm of multicellular ones) are the easiest genetic changes to understand. Due to random chemical events, toxins, or radioactivity, a point mutation will take place at one nucleotide. This mutation usually causes no ill effects, as much DNA is "noncoding" DNA, and, more importantly, protein transcription mechanisms have some redundancy built in. Sometimes point mutations provide advantageous effects; in this case, they stay in the gene pool (and might spread).

But point mutations are less of a factor in causing biodiversity than we once thought. Exchange of DNA and copying/deleting events are much more common, and thus much more important. In animals, much diversity arises from the "mix and match" that arises during chromosomal crossover, which makes sure each of your sperm and eggs is slightly different, and is the reason why brothers (of the same mother and father) are not identical twins.

Another VERY important factor in biodiversity is gene duplication, which can increase the amount of a certain protein in an organism. Building better beings is all about having the right proteins at your fingertips, and protein duplication is like adding more of a certain color to your biochemical easel, to give the painter more options in his work.

I hope I was able to help.

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aazav•

Instead of answering your question, I would like to point out the dangers of this from a biology of populations point of view, before people start thinking that we can pollute as much as we need and biology will fix everything.

There is this lovely concept called "biological magnification of toxins". Generally a bunch of critters eat 10 times as many smaller critters, who eat 10 times as many smaller ones as they. At some point in the chain (or web), chemicals or heavy metals are consumed and enter the food chain. In many cases, as in Minimata poisoning, creatures at the top of the food chain end up with concentrated levels of heavy metals (mercury, etc…) or chemicals (dioxin, PCBs) in their fat tissue. If you consume them, you're eating concentrated badness. This is why the top level predators die fastest, when their tissues are loaded with DDT, polychlorinated biphenyls, mercury or cadmium. While the fish living above the carefully layered dioxin and PCB infused substrate (bottom) of the lovely Hudson have been fortunate enough to develop a gene that sequesters the PCBs in their liver, this means that it is being concentrated in their liver. Normally, various toxins end up being in specific tissues, but this mutation allows the liver to take its "purifying" role to an extreme.

While this is great for the tomcod mentioned, what this does is pass more PCBs up the food chain to higher level predators and if the fish survive to be harvested, to us.

What is SO BAD about the Hudson River situation is that to get rid of the PCBs in the riverbed muck, is multifold. 1) It will cost a lot to dredge or scoop the muck up and B) lots of PCBs & other chemicals will be released in the process.

What's the moral of this story? If given the option, always live upstream and upwind of a power plant, chemical plant or manufacturing plant. Oh that and those eat at the top of the food chain run the risk of being the first to be poisoned through concentrated chemicals and heavy metals through biological magnification of toxins.

Happy. Happy. Joy. Joy.

http://www.cartage.org.lb/en/themes/Reference/dictionary/Biologie/B/189.html

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skeeter_wranglerVirology | Immunology | Vector Biology•

In my opinion, the phenomenon appears to be more closely related to the famous "Peppered Moth" story. For now, this may amount to nothing more than phenotypic plasticity, since variations of the allele exist in other populations (it's difficult to make the claim of evolution over this time scale). However, this is usually the theoretical "beginning" of natural selection and speciation, and thus makes for a good story.

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Ryguythescienceguy•

Isn't this nothing more than a culling?

Biologists normally use more delicate language, but evolution is basically a slow-motion culling.

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tellme_areyoufreeMedicine | Public Health•

Evolution is simply a change in the frequency of genes in a population over time. So yes, this is evolution by natural selection.

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aazav•

Oh and evolution is when a gene change that results from an external force enters a part of the population of creatures and is propagated onwards through reproduction.

This change GENERALLY has some positive affect on survivability of the creatures having it, or enough to get the creatures to reproduce and pass the gene on.

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