Pollution Is Now Driving Evolution

A water snake slides into the reeds under the hanging branches of a sweetgum tree.  Rotting logs lie half submerged in Blackwalnut Creek, a Chesapeake Bay tributary south of Annapolis.   And in the water around the logs dart little fish.
 
The striped Atlantic killifish –- also known as mummichogs -- swarm in brackish water up and down the East Coast. They are as common as pebbles, but also extraordinary.
 
Scientific research, explained in toxicologist Dr. Emily Monosson's fascinating and meticulous new book, Evolution in a Toxic World, suggests that toxic chemicals like PCB’s and dioxins that poison other species do not kill killifish, which have developed resistances to these deadly chemicals.  Killifish are an example of how pollution is driving evolution.
 
This may be surprising to some people, who thought evolution was something that progressed gradually, over thousands or millions of years. But as it turns out, some species adapt their DNA rapidly, sometimes in just a few years, when threatened with chemical annihilation, Dr.  Monosson writes in her book, published last month by Island Press.  Other species do not, and some die.

But the phenomena of rapid evolution in response to toxic threats may be much more common than people think, Dr. Monosson writes.  “The chemicals we have unleashed on the earth are causing genetic change, possibly on a large scale, and we are only just now grasping the reality,” said in an interview.

There are almost 100,000 industrial chemicals in use around the world. And only a tiny fraction of them have been tested for toxicity, Dr. Monosson writes.
 
“We are a society built on chemicals, and there is no turning back. We can certainly improve how we produce, use and release chemicals by striving for better understanding of how they affect wildlife and human health….and an evolutionary perspective can help,” Dr. Monosson said.
 
Why do some creatures evolve when challenged by pollution, and others perish?  Lake trout, for example, may have been wiped out in the Great Lakes in part by dioxins, chemicals released by the burning of trash and manufacturing. But killifish and a freshwater species called the tomcod have developed resistance to dioxins, according to Dr. Monosson and other toxicologists. 
 
In general, the most adaptable critters appear to be those that reproduce fast.  Staphylococcus aureus bacteria, for example, quickly evolved to overcome methicillin antibiotics.  Many mosquitoes adapted to organophosphate pesticides.   Freshwater worms breeding in the Hudson River where battery makers dumped cadmium learned to live in harmony with the toxic metal, according to Dr. Monosson’s book.
 
All this resilience in the natural world does not mean people should care less about releasing toxic pollutants, or cleaning them up, according to Dr. Richard Di Giulio, Director of the Toxicology Program at Duke University.
 
“To have pollution basically driving the genetic structure of a population is an inherently worrisome matter, I would argue,” Dr. Giulio said. “And the long term consequences of that for the survival of that species and population is difficult to predict.”
 
For example, the killifish that Dr. Di Giulio studied were resistant to polycyclic aromatic hydrocarbons dumped by a wood treatment factory on Virginia’s Elizabeth River, a Chesapeake Bay tributary. But their bodies appeared to be so focused on combating these toxins, they became weak in other areas. The fish were more sensitive to low-oxygen conditions, and had impaired immune systems.
 
It is hard not to admire the stubborn and resourceful little killifish, as I watch them flitting about in the shallows, heedless of a Styrofoam cup floating nearby.  They make the best of what we throw at them.  But they may pay a genetic price for adapting to a toxic landscape.  

And future generations of humans may also pay a price if we don’t clean up our mess in the Chesapeake Bay region.

By Tom Pelton

Chesapeake Bay Foundation

(Photo of Atlantic killifish at top from Virginia Tech College of Natural Resources)

 

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Hey, Jodell.Thanks for the suggestion! I will evaltuelny want to euthanize the killifish and it would be easy to get a homogenous lab strain of the parasite (they reproduce asexually in their snail hosts and they reproduce continually, so as long as I keep track of snails I can keep track of genotypes). While I haven't extracted DNA from cercariae (I'm relatively new to molecular techniques), I know that it can be done. Unfortunately, I don't have money for the preliminary work necessary to run PCR of aflp on my parasite, nor do I have money to process the 100+ fish that I'll be running through experiments. Hopefully some of the grants I wrote go through and then perhaps this could be an option! Hmm, why would red light degrade slower than white (although I was actually using a green dye)?

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