Think like a scientist: human-driven selection

A salmonella outbreak on chicken has hospitalized over a hundred people so far. Salmonella is on a lot of chicken; if you cook chicken at all regularly, you have definitely purchased and handled salmonella-tainted chicken. But that’s okay, because you cook it, and the bacteria die from the heat, and then the chicken does not kill you. No worries!

This chicken might want to kill you... Photo by Ido Mor

This chicken might want to kill you…
Photo by Ido Mor

Except that those 100+ sick people probably weren’t eating chicken sushi. Even if they did all manage to undercook their chicken, there’s this: a Costco found salmonella on its rotisserie chicken after they were cooked at 180 Fahrenheit. Chicken is “safe” when it’s cooked at 165 Fahrenheit, so 180 should be extra safe. Now, I’m not a salmonella investigator; maybe Costco lied about its cooking temperature, or maybe someone handled raw chicken and then the rotisserie chicken and that’s how they got contaminated. But there is a third option: maybe a strain of salmonella has evolved, under selection driven by you and me and everyone else who cooks their chicken, to survive cooking.

We all know what natural selection can do, how the pressure of competing with other individuals and evading predators and finding food and staying the right temperature so that you can make the most babies can drive the evolution of “forms most beautiful and most wonderful” (Darwin).

A form beautiful and wonderful: male greater kudu in Kenya.

A form beautiful and wonderful: male greater kudu in Kenya.

Too, we’ve seen the power of human-directed selection, usually called selective breeding: wide-bodied bulldogs and sleek greyhounds descended from the same lupine ancestors; cows bred for milk production or for beef; sheep bred for wool or meat; chickens bred to lay large white eggs or to grow as large and as quickly as possible.

Young beef heifers: big, stocky, shaggy.

Young beef heifers: big, stocky, shaggy.

Dairy calf: slender and gangly. (And thirsty.)

Dairy calf: slender and gangly. (And thirsty.)

We rarely think about the other kind of selection by humans: unintentional human selection. But as much as we award breeding opportunities to the cow that gives the most milk and the dog that has the squashiest face, we also constantly try to prevent the breeding of other organisms. We hunt and fish. We spray pesticides to kill bugs on our crops. We cook meat to kill germs on our food, and we take antibiotics to kill bacteria in our bodies. We’re not trying to select these things, of course—we’re trying to kill them. But if our kill rate isn’t 100%, then instead of elimination, we can have very, very strong selection.

Take fishing. People don’t fish randomly: they take the larger fish. Nets do this size-sorting automatically, since a too-small fish just swims through the net. On the face of it, this seems pretty logical. It would be dumb to go after the tiny fish when big fish give you more meat per individual, sell for more money, etc.

Big catfish = delicious.

Big catfish = delicious.

But do you see the problem? If we kill big fish, we are selecting for smaller fish, because it’s the smaller guys who will live and reproduce. A big fish will get fished, but his smaller cousin will survive and have small babies, and the average size of the next generation will be just a bit smaller. This is happening: many heavily-fished species have started growing slower (to be small for longer), reproducing at smaller sizes, and/or reproducing younger. This pattern has been seen in wild populations of salmon, cod, halibut, plaice, flatfish… We are making the fish we eat smaller (Law 2000).

It gets a lot worse when you start thinking about pests and bacteria. The more offspring an animal can have, the more quickly it can respond to selection: kill all but a pair of salmon, and you’ll have to wait a while for the ocean to be repopulated with salmon; kill all but one single bacteria, and pretty soon you’ve got a petri dish full of bacteria again.

This is why the overuse of antibiotics is a really big deal. Antibiotics don’t always work on every single bacterium. If even one bacterium—out of millions, because these are bacteria, so they come in large numbers—happens to have evolved so that it can survive the antibiotic, then this is what happens: the antibiotic kills off all its competitors, the survivor bacterium has lots of resources for its babies, and now you’re infected with bacteria that you can’t kill. Woohoo!

Fortunately you still have some options. Your immune system attacks bacteria, so maybe your immune system tracks down this super-mutant and kills it before it can reproduce too much. Medical science has come up with several different antibiotics, so you can try a different one and hope that it works. Most of the time, you’re fine. Say, 99.9999999% of the time. That looks like a pretty safe number.

Except remember how bacteria come in gazillions. And remember how we dose the animals we eat with antibiotics constantly, even when they’re not sick. Think about how many chickens, cows, and pigs are being kept in cramped, crowded quarters just perfect for breeding disease, and being dosed regularly with antibiotics. Each one of those animals is like a little experiment: will we get the super-mutant antibiotic resistant bacterium this time?

Maybe in one of these piglets?

Maybe in one of these piglets?

We’re already seeing rising numbers of cases of bacterial infections that can survive antibiotics. Doctors keep some super-strength antibiotics in reserve, only prescribing them if the regular meds don’t work, for just this reason: the hope is that bacteria won’t be selected for resistance to these super-antibiotics if they’re deployed only rarely. But they’re needed more and more, now, and sometimes we find bacteria that are resistant to everything. Some of those cases of salmonella aren’t responding to antibiotics.

This should be scary. Worrying about our selecting for bacteria that we cannot fight is practical, not paranoid. If you can, support efforts to reduce or eliminate “preventative” antibiotic use in livestock. (Sure, it prevents the cow from getting sick—which the cow probably wouldn’t, anyway, if only it was housed humanely—but wouldn’t you rather prevent an antibiotic-proof epidemic?)

This is also why you should 1) not take antibiotics if you don’t have a bacterial disease (a “common cold” is caused by a virus; antibiotics will not do anything to cure you, but they will select in your body for super mutant killer bacteria), and 2) always take the full prescription of antibiotics. A lot of people don’t take all of their prescribed antibiotics. This makes it a lot more likely that you’ll select for resistant bacteria, because instead of dosing your bacteria with lots of death-meds, you just dose them with some death-meds, which is easier to survive. Then you end up with moderately-resistant bacteria, which can more easily mutate into really-resistant bacteria. If you take the full dose, you stand a much better chance of just killing them all.

Human-driven selection is not completely avoidable, but knowing about it, we can strive to mitigate the damage.

Frolicking New Zealand lamb only needs antibiotics if she gets sick, thanks.

Frolicking New Zealand lamb only needs antibiotics if she gets sick, thanks. Photo by M. LaBarbera


Law R. 2000. Fishing, selection, and phenotypic evolution. ICES Journal of Marine Science 57:659-669.

6 thoughts on “Think like a scientist: human-driven selection

  1. I am more inclined to say they handled the cooked chicken with contaminated hands, bacteria according to scientists (or evolution period) requires millions of years to adapte not the short time we have been cooking chicken. of course I do not believe in evolution, I mean how can you accidentally build a house or a computer which is not as complicated as living cells after all can a computer reproduce itself (have computer babies? like living cells can?) anyway I doubt we have to worry about bacteria becoming heat resistant, maybe antibiotic yea, but not heat. in fact bacteria do not adapt to antibiotics their are always individuals with a hardy composition toward the antibiotic and if you stop the antibiotic too soon the weakened bacteria start to breed. it is improper use of antibiotics that is the problem not to mention the lowered resistance of the animal whos immune system is compromised which normally would assist the antibiotic that is why some people can get sick again after taking a course of antibiotics, their immune systems were still weakened and did not help the antibiotic work better. or like my brother didn’t take the whole prescribed antibiotic and had to retake it for a longer period. just make sure when you cook any meat products that you keep your counters clean, your hands clean when touching utensils and veggies and other foods and make sure to focus on cleanliness and cooking properly and I doubt you will ever have a problem. people have been dealing with animal products for thousands of years and I am sure the bacteria then is the same now, so no worries mate,

    • Thanks for your comment! You’re right that it’s certainly possible, maybe probable, that Costco mishandled the chicken. It is not impossible that salmonella has evolved high-temperature tolerance, though. Evolution can happen quickly, especially in bacteria, which can run through several generations in a single day (!). The relevant unit of time for evolution is not actually time itself, but the number of generations: hence bacteria may evolve much faster than, say, an elephant.

      Certainly you could not accidentally build a house or a computer. But evolution by natural selection (which is what you’re talking about when you say you don’t “believe in evolution,” I think, because evolution alone merely means “change in gene frequencies over time”) does not operate by accident – this is a major misconception and a problematic one, and science programs need to work harder to dispel it. The variation on which natural selection acts arises randomly: a bit of DNA isn’t copied correctly, and suddenly you have a tan mouse instead of a brown one, say, or three eyespots on a flatworm instead of one. If this new form is worse at surviving and reproducing, then nothing much happens. However, maybe the tan mouse matches the background better and so is seen by predators less; maybe your three-eyespotted flatworm can more easily find mates. If the new form is better at reproducing, then it will do just that – reproduce – and more of the next generation will be like it. Over time, the population will be made up more and more of the new form. So although the new form arose randomly, its persistence and success, and the eventual change of the entire population, is NOT random: it’s influenced by habitat and predators and mates and food and so forth. This is how you build an eye: you start with a random eyespot that, nonrandomly, lets you reproduce better than your blind compatriots; randomly, more receptors arise in the eye and, nonrandomly, spread throughout the population; randomly, some of those receptors start to detect only a subset of light wavelengths, giving your color vision which nonrandomly gives you an advantage; etc, etc. Highly complex forms arise in evolution through selection as long as each step along the way improves the animal’s fitness. Even something as complex as an eye arises from simple, primitive forms that, through successive small changes that improve function, eventually give rise to the complex form.

      Which is why I’m going to keep worrying about antibiotic resistance in bacteria.

      • yea but the tan mouse is still a mouse it didnt change into another species, that is the point, people are confusing natural variations within a species with evolution, this is not true evolution, evolution says animals change from species to another, (like a dinosaur documentary I seen last night) this dinosaur they talked about changed from a meat eater to a vegetarian, changed from a sleek fast crittor to a slumbering bulky hooved animal, simply changing from one species to another that is not interbreedable at all dna is totally different hence dna complete changes are what evolution claims, as as for each step being condusive to an animals survivablity it still doesn’t explain why it was surviving so well befoer all the parts were complete, a part is useless until it is complete hence evolution says animals adapt to changes in enviromental stressors but it doesn’t explain how the dna or animal knows what parts to start changing gradually into something else that would require seeing into the future and no dna can do that unless it was programmed to do so and we all know how you cant get a program without a programmer which requires intelligent direction. I mean a house is no where near as complex as dna and yet no one is suggesting it happend simply because of enviormental stressors are they? if something as simple as a house requires intelligent direction and work why do we suppose dna which scientists cant even copy with intelligent direction ( which they surly should be able if accidental changes or enviormental changes can form and direct dna without intelligent direction)dna cannot come into existance out of thin air on it’s own. that is impossible. nothing comes from nothing not something comes from nothing, there has to be intelligent direction otherwise one is entering a world of fantasy and wishful thinking. imagination is a powerful thing but even that requires intelligence direction, in order to achieve it. I mean do you really think something just came about by chance out of thin air and is directing itself without having intelligence and ability in itself without a intelligent designer to make it so? and if dna does have intelligence to direct evolution then where did the dna get it’s program/intelligence from then?

    • (Replying to your second comment, since there’s no reply button down there)
      Evolution is not directed – animals don’t “know” how they want to change and then do it. Selection simply makes it seem like this happened after the fact. Change that doesn’t help the animal doesn’t increase the animal’s contribution to the next generation, while change that does help, leads the animal to have more babies, increasing the number of animals with that trait in the next generation. Bad change is eliminated, while good change is passed on, so eventually it looks like only good change happened.

      Imagine that you give me a giant bag of M&Ms and I don’t like to eat red M&Ms. I eat all the other colors and eventually only reds are left. How did the M&Ms “know” to be red so I wouldn’t eat them? They didn’t, but since I ate all the others, it now appears as though they somehow predicted my distaste because they’re the only ones left. But they were just lucky to be the right color.

      Small changes add up to big ones, and eventually lead to species divergence. The difference between changing the color of the mouse and it becoming a new species may not be a big difference at all; in fact, if the mice begin to choose their mates by color, so that tan mice only mate with tan mice and brown mice with brown, even that one change may be enough to split the one species into two. Some species are very different from each other, like dogs and sea lions, but that’s because they’ve had time to accumulate a lot of small changes. Other species can be quite similar – look in a bird book at warblers: many of the species are almost identical.

      I don’t think I’m going to be able to able to convince you in the comments section of a blog post – this is a huge field, and you deserve evidence much more solid than just my words. There’s a great resource from my own university called Understanding Evolution; their “Evolution 101” is here: and their “Lines of evidence” for evolution is here:

      Scientists aren’t trying to pull the wool over your eyes or sell you some nonsense they thought up on their coffee break. If it sounds obviously wrong to you, then you have probably been told an incorrect explanation (like that it is “random,” or that the animals must somehow “know” how to change – both of those sound ridiculous and untrue because they are! That is not how evolution works). Evolution is a complex thing and we are still studying its nuances – hence “evolutionary biology” as a huge field.

      Thanks for taking the time to talk about this! I really appreciate your comments – it’s very important to talk about what evolution is.

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