Science League of America series on Evolution

Misconception Monday! Survival of the Fittest, Part 1

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... But it’s also an absolutely marvelous way of getting to understand what misconceptions are out there among our students.

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Let’s start with this one: Natural selection eliminates all bad variations.

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Where does this misconception come from? Well, that’s easy. Ask people on the street what comes to mind when you say natural selection or Darwin and I bet that a significant number will answer survival of the fittest!* The problem is that most people on the street can’t accurately explain what that phrase means.

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So it’s no wonder that our students come to think that if, for example, an environment favors dark-colored mice because they are harder for predators like owls to find, then all of the light-colored mice will die. Extinct! Poof! See ya! Good night! But this isn’t how it works.

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When it comes to natural selection, the fittest really is the good enough. As long as you get a chance to reproduce and pass on your genes, you win. And even when faced with very strong selection pressures, it just isn’t the case that onlycertain individuals will be able to reproduce. A few not-so-ideal individuals are bound to sneak in there. And on top of that, due to the constant shuffling and reshuffling of genes involved in sexual reproduction, phenotypes not present in the parents can show up in the offspring.

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So it's "survival of the fit enough" rather than the most fit. Misconception Monday! Survival of the Fittest, Part 2

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... I want to get into some ways that this misconceptionthat natural selection eliminates all bad variationscould be tackled, or even headed-off, in the classroom.

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Here are a few of the routes you can take:

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1) Follow the well-trod path. Use your old friends the peppered moths. The basic outline of this example is well-known, but with some well-chosen emphasis points, it can be used to demonstrate the mis of this misconception. Until the industrial revolution in England, light-colored moths were far more common in England than dark-colored moths. By the early 19th century however, the dark-colored form came to predominate. In one highly polluted city in 1895, 98% of moths were reportedly dark98%, not 100%. ...

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2) Invoke our fascination with disease. The human disease sickle-cell anemia and the equine disease of lethal white overo are both great examples of how a deleterious or even lethal recessive allele can be maintained in populationthe former by natural selection and the latter by artificial selection. In both cases, individuals that carry the disease allele have an advantage over individuals that do not. Natural selection therefore favors individuals that carry the allele, increasing the number of disease alleles present in the population. Any time two carriers mate, their offspring have a 25% chance of having the diseaseso the disease not only persists, but it is present in higher-than-expected numbers.

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3) Demonstrate how probability works. Smoking causes lung cancer, right? But not all smokers die from lung cancer; in fact, most don't. Similarly, a trait that is "harmful" does not uniformly result in a reduction in fitness for those individuals possessing it. If you have the time, a simple classroom activity can help to get this important point across. Scatter a few hundred beans of different colors on a lawn and give your students 20 seconds to pick up as many as they can and put them in the cup. Some of the beans will be better camouflaged than others, so you should see some differential bean fitnessbut they won’t be able to find all of even the most brightly colored beans if given limited time. Natural selection works the same way.

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As a point of reference, I met a young man with sickle-cell the last time I was in for chemo -- he had a morphine pump to administer against the pain. I'll take cancer over sickle-cell, thanks.And then there is my mother: smoked a pack of cigs a day and lived to 91 and died of dementia.Enjoy(links and information?)

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