Misconception Monday! Natural Selection and Evolution, Part 1

This perhaps should have been the first up in my exploration of common misconceptions, since it is arguably more fundamental than what “survival of the fittest” means (or doesn’t mean). But all of these misunderstandings and ideas are so entangled that there really can’t be a “right way” to present them, can there? So let’s get to it! Misconception #2, come on down!

Natural selection and evolution are the same thing.

No, they aren’t; they really aren’t. Evolution is descent with modification, heritable change in populations of living things. A common definition of evolution is change in a population’s allele frequencies. (Quick refresher: alleles are versions of genes. The gene that codes for a certain protein on the surface of human red blood cells, for example, has three variations, or alleles, A, B, and O, which in turn contribute to overall blood type.) Natural selection is a mechanism of evolution, a process that leads to shifts in allele frequencies within a population. But natural selection is just one way—albeit a really powerful way—that such change can occur. The others are genetic drift, gene flow (migration), and mutation. Let’s look at each one—genetic drift here, and then gene flow and mutation in part 2 later today.

Lots of M and MsEvolution that occurs by genetic drift could be called “evolution by sampling error.” It happens when some random event results in a new population that is not representative of the original population. It's like reaching into a giant bag of M&M candies and without looking, pulling out ten red and one green. That subpopulation of candies is not representative of the original bag population, which includes a relatively even mix of red, green, brown, blue, orange, and yellow morsels of chocolate goodness.

Genetic drift is common in populations, and can sometimes have dramatic effects. Cohen syndrome, for example, is a very rare disease that has been reported in about 1000 people worldwide. But one family in Ohio has three affected daughters. Throughout the entire Amish population in Ohio, the frequency is an estimated 1 in 5000. Why is the disease so much more common among the Amish than the general population? (Bad) luck. The more than 150,000 American Amish can trace their ancestry back to just a few hundred European immigrants in the 1800s. Just by chance, a few of these immigrants that settled in Ohio carried the allele for Cohen syndrome.

The disease is autosomal recessive, so it takes two copies of the allele—one from each parent—for the disease to manifest in a child. Individuals with the disease do not live to reproduce, so to appear in a population, two healthy adults, each carrying the allele, would have to have a child; and even then, the chance is just 25% that the child will be affected. In the general population worldwide, the chance of this happening in any individual case is very low. But the founding population of the Ohio Amish was not representative. So when two of them had a child, the chance was much, much better that they’d both be carriers. It’s important to emphasize that the event that precipitates evolution by genetic drift is not a selective event. The founders of the Ohio Amish population were not somehow more or less adapted to their environments. They were just the ones that got on the boat.

Stay tuned for Part 2, coming in a bit, in which I discuss Neanderthals, ice fish, and NGSS—oh my!

Stephanie Keep
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Stephanie Keep is the former Editor of Reports of the National Center for Science Education