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Understanding Evolution: What is Natural Selection?

December 31, 2013 Evolution and Biology
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“When it was introduced in the 1940s, penicillin was a miracle drug, especially effective at curing infections caused by the bacterium Staphylococcus aureus (‘staph’). In 1941, the drug could wipe out every strain of staph in the world. Now, seventy years later, more than 95% of staph strains are resistant to penicillin.”

— Dr. Jerry Coyne, ‘Why Evolution is True’

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In ‘Why Evolution is True’, Dr. Coyne used this example in the chapter entitled, “The Engine of Evolution.” The engine he’s referring to is none other than natural selection, and this little insight regarding the adaptability (evolution) of Staphylococcus aureus is excellent evidence for the theory of evolution by natural selection. It’s my go-to “proof,” if you will, when someone asks me to provide proof of evolution because (a) it’s simple and easy to understand, (b) it definitively shows that an organism can adapt to a new environment (in this case, a human body injected with penicillin), (c) it doesn’t require any discourse regarding fossils or the fossil record (which is probably the best evidence for evolution, but it’s very difficult to explain fossil evidence to someone who wants to “see” evolution if you don’t have fossils on hand!), and (d) bacteria are excellent for ‘showing’ evolution in action because they reproduce quickly, meaning that they can potentially evolve much faster than, say, a dog or a dolphin.

Let’s expand on point (d) for further clarification:

Bacteria can reproduce in as little as 20 minutes, so if you leave some bacteria alone for 24 hours, they will have gone through 72 generations. 72 generations in regards to a human generation (~25 years) is equivalent to ~1800 years. So, you can see that, in bacteria, you can get a good amount of change in a very short amount of time. In regards to the above example (Staphylococcus aureus and penicillin), the bacteria had 70 years to evolve and become resistant. That’s equivalent to, if I did my math correctly… ~74,000 human generations, which would be about 1.8 million years for us! To put that in perspective, our species, Homo sapiens, has only been around for about 200,000 years, and Homo erectus, a long extinct hominin of our genus, would have still been walking around that many years ago! And, 1.8 million years just so happens to coincide with the approximate distance you’d have to travel into the past to see the first hominids make their way out of Africa!

Now, I’d like to point out a statement made by a famous biologist, geneticist and Nobel Prize winner, Jacques Monod: “A curious aspect of the theory of evolution is that everybody thinks they understands it.”

I added that quote for an important reason, and now I’d like those who have little to no background in evolutionary theory to stop for a moment, take a breath, and forget everything you think you know about evolution — notable exceptions being what I wrote in the first two paragraphs! I say this because many people believe they know exactly what the phrase “evolution by natural selection” means. You may have heard people talk about it, or maybe it was being discussed on the news. For many, the words “random” and “chance” (or “random chance”) come to mind. Also, the phrase “survival of the fittest” may pop into your head along with the notion that natural selection is some type of force (dynamism is the word I hear too much of in this respect). Forget all of these things. None of these things are true. “Survival of the fittest” is technically *close* to the truth, but not a truth in and of itself. Now, let us “define” (describe) natural selection.

According to the book, “Why Evolution is True,” “The idea of natural selection is not hard to grasp. If individuals within a species differ genetically from one another, and some of those differences affect an individual’s ability to survive and reproduce in its environment, then in the next generation, the “good” genes that lead to higher survival and reproduction will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.”

To show you how simple of a process this is, let’s take a look at a hypothetical example in nature (I say “hypothetical,” but it’s entirely plausible that something similar to this has or could have occurred).

Picture in your mind a stick insect. Now, imagine that this stick insect carries a gene mutation which makes it somewhat more “stick like” than another similar stick insect. That is to say, your imagined stick insect is better camouflaged than another stick insect. It follows, then, that the stick insect which is somewhat better camouflaged, that is better adapted to its environment, will have a better chance to survive and reproduce than the other stick insect. Ultimately, the better-camouflaged is less likely to be seen and thereby eaten by a predator such as a bird. When this stick insect reproduces, it’s likely to pass on this “good” mutated gene to its offspring. These offspring will then be better camouflaged (adapted) and therefore more likely to survive and reproduce. They’ll produce more offspring, who will produce more offspring, and so on and so forth. Over a long period of time, the stick insects with the original gene mutation will come to outnumber those without it due to being better adapted to their environment, i.e., in this example, they are less likely to be eaten by predators and therefore more likely to survive, allowing them to produce more offspring. These stick insects are then said to have been “naturally selected” as they are more adapted to their environment (they are “fitter”) than their predecessors, making them more likely to survive and reproduce. So, essentially, the phrase should be “survival of the ‘fitter’,” not “survival of the fittest.”

Now, the reader may have noticed that I’ve yet to define or describe evolution itself. I realize this, and I also realize that it’s quite unorthodox to begin a series on evolution by describing natural selection. But, it’s been my experience that most of the controversy and confusion surrounding evolution regards natural selection. In the future, we will go more in depth into evolutionary theory, but I hope this post has dispelled any misconceptions you may (or may not) have had regarding natural selection, and I sincerely hope that you’ve learned a thing or two that you can go and share with others!

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