Natural Selection

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Have you ever had strep throat? Not all sore throats are strep, but strep throat is caused by bacteria. And if you have had it before, it typically really hurts.

Chances are, if you have had it, you have taken a course of antibiotics. Antibiotics are specifically designed to destroy prokaryote cells---which in your body will be bacteria---instead of your own cells which are eukaryote cells. We have another video all about bacteria and antibiotics, but this video is going to mention a concern we all have right now about antibiotic resistance that can happen in bacteria.

It’s also the perfect opportunity to illustrate a process that usually takes a very long time to see----natural selection. Let’s explain a basic example of natural selection. Consider these frogs sitting on logs in their habitat.

Let’s assume these are the same species of frog so they can breed with each other and they pass down their DNA to their offspring. Naturally, there is variety in these frogs. Some of these frogs are darker green---maybe almost a brown.

Some of them are lighter green. There is a variety of traits even in the same species. Just like in people.

So back to the log. You know what else is in this habitat? Predators.

And the predators are finding here that the lighter frogs are much easier to see in this habitat than the darker frogs. So, in this particular environment, the darker frogs are having an easier time surviving and potentially more fitness if they can breed. Fitness, in the biological sense, is determined by not how long they live---but by how many offspring they have.

These darker frogs pass down their DNA to their offspring so that the new baby frogs will have DNA from their parents. The lighter frogs are being selected against since they are easier to see in this habitat. Over a long period of time, you could expect to see a higher frequency of darker frogs.

If it continues for a very long time, it could even result in all frogs in this area being darker. Evolution----which is change over time----could take place because natural selection has occurred. Natural selection is a mechanism of evolution.

One thing to point out…it doesn’t necessarily mean that the allele for lighter color is gone completely. The allele could be recessive and carried within the population. And because of processes like crossing over and mutations, there are opportunities for variety.

But if this habitat and predators do not change, darker frogs will continue to have more fitness. Now we do want to point out that variation or mutations are not things that a frog can “will” itself to have. Frogs can’t just think, “You know, it’s better for me to be darker so I’m just going to have some variation to be darker.

” Because if that was true, well, let’s just say I might be a bit different. Mutations and variation are RANDOM. It’s possible they might not have any effect on an organism’s fitness so in that case, the genes are be passed on if that organism happens to be able to reproduce.

Or variations and mutations could be negative. If they are negative and negatively affect the organism’s fitness----meaning no babies----well then that trait will not be passed down. But if they have a positive effect on the fitness, then that frog may have more babies than average because that trait is helping them survive and reproduce.

More babies will receive the passed those genes. Over time, that trait that is an advantage will be more frequent in the population. So what does this have to do with natural selection in bacteria that occurs from antibiotics?

Well keep in mind, there’s variation in bacteria as well. For example, some of them might have an enhanced cell wall that makes it hard for an antibiotic to enter or they could contain enzymes that could assist in breaking down an antibiotic. Now remember the bacteria didn’t “will” themselves to have those traits.

Like our frog example, they are random variations. However when you take antibiotics, the environment is being altered. The bacteria that have traits that allow them to survive that particular antibiotic have higher fitness and can reproduce, passing on their DNA.

Bacteria that do not have traits to help them with the antibiotics do not have very much fitness. Because they’re dying. They can’t have fitness if they’re dead.

Selection has taken place and the resistant bacteria are the survivors. Once all of their competition is killed off, they can reproduce easily with few competition and their offspring can inherit the gene that allows them to be resistant to that specific type of antibiotic. Bacteria also have this amazing ability to transfer genes to other bacteria, so they could share that resistant gene with other bacteria.

It is possible that this specific type of antibiotic may not be as effective for this person in the future. Hospitals especially have this challenge because they are treating multiple patients with bacterial infections in a closed environment. Healthcare workers must have good hygiene practices so that they do not spread resistant bacterial infections from one infected person to another.

So while this is an example of natural selection that is observable in a shorter amount of time---which can result in evolution in bacteria, what can be done about this resistance issue? Scientists are often in the process of developing new antibiotics----because we do have many types of antibiotics available---so they can stay one step ahead of bacteria. But there are still bacterial strains out there that have developed a resistance to commonly used antibiotics making it very difficult to treat.

So it’s important to only take antibiotics for bacterial infections. Antibiotics don’t work against viruses, and you don’t want to be introducing antibiotics in your body when you don’t need them. That will just further select for bacteria that have some form of resistance.

It’s also important to mention that many vaccines like the DTaP vaccine can protect you from serious, potentially deadly bacterial infections such as diptheria, tetanus, and pertussis. Well that’s it for the amoeba sisters and we remind you to stay curious!