Mutation as a source of variation | Gene expression and regulation | AP Biology | Khan Academy

In many videos when we've discussed evolution and natural selection, we've talked about how variation in a population can fuel natural selection and evolution. So if you have a population of circles, obviously a very simple model here, maybe some of these circles are that off-white color, maybe some of them are blue, and maybe some of them are this salmon color. For certain traits, your environment might make certain of them better for reproduction, better for survival, or better for evading predators, and better for finding food.

Let's say these circles, for whatever reason, are in an environment where maybe being blue makes it a little bit easier to evade predators and a little bit easier to reproduce and find food. Well then, in the next generation, because the blues are more likely to be able to get to reproduction because they weren't eaten, you're likely to have more blues. So let me draw a few more blues and maybe a few a little bit less of the other ones because they're also competing for resources amongst each other, at least in this model that I'm doing.

Over time, if this blue phenotype—remember, phenotype is the expressed trait that's actually observable versus the genotype, which is the underlying genetics, which is sometimes observable and sometimes not—starts to dominate. As you can see, if in this case, in this environment, blue seems to carry some advantage, even if it's a slight probabilistic advantage over many generations, blue will start to dominate.

So you start to see that evolution of this population to being more blue as a species. One way to think about it is that variation in a species is really what natural selection is based off of. Certain variants might be more favorable than others, so that is what's really necessary for natural selection to fuel evolution.

Now, a key question is: where does this variation in a population come from? To think about that, we just have to remind ourselves where our phenotypes come from. How do these expressed traits get expressed? Well, in all the living organisms we're aware of, we have DNA. As human beings, we have 23 pairs of chromosomes, and each chromosome, you could view as just a very, very, very, very long strand of DNA.

Sections of that DNA code for various traits, and each of those sections that code for, say, a certain protein or a part of an enzyme, we call those things genes. So, chromosomally, we have multiple chromosomes; we have 23 pairs of chromosomes. Each chromosome can be viewed as a long strand of DNA. Parts of that DNA code for specific genes, and then, if you were to zoom in on those genes, you would see these nucleotide sequences.

This is all a review—we've seen this in other videos—where you see your adenine, your guanine, your cytosine, your thymine in an order that carries the information that will eventually be coded into mRNA, which then gets coded into protein. Now, there's two primary sources of variation. One source of variation is sexual reproduction. Not all organisms reproduce sexually, but many of the ones that we know, including human beings, do.

Where a male member of the species and a female member of the species each contribute a random half of their chromosomes to the next organism. So one way to think about sexual reproduction is it keeps shuffling the different versions of the genes that you have in the population into different combinations of those versions of genes, and so that generates variation.

But sexual reproduction by itself will not create new versions of genes, which we call alleles, or new genes entirely. And so the primary way that that happens is through mutations, and you might have guessed that we're going to talk about that because I had this title up here.

So another source of variation—and you could almost view this as a more fundamental one because this would happen even in organisms that aren't reproducing sexually—is that over time, there could be just random mistakes. There could be edits to these genes, and it could be a random, maybe this G gets turned into a C randomly, or maybe this T and A get cut out during the DNA replication process.

These mutations, which are all about genotype, and let me make this very clear: when we're looking at the sequence, we're thinking about genotype differences. Differences in genotype are not always obvious from expressed traits. Sometimes they do change phenotype or they're observable in phenotype; sometimes they're not. But when they are observable in phenotype, as I just mentioned many times, it could be a negative change in phenotype where it makes it less viable for that organism, or it's harder for them to survive and reproduce.

But every now and then, it could result in a variation in phenotype that is maybe neutral or even confers some type of advantage. So it might have been a random mutation that somehow turned one of these white circles into a blue circle, and there might have been another mutation that turned a white circle into a square, and that just wasn't even viable as an organism. Yet, the blue circles happen to be in the environment therein, and happened to be a favorable variation.