We've been talking about the sources of genetic variation, and now we want to focus on mutations. Mutations are just those changes in the DNA that introduce that genetic variation. Remember, genetic variation is required for evolution. Think of it as the clay that evolution is going to mold. Now before we go on, I just really want to stress that mutations are going to be random, and they are rare.
But those random rare mutations that do enter the population, we can kind of break them up into three buckets depending on how they affect an organism's fitness. We can start off talking about beneficial mutations. Those are the ones that increase the fitness of an organism, as I'll indicate there with an up arrow. Now if mutations are rare, beneficial mutations are kind of the rarest of the rare. Right?
The idea that you can randomly change DNA and make an organism better is pretty unlikely. But when it happens, these are the mutations that natural selection can kind of grab a hold of and make more common over time. Now what's much more likely is that there is a neutral mutation. A neutral mutation is a mutation that just doesn't affect the fitness, as I'll indicate here with sort of sideways arrows. There are actually all sorts of ways that DNA can change that just has no effect on any proteins that it makes or, maybe changes of protein coding sequence, but the change in that protein again just doesn't matter.
Since these don't change fitness, natural selection has no effect on neutral mutations. And then the third bucket we have here are going to be harmful or, you'll often see them called deleterious. Deleterious mutations, these are the ones that decrease fitness as I'm indicating with the down arrow there. Make a random change to a DNA sequence, there's a good chance you're gonna break something.
That's those deleterious mutations. And because they reduce fitness, these are the ones that natural selection can sort of grab a hold of and remove from the population over time. Now we want to note that mutations can introduce both new alleles. Right? They can introduce new versions of genes, alleles, and they can introduce whole new genes.
Now as we go forward in this section, we're often to be talking about the evolution of a single gene with multiple alleles. So we're often to be focusing on that idea of where do new alleles come from. But we also just want to, in broad scopes, understand where do whole new genes come from because that's important for evolution as well. Alright. So let's first talk about these new alleles.
New alleles come from the changes to the DNA sequence, and these changes may affect coding regions. They may change the protein itself, or some people don't realize that often they just change the regulatory regions. A lot of changes between organisms is not the proteins themselves, but just when proteins are expressed, how much they're expressed, when genes are turned on and off.
These changes, well the simplest change that you could get is a point mutation. And a point mutation is just a change in a single DNA nucleotide, and we can see this in our image. We have two different alleles for the a gene here, the big A and the little a. And when we look at them, what is different about them? Well, there was a point mutation.
There's a single difference in those DNA sequences. Alright. So that's where new alleles can come from. But what about whole new genes? Right?
Humans have something like 20,000 genes. Where did they all come from? Well, new genes can come from a chromosomal level mutation or chromosome level mutation, as it says there, and this is going to be a change in the arrangement or the number of chromosomes. Sometimes a piece of a chromosome gets sort of switched around, or sometimes two chromosomes fuse to make one, or a chromosome breaks apart into two.
These are all important things that happen in evolution. But for new genes, we want to focus on the idea of gene duplications. The idea that a piece of a chromosome might get duplicated, and that can lead to the evolution of new genes. So we see here, this chromosome had a D and an E gene. There was a duplication.
Now it has two D genes and two E genes. This sort of gives the space where mutations can come in and change one of those copies and give it a new function because the original copy is still there, still doing the job of that gene. Well, new genes can also come from horizontal or, as we sometimes call it, lateral gene transfer. And this is the movement of alleles from one species to another. Alright.
Horizontal or lateral gene transfer is normally what we think of vertical gene transfer, right? That sort of vertical transmission is the passing of genes from parent to offspring. But horizontal transfer is from one organism to a completely unrelated organism. Now this is going to be most common in bacteria and also in archaea.
But it does happen in eukaryotes rarely, and there are some really important cases of it happening in eukarya evolution. Now in bacteria, this can be really important for things like the evolution of antibiotic resistance. Because one of the things that causes antibiotic resistance to spread so fast is that these bacteria can pass genes between unrelated species. Now in eukaryotes, as we said, it's less common, but my favorite example of this is the vertebrate eye. There's a gene in the vertebrate eye that allows vertebrate eyes to work differently from other animal eyes, and that gene entered vertebrates sometime early in vertebrate evolution.
It jumped in from bacteria. It gained a new function and now allows vertebrate eyes to operate differently. Now to illustrate this, we have this sort of tree here, this evolutionary tree, and we see this sort of normal branching pattern that we normally see in a tree with these branches going sort of vertically. But then we see some branches that are sort of jumping across laterally or horizontally across this tree. That represents the idea that sometimes a gene will jump from one species all the way across to a different species on this tree.
Alright. So remember, mutations introduce that genetic variation, that sort of fuel for evolution. We'll look at all this more, and examples and practice follow. I'll see you there.
