Hi. In this video, we're going to be talking about comparative genomics. So, comparative genomics is the study of genomics from an evolutionary perspective. And so, first, let's understand, you know, what are the terms of the different genes that are related between organisms. First, we use the term homolog to define genes that have similar DNA sequences and usually some type of similar evolutionary origin. Now, orthologs are genes inherited from a common ancestor, and paralogs are genes related by gene duplication. So if we look at what this looks like, we have some kind of early gene, there was a gene duplication that creates the alpha form and the beta form, and then this evolves further into chicken, human, and mouse, mouse, human, chicken. A homolog is going to represent every version of this gene because it originally came from this early gene, and therefore they're all going to have similar DNA sequences. Orthologs are going to be genes that are similar because they are derived from the same alpha gene, but they're actually found in different organisms. Paralogs are going to be in the same organism, so these are both mice, but these are two genes that arose from original gene duplication, whereas the alpha gene became the alpha and then evolved into orthologs in different organisms, whereas paralog underwent gene duplication, but found in the same organism. So that's kind of the first cell. It's understanding that terminology.
Then, you have phylogeny, and phylogeny is going to be the evolutionary history of a group of organisms. And so, oftentimes, evolutionary biologists are presented with some kind of question. That question is, how are these organisms related? Or how did this gene evolve between these different organisms? And they have to answer that question. And so to answer that question, they always apply 2 kinds of principles. The first is biological inference, and that is sort of inferring how genes came about. And to do that, they use a concept called parsimony, and that is a principle that says to always choose the simplest explanation. So the best example of this is we have mammals. So mammals don't lay eggs. Right? They do not lay eggs, we know that. But there are certain mammals that do, and so, platypuses, which is literally the plural of platypus, which bothers me for whatever reason. But anyways, platypuses are mammals that lay eggs. So how do they do that? Mammals don't lay eggs, but platypuses do. And so, therefore, platypuses have to contain genes that are important for egg development, including like egg yolks. They contain yolk genes, and other mammals don't have them because they don't produce eggs. So the question here, the biological inference that they have to make is, where do these yolk genes found in platypuses come from? And so there are 2 options here. Right? So what's the first option? Right. So that means that there was some kind of common egg-laying ancestor that produced all mammals, and then eventually, the other mammals just lost the yolk genes, and platypuses contained them and remained having them. So that's one potential. The second option, right, is that there wasn't this common egg-laying ancestor, but instead, these egg genes in platypuses evolved independently of other egg-laying organisms, like birds, for instance. Right? Because platypus is not the only thing that lay eggs, multiple organisms out there lay eggs. So here are your 2 options. So just using parsimony, which one would you pick? Would you pick A or would you pick B? Which one is the simplest? Is it simpler that pretty much all these organisms laid eggs, and then that evolved, and platypuses kept that, but mammals eventually lost it? Or is it that you have some organisms that evolved eggs and some that didn't, and then mammals evolved, and these kept evolving eggs. The mammals, well most of them didn't, but then a couple of the mammals actually evolved eggs separately from these other organisms over here. Hope it's clear that A is the simplest, that there was some kind of common egg-laying ancestor that the birds evolved from, that platypuses evolved from, and that mammals also evolved from, but mammals eventually lost it. And so, this is an example of how this type of genomics can sort of look at these evolutionary perspectives and figure out, you know, how the genomes compare, where these genes are coming from, and how they all are evolving. So with that, let's now move on.
