The mtDNA sequence of Neanderthals is more similar to that of modern humans than to that of Denisovans. However, analyses of nuclear DNA clearly indicate that Neandertals and Denisovans share a more recent common ancestor than either of these hominins shares with modern humans. Propose a hypothesis to resolve the discrepancy between the mtDNA and the nuclear genome.

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Hi, everyone. Welcome back. Let's look at our next question. It says, which hypothesis could potentially explain the discrepancy between the mitochondrial DNA M T DNA and nuclear genome analysis in chimpanzees, Bonobos and gorillas. And we have four answer choices that are four different hypotheses that might explain this discrepancy. But before I walk through each of these answer choices and read them, and we think about them, let's first describe what this discrepancy is so that we're able to evaluate these answer choices. So the discrepancy in this case is that the mitochondrial DNA shows a closer relationship between these species indicating a more recent common ancestor than their nuclear DNA would indicate. So specifically, in the case of gorillas, the DNA, the nuclear DNA indicates that they diverged from chimpanzees and bonobos quite a long time ago compared to the mitochondrial DNA, which would indicate that they had a more recent common ancestor. Well, what would explain this? Why would you see this discrepancy? Why wouldn't they correspond? So let's go through our answer choices. Choice. A says intro GREs hypothesis. And that says that Chimpanzees and Bonobos interbred with gorillas leading to the transfer of M T DNA between these groups. Well, this would definitely be an excellent explanation for this discrepancy if you had guerrillas as the nuclear DNA indicates diverging from this line very far in the past. And then after the two lines diverged, you had interbreeding. So, interbreeding after speciation, then as we recall with mitochondrial DNA, it's inherited solely from the mother. So imagine a chimpanzee father and a gorilla mother, that offspring would have 100% gorilla, mitochondrial DNA. Even though it's actually a hybrid of the two species. If it's a female, it passes on to its offspring 100% gorilla, mitochondrial DNA. Now imagine in a population of mostly chimpanzees, but you have some individuals descended maternally from a gorilla, you then have this gorilla mitochondrial DNA indicating a more recent common ancestor, specifically the gorilla that interbred with the chimpanzees. So is a very good explanation for this discrepancy. I'm not going to choose it as the answer for sure. Just yet, just in case there's a better one. But I'll put a little dot here to indicate this, but I think we can agree. And in fact, if I were in a hurry, I would choose that and move on. But to be thorough, let's just look whenever we have what could potentially explain, it's a little bit nebulous. It doesn't necessarily mean there's single right answer and the others are completely wrong. So to be careful, let's look over these choice b genetic drift hypothesis. Random genetic mutations occurred in the mitochondrial DNA of chimpanzees and bonobos causing greater similarities to gorillas. Well, the genetic drift hypothesis does involve the random changes to the genome. So they're not caused by natural selection. It occurs when you have in a small population, certain genes get lost due to individuals dying out. And again, if you have a small population, that's gonna have an oversized effect on the representation of those genes in the population or to just certain individuals failing to reproduce. So that does involve random genetic mutations, but it involves the loss of genes through these random mutations. So you wouldn't expect we're not talking about just the loss of certain genes. We're talking about overrepresentation of certain genes in the mitochondrial DNA indicating a more recent common ancestor. So this isn't a good explanation for the discrepancy. We see choice is the mutational rate hypothesis which says the mitochondrial DNA of chimpanzees and Bonobos experienced a higher rate of mutation resulting in greater divergence from their nuclear DNA. Well, mitochondrial DNA does mutate at a higher rate more quickly than nuclear DNA. This is correct. But the mitochondrial DNA of gorillas would also be mutating more rapidly than nuclear DNA. So that won't explain why they seem to be more closely related because all three species would be experiencing this mutation. So choice C is not a good explanation. And finally, choice d the maternal inheritance hypothesis, which says the unique inheritance patterns of M T DNA resulted in a slower rate of genetic change compared to nuclear DNA in chimpanzees, bonobos and gorillas. Well, this just isn't correct. As we discussed in choice. C mitochondrial DNA changes more rapidly then nuclear DNA. So choice D cannot be correct. So we will eliminate that. And we can definitely say choice A is the best explanation. So which hypothesis, hypothesis explains the discrepancy uh in terms of when the lines diverge from each other. Choice. A the Intres hypothesis, chimpanzees and bonobos interbred with gorillas leading to the transfer of mitochondrial DNA between these groups. See you in the next video.

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Key Concepts

Mitochondrial DNA (mtDNA) Inheritance

Nuclear DNA and Phylogenetic Relationships

Introgression and Gene Flow Between Hominin Groups