Endogenous Retroviruses and Homologous Pseudogenes

One of the most convincing, albeit complicated, pieces of evidence for evolution is the endogenous retrovirus. Endogenous retroviruses are molecular remnants of a past parasitic viral infection. Occasionally, copies of a retrovirus genome are found in its host’s genome, and these retroviral gene copies are called endogenous retroviral sequences. Retroviruses (like the AIDS virus or HTLV1, which causes a form of leukemia) make a DNA copy of their own viral genome and insert it into their host’s genome. If this happens to a germ line cell (i.e. the sperm or egg cells) the retroviral DNA will be inherited by descendants of the host. Again, this process is rare and fairly random, so finding retrogenes in identical chromosomal positions of two different species indicates common ancestry.

Image

This may be the most boring picture I’ve used on this site.

This diagram shows a phylogenetic tree of several primates, including humans, from a recent study which identified numerous shared endogenous retroviruses in the genomes of these primates. The arrows designate the relative insertion times of the viral DNA into the host genome. All branches after the insertion point (to the right) carry that retroviral DNA – a reflection of the fact that once a retrovirus has inserted into the germ-line DNA of a given organism, it will be inherited by all descendents of that organism.

An easy way to disprove this idea would be to show a retrovirus that humans have and chimps do not have that is found in another, far more distant relative (in a dog or a duck or a zebra). To date, this has not been the case.

From Ken Miller, during the Dover trial:
“…hemoglobin. It’s made up of four parts. Those parts are called polypeptides, but we can think of them essentially as four subunits. It has two copies of a part called alpha-globin and two copies of a part called beta-globin…And as our genome does for many genes, we have multiple copies of these, so we have backups. We’ve got extra copies of the alpha-globin genes and extra copies of the beta-globin genes…I’ve zeroed in on the six copies of the beta-globin gene sequence.

Each of these copies is a set of instructions for how you build this polypeptide. Five of them work, but one of them doesn’t. It’s given the Greek letters psi, beta, and then the number one. And the psi-beta-1 sequence isn’t a gene. It doesn’t work. It’s a pseudogene, and a pseudogene is recognized as a gene because it’s so similar to the other five in its DNA sequence, but it has some mistakes. It’s broken, and it has a series of molecular errors that render the gene non-functional…there are six distinct mistakes in this gene…Now, the reason that this is important in evolution is actually very simple, and that is, these errors appear in a gene, they have no functional purpose.

And you might ask yourself, what would I do, what would you do if we were to find another organism that didn’t just have similar genes but also had a pseudogene in the same spot and had the same set of errors?  There’s no reason why evolution would produce a duplicate set of mistakes in two copies of things. It must mean that these two organisms are descended with modification from another organism that had the same set of mistakes.

And if you go on to the next slide, what I’d like to show you are three organisms, the gorilla, the chimpanzee, and the human being that share the exact same set of molecular mistakes.  Now, why is this significant? One of the core principles of evolution is common descent. One could always argue that because the three species that I’ve depicted on this slide are all African species, that’s where they all come from, they’re all primates and they all probably started out living in similar environments, that the functional parts of this gene locus, they might work the same. But you cannot argue that the mistakes should match.  And the fact that all three of these species have matching mistakes leads us to just one conclusion, and that’s the same conclusion that Charles Darwin predicted almost a century and a half ago, and that is that these three species share a common ancestor. Matching mistakes are evidence of common ancestry.”

How could anything but common ancestry explain endogenous retroviruses and homologous pseudogenes?

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~ by kriskodisko on September 4, 2013.

One Response to “Endogenous Retroviruses and Homologous Pseudogenes”

  1. Pretty dang cool.

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