PSEUDOGENES 

        What are they, where do they come from and what can we learn from them?

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Creation vs. Evolution:

Evolution vs. Creationism: The Debate Continues (?)

It is well known fact that evolutionary scientists and creationist differ over the origins of life.  Many arguments have been used by both sides to validate their views; so it is of no surprise that in the age of molecular biology that the two sides have found new ammunition -- namely pseudogenes -- to support their respective causes.

Evolutionary View:  

Evolutionary scientists believe that DNA structure similarities “agree remarkably well with the evolutionary trees derived earlier from anatomic similarities.”  Many genes, including the GLO gene (L-gulono-gamma-lactone oxidase) required for synthesizing ascorbic acid have been studied.  Guinea pigs and primates lack a functional GLO gene, while most other species can synthesize their own ascorbic acid and do not require this molecule in their diet.

 Molecular geneticists know that large gene deletions are rare, so scientists expected that a non-functional pseudogene of the GLO gene might be present in primates (ie. humans) and guinea pigs as relics of the functional ancestral gene.  Creationists, on the other hand, believe that humans and guinea pigs were created independently of all other species and must have been “designed” to function without GLO.  As such, these two species would not be expected to carry a defective copy of the GLO gene.  But pseudogenes have been detected in both guinea pigs and humans (Nishikimi et al., 1992; Nishikimi et al., 1994.) which is consistent with the evolutionary view.  The evolutionary model explains the presence of non-functional pseudogenes as the natural consequence of mutations that fail to be eliminated by natural selection because the function of the gene product has become unnecessary.  This model predicts that many pseudogenes should be found if scientists examine the genes of vestigial structures (ie. the genes encoding eye structures in blind species such as moles or cave-dwellers).  These vestigial structures serve no apparent function that could explain their design by a creator, but they can be understood in the evolutionary perspective as deriving from structures that were functional in ancestral species.

Creationists argue that features observed in the DNA of species, including tandemly repeated sequences were designed specifically by an intelligent creator; whereas evolutionists view them as resulting from accidental DNA duplications.  An argument in favour of the evolutionary view is that genetic accidents occurring in the DNA can be observed in the laboratory without divine intervention.

It has been stated that some processed pseudogenes are functional, but Edward E. Max (evolutionary scientist) argues that creationists are erroneously lumping together two distinct classes of retroposed gene copies: processed genes and processed pseudogenes.  As such, this argument is really just a matter of semantics.

How are non-functional pseudogenes preserved in all individuals of a species?

One possibility is that these sequences lie close to an advantageous gene and became prevalent in a population by natural selection.  Also, the burden of carrying even a large pseudogene sequence (ie. 100,000 nucleotides) is insignificant for a mammalian cell with three billion nucleotides worth of information.  In any case, there is no known mechanism for the cell to distinguish non-functional from functional DNA and selectively eliminate what it doesn’t need.

Deletions that by chance do not remove any functional genes could eliminate some useless DNA like pseudogenes; but an individual with such a deletion would not have any particular selective advantage as a result of the deletion.  Therefore the spread of DNA copies with this deletion into the population would be no more likely than the spread of any other inconsequential mutation (Max, 1986).  

Why aren’t primitive pseudogenes scrambled beyond recognition as a result of random mutations?

A possible answer is that our ancestors were under selective pressure to suppress retroposition, since high frequencies of retroposon insertion would increase the rate of genetic damage caused by crippling insertions into genes.  Also, the rate of mutation estimated for the complete obliteration of a typical retroposon requires over 100 million years.  Therefore, it’s not surprising to see these pseudogenes (Max, 1986).

How does one explain occurrences where pseudogene derived phylogenies do no match to the evolutionary phylogenies derived from anatomic similarities?

Edward E. Max argues that there are many exceptions in the long and complex history of life.  To disregard evolution because of specific cases that violate a simplistic interpretation of evolution and to ignore the vastly greater number of examples supporting evolution is foolish and improper science.  Cases are to be expected in which a pseudogene that arose in the ancestor of three modern species may get deleted in one, suggesting a closer relationship between the other two than is warranted.  As such caution should be encouraged in drawing generalizations from exceptional cases (Max, 1986).

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Last updated: March 21, 2002.