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Mechanisms for Change: Mutation
By D. Jon Scott

What is a Mutation?  
A mutation is nothing but a random change in the gene sequencing of deoxyribonucleic acid (DNA) strands, such as those within the nuclear envelope of single-celled and multi-cellular organisms. There are any number of causes which can contribute to these changes, many of which are simple copying errors.  

Gene Transfer: Vertical Vs. Horizontal  

  • Vertical gene-transfer is linear. Simply stated, vertical transfer is when genes are passed from father to son, mother to daughter, etc. "Vertical" mutations occur only during this type of gene-transfer. Once a mutation is passed to the next generation, it ceases to be a mutation because the "mutant gene" was passed on succsessfully. E.g., it is no longer an error in gene transfer. For example, albinism is a common point mutation which occurs during horizontal transfer, however if it is passed from one generation to the next it is no longer a mutation because it was inherited rather than being a copying error. This is the mechanism for Darwinian/Neo-Darwinian evolution.

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  • Horizontal gene-transfer is very rare in eukaryotic, sexually-reproducing organisms. It basically occurs at the brother-to-sister level. That is to say, it is a change that occurs during the life duration of the cell/organism and is not the result of an error in vertical gene-transfer. This is the mechanism for Lamarckian* evolution.

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  • It should also be noted that all horizontal gene-transfer is mutation and horizontally-transfered genes can be passed vertically.
  • * Lamarckian evolution in this context refers to changes within the liftetime of an organism which may be passed down to the next generation. Perhaps a better term might be "neo-Lamarckianism". By this term I am not refering to, for example, cutting off a rat's tail and having it produce offspring without tails.

Types of Mutations  
  
1. Point Mutation  
Transfer: Vertical
Description: The single most common type of mutation. A simple error in the copying of one set of genes to another (such as in sexual reproduction). The average human carries about fifty to one hundred point mutations which were not inherited from their parents. Three of these change the 'behavior' of certain proteins and genes.
Cause(s): A nucleotide in one chromosomal position is replaced by another nucleotide, possibly from a different chromosomal position.
Effects: Point mutations which occur in the genotype have no externally observable effect on the organism it occurs in. However, occasionally a point mutation will occur within the phenotype DNA and alter the expression of certain genes - thus changing the physical form/function of  the organism as a whole. These mutations, however, are often very minor and virtually undetectable.
 
2. Additions and deletions   
Transfer: Vertical or Horizontal
Description: Another very common type of mutation. These occur when cells are dividing/copying DNA. Typically, breakage and realignment cause entire segments of DNA to be either lost completely, or less often appear more than once within the DNA strand.
Cause(s): Breakage and realignment occurs by duplicating/subtracting gene segments during vertical gene transmission, and certain type of horizontal transfer - on which case any additions might be parts of another organism's genome.
Effects: If the length of the added/deleted segment is not a multiple of three, the reading of the genes will become obscured or garbled within that segment, usually (but not always) resulting in harmful or lethal mutations. This occurs because the frame-reading of the DNA becomes misaligned - a process known as frame-shift mutation. Sometimes segments of DNA may be duplicated in block form. This is known as tandem duplication.
  
3. Chromosomal duplication   
Transfer: Vertical
Description: Duplicated copies of pre-existing chromosomes added to DNA sequence.
Cause(s): Occasionally, one or more chromosomes are duplicated during germ-line cell-replication, and the offspring inherit extra copies of those chromosomes.
Effects: Duplicating only one chromosome (or only one set of chromosomes) generally results in harmful mutations such as Down's syndrome in humans. When an organism recieves duplicate copies of every chromosome, it is known as polyploidy. Polyploidy is rare in animals and fungi yet very common in plants. It is estimated that 20% to 50% of new plant species arise as a result of polyploidy.
Example: This is the most rapid cause of speciation - two north american frog species (one a polyploid of the other) appear morphologically identical and can only be distinguished by their mating calls - can not interbreed and are therefor different species because of this one mutation in an individual in the recent past. 

This is particularly interesting in the creation/evolution controversy because morphologically the two animals are identical (and should be considered the same 'kind' by creationists), yet have become completely different species (one with more than double the genetic information) because of a single mutation! 
  
Note: It is interesting that this species would still exist, since the chances of it happening separately in both a male and a female are very low indeed. It seems that either this mutation arose in many offspring of a single pair of frogs, or a bit more likely, the mutation originated in a female who then reproduced parthenogeneticly (which occurs today in lepidosaurs - howeve rarely).

  
4. Chromosomal breakage and realignment   
Transfer: Vertical
Description:
Cause(s):
Effects:
  
5. Retroviral infection  
Transfer: Horizontal
Description: Retroviral infection occurs when a segment of a retrovirus' genome is added to the total genome of the host organism - sometimes (but rarely) replacing part of the host genome.
Cause(s): A retrovirus (such as HIV and some strains of luekemia) will try to get a cell to do the work of reproducing for it. To accomplish this, the retrovirus will insert part or all of its own DNA into the cell's genome.
Effects: Occasionally the inserted gene will mutate and become a permanent part of the host cell. Depending on where the gene is inserted, it can change the function of the cell.  

If the retrovirus infects a germline cell (sperm or egg), the new genes can be passed down vertically. The organisms offspring will carry the retroviral DNA, or "retrogenes" in every single cell of their body. The chances of this occuring are very slim.

Example: Scientists have identified three different insertion points in both humans and chimpanzees. They occur in exactly the same chromosomal position and must have occured at three different times while both chimpanzees and humans were a single species. And it must have occured in three direct ancestors within this species. 

There is simply no way these common insertion points could have been made by chance. Given the extreme complexity of the DNA strand and the likelyhood of vertical retrogene transmission, as well as the fact that if the retroviral DNA alters the expression of other genes and may be destroyed and replaced by neighboring cells, and the fact that all of this must occur in a germline cell, the probability of humans and chimpanzees sharing these retrogenes - in the exact same chromosomal positions - without having descended from a single ancestor (even if they were both infected by the same three viruses) are about equal to a tornado blowing through a junkyard and putting together two identical 747's!

  
6. Plasmids 
Description:
Cause:
Effects:
Example:
 
7. Bacterial DNA exchange   
Transfer: Horizontal
Cause:
Effects:
   
8. Higher level transfer 
Cause:
Effects:
  
9. Symbiotic transfer   
Transfer: Horizontal
Cause:
Effects:
   
10. Transposons  
Transfer: Horizontal
Description: Transposable gene segments. That is, segments of DNA which can cut themselves out of, and insert themselves into parts of the host organism.
Cause(s): Bacterial DNA exchange, some types of additions and deletions.
Effects:
Example:
  
11. Retro-Transposons 
Description: DNA or RNA inserted into a chromosome via retroviruses. These genes can transpose themselves from one organism to another.
Cause: Retroviral infection, Symbiotic transfer.
 
For more information, please see:
http://www.talkorigins.org/faqs/mutations.html by Richard Harter 
Much of the information on this page comes from that URL

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