In several threads now, creationists have made the accusation that scientists only assume that the genetic differences between species is due to mutations. This is absolutely false, as I will show with the following example.
The first thing we need to establish is why scientists call them random mutations. What scientists mean is that mutations are random with respect to fitness. This means mutations can be neutral, beneficial, or detrimental. Due to negative selection, detrimental mutations tend to dwindle in number within a population while neutral mutations prosper or fail based mostly on chance.
So what does this mean with respect to our topic? Knowing that mutations are random with respect to fitness, and that natural selection will remove detrimental mutations in functional genes, we can predict what types of patterns we should see when we compare the genomes of different species.
What we should see is sequence conservation in functional DNA. This is observed as having more similarity between species in the coding region of genes and less similarity in non-coding regions of a gene where the sequence doesn't matter. So do these types of regions exist?
That takes us to a discussion of introns and exons. In eukaryotes, the functional part of a gene is broken up into the coding sections called exons, and the non-coding parts called introns. The mnemonic device I use to keep them separate is that the introns are like interstates that connect the cities, which are the exons. When the gene is transcribed into RNA the introns are clipped out and only the exons are used to make the protein.
Therefore, mutations in the introns do not affect the amino acid sequence of the resulting protein whereas many of the mutations in the exons will result in changes in the amino acid sequence.
If random mutations are responsible for the genetic differences between cities, then we should see more differences in the introns than in the exons when we compare these genes. This is because detrimental mutations can occur in exons, but they would be very rare in introns of a gene. Negative selection would remove more changes from the exons than the introns, creating little islands of conserved sequence.
So is that what we see? Sure is. This is a section from Dr. Francis Collins (former head of the NIH Human Genome Project, and current director of the NIH) excellent essay entitled "Faith and the Human Genome":
http://www.asa3.org/ASA/PSCF/2003/PSCF9-03Collins.pdf
Those little islands of conserved sequence that you see towards the bottom of the figure are the exons. As we increase evolutionary distance we see less and less conserved sequence except in the exons, right where we should see it. As Dr. Collins says, this is smoking gun evidence for evolution.
More to the point, I would like to see any ID/creationist explain why there should be more differences in the introns of genes as compared to the exons, if they want to claim that evolution is not supported by these observations.
The first thing we need to establish is why scientists call them random mutations. What scientists mean is that mutations are random with respect to fitness. This means mutations can be neutral, beneficial, or detrimental. Due to negative selection, detrimental mutations tend to dwindle in number within a population while neutral mutations prosper or fail based mostly on chance.
So what does this mean with respect to our topic? Knowing that mutations are random with respect to fitness, and that natural selection will remove detrimental mutations in functional genes, we can predict what types of patterns we should see when we compare the genomes of different species.
What we should see is sequence conservation in functional DNA. This is observed as having more similarity between species in the coding region of genes and less similarity in non-coding regions of a gene where the sequence doesn't matter. So do these types of regions exist?
That takes us to a discussion of introns and exons. In eukaryotes, the functional part of a gene is broken up into the coding sections called exons, and the non-coding parts called introns. The mnemonic device I use to keep them separate is that the introns are like interstates that connect the cities, which are the exons. When the gene is transcribed into RNA the introns are clipped out and only the exons are used to make the protein.
Therefore, mutations in the introns do not affect the amino acid sequence of the resulting protein whereas many of the mutations in the exons will result in changes in the amino acid sequence.
If random mutations are responsible for the genetic differences between cities, then we should see more differences in the introns than in the exons when we compare these genes. This is because detrimental mutations can occur in exons, but they would be very rare in introns of a gene. Negative selection would remove more changes from the exons than the introns, creating little islands of conserved sequence.
So is that what we see? Sure is. This is a section from Dr. Francis Collins (former head of the NIH Human Genome Project, and current director of the NIH) excellent essay entitled "Faith and the Human Genome":
http://www.asa3.org/ASA/PSCF/2003/PSCF9-03Collins.pdf
Those little islands of conserved sequence that you see towards the bottom of the figure are the exons. As we increase evolutionary distance we see less and less conserved sequence except in the exons, right where we should see it. As Dr. Collins says, this is smoking gun evidence for evolution.
More to the point, I would like to see any ID/creationist explain why there should be more differences in the introns of genes as compared to the exons, if they want to claim that evolution is not supported by these observations.
