Thermos aquaticus
Well-Known Member
Why do scientists conclude that random mutations coupled with natural selection are responsible for the genetic differences between species? The answer is the evidence. One of those pieces of evidence is the bias found in the types of mutations that separate species, and a great rundown of that evidence is found in an article written by Stephen Schaffner over at BioLogos:
Testing Common Ancestry: It’s All About the Mutations
First, let's look at the nucleotides themselves:
The four DNA nucleotides (G, C, A, and T) are pictured above, and you will notice that G and A are chemically similar (two rings) while C and T are chemically similar (single ring). Due to these chemical similarities it is easier for a G to be accidently changed out for an A when DNA is being copied, and the same for C and T. Again, this has to do with the basic chemical properties of the nucleotides. A mutation that switches out similar nucleotides is called a transition while a mutation that switches out dissimilar nucleotides is called a transversion.
Therefore, if random mutations are responsible for the differences between species then we should see this bias when we compare genomes. Let's start with just a comparison between human genomes:
As we can see, the most common types of substitution mutation are transitions which is a mutation that switches out similar nucleotides. This is exactly what we would expect to see of the known and observed mechanisms that produce random mutations were active in the past in the human genome. What gets more interesting is when we compare the human and chimp genomes:
We see the same exact pattern as we see when we compare human genomes. Transitions are the dominant type of mutation as we would expect to see if the known and observed natural mechanisms of random mutagenesis were responsible for the differences between the human and chimp genome. This pattern also extends to even more distantly related species as discussed in Dr. Schaffner's essay.
There is also another special type of mutation called a CpG. This stands for 5'--cytosine--phosphate--guanine--3'. In other words, this is mutation where a C turns into a T just upstream of a G in a DNA sequence. The C at this position is very susceptible to being methylated which causes it to mutate into a T. These are, by far, the most commonly observed random mutations, and once again this observed pattern matches what we find when we compare genomes:
This type of evidence is why scientists conclude that random mutations are responsible for the differences between genomes. It has nothing to do with faith or worldview. It has everything to do with observed facts.
Testing Common Ancestry: It’s All About the Mutations
First, let's look at the nucleotides themselves:
The four DNA nucleotides (G, C, A, and T) are pictured above, and you will notice that G and A are chemically similar (two rings) while C and T are chemically similar (single ring). Due to these chemical similarities it is easier for a G to be accidently changed out for an A when DNA is being copied, and the same for C and T. Again, this has to do with the basic chemical properties of the nucleotides. A mutation that switches out similar nucleotides is called a transition while a mutation that switches out dissimilar nucleotides is called a transversion.
Therefore, if random mutations are responsible for the differences between species then we should see this bias when we compare genomes. Let's start with just a comparison between human genomes:
As we can see, the most common types of substitution mutation are transitions which is a mutation that switches out similar nucleotides. This is exactly what we would expect to see of the known and observed mechanisms that produce random mutations were active in the past in the human genome. What gets more interesting is when we compare the human and chimp genomes:
We see the same exact pattern as we see when we compare human genomes. Transitions are the dominant type of mutation as we would expect to see if the known and observed natural mechanisms of random mutagenesis were responsible for the differences between the human and chimp genome. This pattern also extends to even more distantly related species as discussed in Dr. Schaffner's essay.
There is also another special type of mutation called a CpG. This stands for 5'--cytosine--phosphate--guanine--3'. In other words, this is mutation where a C turns into a T just upstream of a G in a DNA sequence. The C at this position is very susceptible to being methylated which causes it to mutate into a T. These are, by far, the most commonly observed random mutations, and once again this observed pattern matches what we find when we compare genomes:
This type of evidence is why scientists conclude that random mutations are responsible for the differences between genomes. It has nothing to do with faith or worldview. It has everything to do with observed facts.