During the second episode of Cosmos, Tyson took a trip to the realm of the very small to show us the genetic machinery. He talked about how a small mutation can have big long-term consequences and how the differences in the genetic code leads to the differences between the species. After establishing the differences he went on to talk about the similarities between all living things. Proteins involved in metabolizing sugars, for example, are the same across all species. The reason, he explained, is that metabolizing energy sources happened too early, and is too important, to leave room for change later.

At this point there was a graphic on the screen depicting the DNA of various creatures and how it is the same in some locations. I thought for sure he was leading in to an interesting piece of evidence for evolution. I also knew, if any creationists address the point at all, they are likely to say something like, “That’s not evidence for evolution. They are the same because they were made by the same God.” There’s a problem with that response though and as we’ll see these shared functions actually provide a nice illustration of the fact of evolution.

Proteins, Amino Acids, and a Degenerate Code
Before we can see why we’ll need some background information (which I’ll try to keep as short and to the point as possible).

Within living cells genes are ultimately turned into proteins. Genes are sequences of nucleotides (guanine, adenine, thymine, and cytosine) and when taken in threes the nucleotides code for a particular amino acid. Proteins are simply chains of these amino acids.

But the code is degenerate which simply means that different three nucleotide sequences code for the same amino acid. For example the amino acid leucine can be coded for by any of the triplets TTA, TTG, CTT, CTC, CTA, or CTG. What this means is that mutations can happen to these sequences and the overall function of the gene, as expressed in the protein, is not changed at all! They are called neutral mutations because they don’t change anything, and they happen all the time, at a steady rate.

Another Way to Grow an Evolutionary Tree
So here we have a nice way to test for evolutionary relationships. Species that shared a common ancestor more recently should have fewer of these types of neutral mutations separating their DNA sequences in the shared genes. Since the genes do the same thing this can’t be explained as serving some different function in each.

So what do you think we find when we compare sequences? Exactly what we would expect to find if species are related by evolution from common ancestors! Evolutionary trees derived from such comparisons very nicely match the evolutionary trees we had already independently deduced from anatomy and the fossil record. Those species that diverged from each other recently have fewer substitutions (human and chimp genes for cytochrome c differ by only 4 substitutions for example) and those that diverged long ago have many.

Things didn’t have to come out this way. We could have found that there was very little difference between sequences or that they diverged in ways that couldn’t be explained by evolution. Ultimately the creationist dodge I noted at the beginning fails. The genes in question differ in ways that allows us to see how they got that way and, unless their god purposely set out to deceive everyone, they didn’t get there by any special creation of each species.

This piece of genetic evidence is just one of many that can be found in our genome. Genetics today provides some of the most powerful evidence we have today for evolution. Our evolutionary history history is literally written in our genes.

Further Info
I do want to point out a couple simplifications I’ve made. First, the type of mutations I mentioned aren’t the only kind of neutral mutation that is found. More drastic changes can happen while still preserving function. This is because all that matters in the end is how the protein folds up to do its job.

The second is that there are exceptions to the rule. Some species, and comparisons between species, differ from the mutation rate we might expect. This is because mutation is a random process. With so many species it is to be expected that unusual things happen from time to time. The important thing is the big (average) picture which can’t be explained except with evolution. For more information and discussion of these technical details see Douglas Theobald’s excellent article and his rebuttal of a misguided creationist response to it. (Unfortunately I can’t link directly to the relevant section of the second link but look for the heading ‘Insufficient Knowledge of Basic Molecular Biology and Genetics’ and continue until you get to ‘Prediction 4’).