quarta-feira, 9 de abril de 2014


Hi again! Where does diversity come from? How did we get so much variation out there in nature (and also even within ourselves o.O)?

Today's subject: mutation!

Even though mutations often have a negative connotation, they are the main - and arguably only - constructive evolutionary force. Mutations are what give all the other forces something to work with: variability.

So, what are exactly mutations? I suppose most people have some understanding of what they are, considering how common words like "mutant" and "mutation" are in the pop culture (e.g. Teenage Mutant Ninja Turtles, X-Men, Hulk, Bioshock, etc.). Well, mutations are very simple. They are nothing more than changes to the genome of any living being (including even viroses!).

There are many ways in which mutations can happen, but these events can be categorised in simple manner in two groups: substitutions and indels (insertion/deletion). As you may know, DNA is formed by a sequence of nucleotides (A, C, G or T). Whenever one nucleotide is replaced by one other nucleotide (e.g. ACACAA --> ACATAA), this is called a substitution (or point mutation). When one letter disappears (e.g. ACACAA --> ACA-AA), it is a deletion; when a letter is added (e.g.  ACA-AA --> ACACAA), that is an insertion. These indels are structural changes and they can happen at large scale: Making copies of big chunks of a chromosome (duplications), Inverting the same big chunk and putting it back (inversions), or simply deleting a big chunk (also called deletions). Also, if two big chunks of DNA swap places, that is called a translocation.

Mutations can also be classified based on the effect they have on the fitness of the individual carrying it. They can be deleterious, nearly-neutral, neutral or beneficial, with negative to positive effect in the order they are listed. Mutations can yet be classified according to the effect on protein sequence (when they happen on protein-coding DNA), which kind of cell they occur in, or even their effect on function (which overlaps a lot with fitness effect).

Something that is particularly interesting about mutations is that, on the long term, they tend to happen with a predictable frequency - e.g. one mutation per nucleotide every million years. This may sound like a very rare event, but bear in mind that, even though this varies a lot in different organisms, genomes tend to have billions of nucleotides! This means that, in a genome of 1 billion nucleotides (or base pairs, bp for short), 1 thousand mutations will occur every year, in average. This property of mutations has been widely used to figure out when evolutionary events happened. If you compare the genomes of two species and observe that they are different in say 2% of their nucleotides, and if you an approximate divergence rate (e.g. 1% per million years), you can assert that these two species are separated for approximately 2 million years (plus or minus a confidence interval).

All in all, evolution would not happen without mutations! They are a necessary evil, one could say. But there is more to them. In the near future, we'll have a look on the effect of mutations in small populations. Spoiler alert: it's not good...

See you next time. Cheers!

Nenhum comentário:

Postar um comentário