Question: are some mutations benefitial?

Penelope Mason answered on 13 Jan 2014:

Hello!

The short answer is yes – mutations that cause an organism to be more healthy or better able to compete with other organisms will be more likely to live to pass on that mutation hence the mutation is transmitted to the next generation and we get evolution. The organism changes because it has mutated, and the mutation has given it an advantage. But as usual, it’s far more complex than that! I say that because something being beneficial usually depends on context.

Mutations can be actively deleterious i.e. cause the organism to be less fit and healthy (or just plain dead), but if the mutation doesn’t affect the organism e.g. it’s lost it’s ability to make a particular essential amino acid but it happens to be living somewhere where it can get more than enough of that amino acid from it’s environment, the mutation acts as if it is neutral. The mutation may get passed to the next generation but there is no selection pressure for or against this so if it does get passed on, it’s owing to chance. However, if the organism needs to make the amino acid to survive, the mutation is much less likely to get passed on because now it’s causing the organism to be less healthy and competitive.

The same is true for beneficial mutations. One example of this is the mutation in the MCR1 gene which is involved in making dark pigment in the skin. This mutation tends to make people lighter. In tropical areas this would be a really deleterious (bad) mutation because people need the protection of dark pigment from the sun’s radiation. Here, if you are lighter, you are much more likely to get cancer so you will be less heathy.

However, the sun’s radiation also helps us make essential vitamin D. People with lots of pigment living in areas where there is little sunshine don’t make enough vitamin D, so when humans originally started migrating from tropical areas to colder places with less sun, having a mutation that made them lighter became more advantageous – it allowed them to make enough vitamin D, and as the sun wasn’t as strong, they were still protected so they didn’t get cancers too early. So the mutation went from being deleterious to beneficial for those people. Of course, people now move countries a lot. But this doesn’t matter because people with light skin can wear sunscreen in hot countries and dark skinned people can take vitamin D tablets in cold countries!

Another interesting example that illustrates the value of context is the mutation involved in sickle cell anaemia (SCA). Your cells have two copies of this gene in each cell and if you have one copy that is normal and the other that is mutated, you have an advantage if you live in particular countries because this makes you more immune to malaria, which is a disease that has killed a lot of people.

This all sounds good, but there is a problem – the mutation causes defects in your red blood cells, which carry oxygen round your body. If both copies of the gene in your cells are mutant, you get sickle cell anaemia, which is a disease of the blood that makes you very weak. So people with two copies of the normal gene have normal blood but are susceptible to malaria, and people with two copies of the mutant gene don’t get malaria but have SCA. The best thing to be is have one copy of each…

Finally, I just wanted to mention mutation in other organisms – for example bacteria can mutate very fast and can evolve ways to become immune to antibiotics. This is obviously very beneficial for the bacteria – and not at all beneficial for us!