Light micrograph of a red blood cell sample showing sickle-cell anaemia

Balancing selection

Natural selection should weed out alleles with a harmful effect from the gene pool. So why do some still hang around?

The sickle-cell allele is a mutant form of the gene coding for a component of haemoglobin, the oxygen-carrying pigment in red blood cells. Given how serious sickle-cell disease is, why does this allele persist?

The answer lies in selective pressures favouring the sickle-cell allele: although having two copies of the allele (one from each parent) is bad news, having one can be an advantage – it protects against infection with the malaria parasite.

So in regions where malaria is present, there are ‘balancing’ selective pressures at work. The losers are those unlucky enough to inherit two copies of the sickle-cell allele.

Old age

Harmful alleles can also survive if they act late in life, as in the case of Huntington’s disease. The disease-causing allele doesn’t show its effects until later life, after the age of reproduction. As it does not affect someone’s reproductive success, natural selection cannot act upon it.

It’s possible that other alleles have negative effects in old age while being advantageous when we are young.

It all depends…

This highlights the importance of context. A gene or allele of a gene may only be ‘bad’ for us because our circumstances change. Some alleles seem to predispose to obesity or cardiovascular disease by altering our metabolism. This may be bad for us now, but in the past it might have been an advantage – with resources scarce, being able to use fats more effectively might have been very handy.

Finally, although there’s still debate about this, natural selection might not have ‘caught up’ yet. For instance, alleles of the ApoL1 gene are associated with an increased risk of kidney disease in African Americans. It’s likely that these alleles once afforded protection against infection with trypanosome parasites.

Although this may have been advantageous for their ancestors in Africa, African Americans are unlikely to encounter trypanosomes and consequently suffer the drawback but not the benefits of the ApoL1 alleles.

Most harmful alleles, though, are quickly lost – a process known as purifying selection.

Lead image:

isis325/Flickr CC BY


About this resource

This resource was first published in ‘Evolution’ in January 2007 and reviewed and updated in December 2014.

Genetics and genomics, Health, infection and disease
Education levels:
16–19, Continuing professional development