It's Wednesday, which means its time to answer a question from you lovely folks. Today's comes from Paul, who asks
More interesting to laymen like me would be if you spelt out how we can tell that a particular allele is being favoured by positive selection pressure
Ok yes, this isn't really a question. But I think it's an interesting topic worth dealing with anyway. Before we can do all of that though lets back up a bit. You need to take Evo 101 before we can tackle Paul's not-question. So settle down for a lecture from Mr Internet.
Evolution is basically three things happening:
- Organisms reproduce with variation.
- Some of those variants do better/worse than others and so have more/less babies
- Over time the frequency of those variants in a group changes (e.g., a successful variant of zebra has more kids so that variant more common in their herd of zebra).
The last example I gave there is a case of positive selection: evolution favouring a beneficial trait, prompting it to be spread throughout the group. And it's worth noting this doesn't just apply to whole animals like zebra. There are also variants of genes (called alleles) and they can also be selected for. For example, we've got a gene for hair colour; with the variations of that gene responsible for black, blonde, red hair etc. being alleles of that gene.
So how to we spot if one of these alleles is beneficial and undergoing positive selection?
Well lets keep going with our little hair example. Imagine a group of cavemen. One third have black hair, one third have blonde hair and the last third have red hair. If there's no selection going on those proportions should remain relatively stable since natural selection isn't pushing them in any direction. Some changes in frequency may still occur by chance (or more formally called " genetic drift "). But all else being equal if we were to return to our cavemen 10 generations later they should still have the same distribution of hair colour.
Now imagine that a new species of lion turns up and it's just terrified by the colour red. Suddenly being ginger has an advantage! So through evolution we'd expect to see that variant become more common, as ginger people survived longer and so had more kids. This same principle is true in genes: if a gene is being influenced by evolution it will have fewer alleles, because most are being out-competed by the beneficial allele.
However, positive selection isn't the only form of selection that can result in this state of affairs. Purifying selection, for example, occurs when there's an important allele. It's so important that even the tiniest change can be harmful, so any new alleles that crop up are killed off straight away. How do we tell the difference between these other forms of selection and positive selection?
The most common method is to look at history. Genes undergoing purifying selection will pretty much always have a few alleles (until something changes). So if you look at genes in older populations, you'll also see few alleles for those genes. Or even if you look at closely related species you should still see the same lack of variation. However, positive selection can take place at any moment, so this conservatism won't necessarily be present.
If a gene has a lot of variation in chimps but none in humans then that indicates that it's been undergoing positive selection in humans.