Question

You are examining human genome-wide variation from a large sample collection. How would you test for evidence of purifying (background) selection and positive selection, distinguishing between them? Include an example.

Answer 1

Positive selection causes increase in frequency of advantageous genetic variants. Whereas, purifying selection is used to remove deleterious mutations to maintain the stability of biological structures.

Positive selection is also called as Darwininan selection. Purifying selection is also called as negative selection.

Purifynig selection is done by purging deleterious genetic polymorphisms that are caused by random mutations.  The incidental purging of non-deleterious alleles due to their spatial proximity to deleterious alleles is called background selection. An example for background sel cetion is when a wild species plant that is used as a source of desired traits, making a crossbreed between the cultivated tomato plant and the wild tomato plant will giev rise to a new plant that will have desired wild tomato plant traits but any other undesired feature will be present along withe the desirable background genetic material.

An example for purifying selection: Found in living fossils. If a species ecological niche does not change for millions of years, their fossils can be indistinguishable from their present day descendants.

Human brain and its advanced cognitive abilities are examples of positive selection.