Beta approximation for the two alleles Moran model by Stein's method

In work on the two alleles Moran model, Ewens showed that the stationary distribution for the number of genes of one type can be approximated by a Beta distribution. In this short note, we provide a sharp error term for this approximation. We show that this example fits perfectly into Doebler’s framework for Beta approximation by Stein’s method of exchangeable pairs.