In an article about Tibetan adaptation, they talk about a population branch statistic. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3711608/
By comparing the three pairwise FST values between these three samples, we can estimate the frequency change that occurred in the Tibetan population since its divergence from the Han population (5, 9). We found that this population branch statistic (PBS) has strong power to detect recent natural selection (Fig. S3).
I was wondering how it was calculated and what that means (lower vs larger values).
It seems that it can tell when there was natural selection at a certain time.
What are the assumptions of this model?