Question

Please show/ explain:

Silversword plants have undergone an adaptive radiation in Hawaii, and they are closely related to a species called tarweed. Researchers used the McDonald-Kreitman test to look for different types of selection of 5,000 different loci in both silverswords and tarweed. They found that in many transcription factors, polymorphism within silverwoods (pN/pS) was less than divergence between silverswords and tarweed (dN/dS). Does this indicate selection, and if so, what type? What does it suggest happened during the silversword radiation?