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?

Answer 1

The amount of adaptive evolution within a species is determined by this test. In this test the amount of variation within a species (polymorphism) to the divergence between species (substitutions) at two types of sites, neutral and nonneutral.

According to the test the positive selection occurs if ratio of nonsynonymous to synonymous variation within species is lower than the ratio of nonsynonymous to synonymous variation between species (i.e. D_n/D_s > P_n/P_s)

It is given that pN/pS is lesser than dN/dS, so it is a positive selection. As it is known that neutrality index quantifies the direction and degree of departure from neutrality.

NI = (Pn/Ps)/(Ds/Dn)

Since our Pn/Ps is lesser so the NI will be less than 1. we knoe that silent mutations are neutral, and a neutrality index lower than 1 (i.e. NI < 1) represents an excess of nonsilent divergence, when there is positive selection.