Recently, there have been several studies to look into the Ka/Ks of alternatively spliced genes. These studies have advanced our knowledge about the selective force on alternatively spliced genes during evolution. Studies on the Ks of alternatively spliced exons also reveal widespread "RNA-level" selection pressure in the mammalian genomes that are not associated with functional constraints on proteins. Synonymous mutations in alternatively spliced exons are NOT selectively neutral, but are under strong functional constraints for proper regulation of alternative splicing.
I'll write about these studies in detail next week.