Conserved regions in mammalian beta-globins: could they arise by cross-species gene exchange?

Comparison of the nucleotide sequences from the coding regions of the four mammalian beta-globin genes shows that different parts of these genes have evolved at two different rates. Those codons designating amino acids 1-20, 41-91 and 109-146 have accumulated substitutions in a random fashion as the molecular clock hypothesis would predict. The codons at positions 21-40 and 91 to 108 behave as if they evolved at a much slower rate. Each of the slowly evolved regions contains an intron. Conservation of the coding sequences flanking the introns are hypothesized to be the result of corss-species gene exchange.