Gene Evolution and the Hæmoglobins

The role of duplications in evolution

“…The finding that these chains can be very similar, but are evidently produced by different gene loci, has led to the suggestion that these particular loci have arisen by duplication (Ingram, 1961;Smithies, 1962;Lush, 1966). Lush (1966) refers to about ten such cases among eighty or more polymorphisms discussed in vertebrates other than man, including that of the enzyme plasma aryl esterase in the pig, where the alleles appear to be zero, one, two, three or four doses of a basic gene.…”

“…Lush (1966) refers to about ten such cases among eighty or more polymorphisms discussed in vertebrates other than man, including that of the enzyme plasma aryl esterase in the pig, where the alleles appear to be zero, one, two, three or four doses of a basic gene. However, the two proteins most closely examined are haptoglobin (Smithies, 1962(Smithies, , 1964 and haemoglobin (Ingram, 1961;Nance, 1963).…”

“…Smithies (1962Smithies ( , 1969 has emphasised the way that partial gene duplication can lead to production of a continuous, uninterrupted polypeptide covering the beginning of the duplication, so that it is distinct from the original polypeptide immediately, enabling the bypass of the possibly very slow process of random mutation followed by selection of mutant types. However, in the case of haemoglobin, Ingram (1961) has emphasised that at least two complete duplications, and possibly a third, have occurred, to produce the , fi, v and 3 chains, since these are all about the same length (140 amino acid residues) and differ, for example in man, from one another in about 10 to 50. Postulates required to explain the present nature of haemoglobin are as follows: as already mentioned, there have been several duplications, followed or accompanied by translocation (so that the 8 and 3…”

“…CQnsidering evolution as the accumulation of information (Kimura, 1961), duplication offers a number of ways of increasing the rate of the process. It can lead immediately to new gene products (Smithies, 1964); it can allow changes at one locus while a still advantageous point is made at the other (Ingram, 1961); it can increase the rate of production of particular products and their polymers (Hopkinson, Spencer and Harris, 1964;Smithies, 1964); and it can probably alter dominance relationships without the intervention of selection (Zimmerman and Gundelach, 1968). This assumes that duplications have occurred, since the chance of two very similar polypeptides arising independently is particularly low (Smithies, 1964), though such convergence could occur, and duplication provides the best explanation of such observations. Thus it may be said that, in so far as they can be met a posleriori, Stephens's (1951) criteria for the demonstration of the occurrence of duplication of evolutionary importance have been met.…”

“…duplication, and on the other hand that some proteins are made up of very similar subunits produced by different genes (Ingram, 1961;Smithies, 1962). Furthermore, Kimura (1 968a, b) has made calculations which suggest that there may be very high rates of neutral mutation of the base-pair replacement type and hence of basepair substitution, so that duplication may be more important than previously thought in making substantial changes unlikely to be neutral.…”