Biology Reference
In-Depth Information
Some genes/proteins show a rather different, discontinuous pattern of evolution.
Thus, for example, in both primates and artiodactyls, the growth hormone (
GH1
;
17q23) gene exhibits a pattern of near stasis punctuated by bursts of rapid evolution
during which the rate of evolutionary change has increased at least 25-fold (Ohta,
1993; Wallis, 1994, 1996). Interestingly, during mammalian evolution, only the cod-
ing region of the growth hormone gene corresponding to the mature protein has
exhibited rapid change whereas other regions of the gene (signal peptide, nonsyn-
onymous substitutions and 5
untranslated regions) have remained relatively
unchanged. This would be consistent with the rapid bursts of evolution being of
adaptive significance and Wallis (1997) suggested that this may have involved what
he termed
function switching
. Briefly, if primate GH were to have had biological func-
tions other than growth promotion viz. lactogenic (prolactin-like) activity, GH
could then also have played a role in maintaining the nutritional balance between
mother and young. Acquisition of its lactogenic function could have involved
changes in GH structure away from that best adapted to growth regulation to one
that represented an evolutionary compromise which optimized the protein for its
dual function. If the secondary function of GH had then changed (e.g. the adoption
of a pattern of seasonal breeding following migration or climatic change, in which
gestation and suckling no longer overlapped), selection for dual function would no
longer have operated and the selection pressure on GH structure would have been
related to its growth promoting function alone. If several amino acid substitutions
had already occurred, then the process of functional reversal involving simply the
reversal of each amino acid change would have been very unlikely. Rather, GH
would have adopted a new structural form, slightly different from the original, but
nevertheless adapted to its primary function. Switching back to dual function
might then have led to a new cycle of structural change, driven by selection but
without any overall change in function. This 'pushme-pullyou' mechanism of
func-
tion switching
can in principle be resolved by gene duplication with subsequent
divergence of the paralogues to perform different functions. Indeed, the gene dupli-
cations giving rise to the GH cluster occurred after the rapid burst of evolution in
primates. Once duplication had occurred, the rapid evolution of the pituitary
expressed
GH1
gene would probably have ceased although the relatively high evo-
lutionary rate would still have been maintained by the placentally expressed genes.
Whilst this idea is very appealing, it should be pointed out that the evolution of the
placental lactogens early on in the mammalian radiation might reasonably be
expected to have removed any selective pressure on the lactogenic properties of GH.
Another example of the intermittent acceleration and deceleration of the
nucleotide substitution rate is provided by the cytochrome
c
oxidase subunit IV
(
COX4
; 16q22-qter) gene at different stages of primate evolution (Wu
et al
., 1997).
In passing, it is interesting to note that conceptually, the idea of function switch-
ing had its origin in the days of the early evolutionists:
and 3
It is an error to imagine that evolution signifies a constant tendency to
increased perfection. That process undoubtedly involves a constant remodel-
ing of the organism in adaptation to new conditions; but it depends on the
nature of those conditions whether the direction of the modifications effected
shall be upward or downward.
T.H. Huxley (1888)
The Stuggle for Existence in Human Society
