132 REAL GENES, REAL INTELLIGENCEexception of categorical disorders, natu ral se lection tends to eliminate
deleterious gene variations, resulting in reduced heritabilities, as described
in chapter 2.Many Alternative Pathways
A defi ciency in the provision of a metabolite, either from the environment
or through a ge ne tic mutation, can usually be overcome by recruitment
or creation of an alternative biochemical pathway. So it is with some
astonishment that experiments have revealed that large portions of the
genome can be deleted without noticeable eff ects on basic functions.
Th e common yeast Saccharomyces cerevisiae has six thousand genes. Ex-
periments have shown that up to 80 percent of them can be deleted with-
out detriment to normal function under optimum conditions. Th is obser-
vation attests to the robustness of biological networks even at that level.^28
Andreas Wagner and Jeremiah Wright studied fi ft een diff er ent signal
transduction pathways and two large networks regulating transcription
and found many alternative pathways between demand, on the one
hand, and response in metabolism, on the other. Th ey concluded that
“multiple alternative pathways... are the rule rather than the exception...
such pathways can continue to function despite amino acid changes that
may impair one intermediate regulator. Our results underscore the im-
portance of systems biology approaches to understand functional and
evolutionary constraints on genes and proteins.”^29
Frederik Nijhout and colleagues were surprised to discover that, in
humans, many of the genes for enzymes in critical metabolic pathways
actually exhibit large degrees of variation. But as they also discovered,
although the eff ects of the gene variations are quite large at the molecular
level, the epige ne tic pro cesses “greatly reduce their eff ect at the pheno-
typic level.”^30 Th ey do not matter.
Identical Genes Do Not Restrict Variation
In complex forms and functions, the relationship between ge ne tic varia-
tion and phenotypic variation is extremely fuzzy. Th us, individuals with
identical sets of genes can be markedly diff er ent from one another. Ver-This content downloaded from 139.184.14.159 on Tue, 17 Oct 2017 13:52:50 UTChttp://www.ebook3000.com