Evolution__3rd_Edition

244 PART 2 / Evolutionary Genetics
Population genotypic distributions
(a) Linear relation (b) “Canalized” relation (c) Threshold relation
Phenotypic
value ( G)
Phenotypic
value ( P )
Population phenotypic distributions
Figure 9.12
The relation between genotype and phenotype. The genotypic
value is for a polygenic character, in which higher values might
be produced by more positive alleles. The shaded frequency
distributions for the phenotypes shown below the x-axes are
what would be observed, given the shape of the graphs, if the
Vein-inducing substance
Phenotype distributions
Population genotypic distributions
0 50 100
Expression ratio
Phenotypic
value ( G)
Phenotypic
value ( P )
Population phenotypic distributions Population phenotypic distributions
Population genotypic distributions
Phenotypic
value ( G)
genotypic frequency distributions were as illustrated.
(a) Linear relation. (b) “Canalized” relation.
(c) Threshold relation. In the canalized relation,
the phenotypic variance of a population is reduced
relative to the variance in the causal genotypic factor,
and vice versa in the threshold relation.
Phenotypic
value ( P )
Figure 9.13
A model of the relationship between genotype and phenotype
for the Drosophila fourth wing vein. The genotype produces an
amount of vein-inducing substance. The phenotype (vein length)
is drawn along the top; six different population frequency
distributions are shown. The graph shows the relation between
genotype and phenotype, and between the genotypic and
phenotypic frequency distributions of a population. The
phenotype in the expression ratio 60–80 range matches a
threshold jump in the genotype–phenotype relation; a bimodal
phenotypic frequency distribution arises there. Redrawn, by
permission of the publisher, from Scharloo (1987). © 1987
Springer-Verlag.
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