FORTY YEARS OF CJENECOLOQY
183
than one latitudinal or altitudinal climatic belt will be found to possess
clines for the physiological characteristics adapting them to the conditions
prevailing in different parts of their range” has now been fully justified.
A special type of clinal variation occurs in polymorphic species when
successive populations show progressive change in the proportion of the
morphs (Huxley, 1955). Such ratio clines frequently reflect directly the
distribution of allelic genes, and several examples have now been described. Thus Harland (1946) detected a cline in Ricinus comrnunis in Peru
in the proportional representation of genes for stem bloom (glaucousness, or waxiness). Bloom is determined by a pair of alleles, B-b. The
castor bean is not native to Peru, but it is naturalized as a weed, as well
as being in cultivation, up to considerable altitudes. Passing inland from
Lima to an altitude of 7 764 ft the proportion of plants bearing bloom
increases from 0-15y0 to lOOyo. I n experimental cultivation in the Lima
district, homozygous B type plants were found to be sterile in the winter,
whilst bb plants fruited throughout the cold season, although at a
diminished rate. The winter sterility of BB plants Harland attributed to
the cold and fog of this season in Lima; the increase of such plants with
increasing elevation inland is associated with an increase in the amount
of sunlight and the diminution of fog. I n what way glaucousness is
physiologically disadvantageous in the Lima climate is not apparent. It
may be that bloom is simply a superficial expression of some unknown
more profound physiological condition, or, as Harland suggests, selection has been for some closely linked gene of physiological significance.
Parallels to Harland’s Ricinus case occur in several species of Eucalyptus in Tasmania, where clines in the incidence of glaucousness have been
described by Barber (1955) and Barber and Jackson (1957). The
glaucousness of leaves and stems is considered to be under oligogenic
control, and the clines, detected by field inspection, are taken by Barber
to represent changes in the allelic frequencies at one or more loci controlling wax development. I n three species, E. gigantea, E . gunnii and E ,
coccifera, parallel clines were observed in a transect passing from altitudes of c. 2 500 ft to 4 000 f t in north central Tasmania. I n E. gigantea
which has the lowest altitudinal range, glaucousness is dominant in the
lowest populations; thereafter to 3 500 ft glaucous types are absent,
when they reappear to achieve a frequency of 60% at 3 750 ft. I n E.
gunnii and E. coccifera a simpler clinal pattern appears, with nonglaucous plants at lower elevations giving place quite rapidly to glaucous
ones with increasing altitude. In E. gunnii there is an additional cline in
glaucousness of juvenile foliage which is said not to be in step with that
in the adult-type foliage. Although the clines described by Barber are
described as involving changes in proportion of glaucous and nonglaucous genotypes, he also indicates that growth conditions affect
183
than one latitudinal or altitudinal climatic belt will be found to possess
clines for the physiological characteristics adapting them to the conditions
prevailing in different parts of their range” has now been fully justified.
A special type of clinal variation occurs in polymorphic species when
successive populations show progressive change in the proportion of the
morphs (Huxley, 1955). Such ratio clines frequently reflect directly the
distribution of allelic genes, and several examples have now been described. Thus Harland (1946) detected a cline in Ricinus comrnunis in Peru
in the proportional representation of genes for stem bloom (glaucousness, or waxiness). Bloom is determined by a pair of alleles, B-b. The
castor bean is not native to Peru, but it is naturalized as a weed, as well
as being in cultivation, up to considerable altitudes. Passing inland from
Lima to an altitude of 7 764 ft the proportion of plants bearing bloom
increases from 0-15y0 to lOOyo. I n experimental cultivation in the Lima
district, homozygous B type plants were found to be sterile in the winter,
whilst bb plants fruited throughout the cold season, although at a
diminished rate. The winter sterility of BB plants Harland attributed to
the cold and fog of this season in Lima; the increase of such plants with
increasing elevation inland is associated with an increase in the amount
of sunlight and the diminution of fog. I n what way glaucousness is
physiologically disadvantageous in the Lima climate is not apparent. It
may be that bloom is simply a superficial expression of some unknown
more profound physiological condition, or, as Harland suggests, selection has been for some closely linked gene of physiological significance.
Parallels to Harland’s Ricinus case occur in several species of Eucalyptus in Tasmania, where clines in the incidence of glaucousness have been
described by Barber (1955) and Barber and Jackson (1957). The
glaucousness of leaves and stems is considered to be under oligogenic
control, and the clines, detected by field inspection, are taken by Barber
to represent changes in the allelic frequencies at one or more loci controlling wax development. I n three species, E. gigantea, E . gunnii and E ,
coccifera, parallel clines were observed in a transect passing from altitudes of c. 2 500 ft to 4 000 f t in north central Tasmania. I n E. gigantea
which has the lowest altitudinal range, glaucousness is dominant in the
lowest populations; thereafter to 3 500 ft glaucous types are absent,
when they reappear to achieve a frequency of 60% at 3 750 ft. I n E.
gunnii and E. coccifera a simpler clinal pattern appears, with nonglaucous plants at lower elevations giving place quite rapidly to glaucous
ones with increasing altitude. In E. gunnii there is an additional cline in
glaucousness of juvenile foliage which is said not to be in step with that
in the adult-type foliage. Although the clines described by Barber are
described as involving changes in proportion of glaucous and nonglaucous genotypes, he also indicates that growth conditions affect