196
J. HESLOP-HARRISON
One aspect of the function of the genetic system is the regulation of
the pheno5ypic expression of genetic variation. Latent variation may be
carried in the heterozygotes of a diploid population in the form of
recessive genes, which will reach expression only with the segregation of
homozygotes. Where phenotypic characters are governed by several
genes each with small individual effects other possibilities for the concealment of genetic variation exist. Mather (1943) has shown how with
such polygenic systems a constant phenotypic frequency distribution in
a population can be maintained by selection with the simultaneous
preservation of a high level of genotypic variation. Because a given
phenotypic expression can be determined by several polygene combinations, the selectively favoured group in each generation may be essentially uniform in adaptive characters, but will always transmit
genetic diversity to their progeny. Moreover, isolated populations may
attain phenotypic similarity under similar selective pressures by the
assembly of different polygene complexes each in a more or less homozygous state ; between them they will then preserve a reservoir of genetic
variation which will be tapped only when they pass back into breeding
contact.
In governing the flow of variability in a population, the genetic system necessarily affects the potentialities for response to selection; it is
thus one of the major factors to be taken into account in seeking to
understand the origin of adaptive variation in plant species. Baker
(1953, 1959) has considered the effects of differences in reproductive
methods on race formation, but he acknowledges that it would be
equally possible to “examine the consequences of the formation of
certain kinds of race upon breeding behaviour of the plants”. Indeed it is
barely justifiable to separate intrinsic and extrinsic aspects, for they are
always in interaction. The genetic system in the short run may determine the pattern of response to the environment, but environmental
selection acting in a retrospective mode will determine what kinds of
genetic system will prevail in the ultimGte )mrvivors in any particular
ecological situation. Nevertheless, however tangled may be the chain of
cause and effect, patterns of infraspecific variation will inevitably show
some relationship with genetic systems and the extrinsic factors impinging upon them.
B. MODES O F SELECTION
Mather (1953) has termed the three basic modes of selection stabilizing, directional and disruptive. Under stabilizing selection the mean
of the phenotypic distribution is favoured at the expense of the extremes; with directional selection, one extreme is favoured; and with
disruptive selection both extremes. It is obvious that in one and the
J. HESLOP-HARRISON
One aspect of the function of the genetic system is the regulation of
the pheno5ypic expression of genetic variation. Latent variation may be
carried in the heterozygotes of a diploid population in the form of
recessive genes, which will reach expression only with the segregation of
homozygotes. Where phenotypic characters are governed by several
genes each with small individual effects other possibilities for the concealment of genetic variation exist. Mather (1943) has shown how with
such polygenic systems a constant phenotypic frequency distribution in
a population can be maintained by selection with the simultaneous
preservation of a high level of genotypic variation. Because a given
phenotypic expression can be determined by several polygene combinations, the selectively favoured group in each generation may be essentially uniform in adaptive characters, but will always transmit
genetic diversity to their progeny. Moreover, isolated populations may
attain phenotypic similarity under similar selective pressures by the
assembly of different polygene complexes each in a more or less homozygous state ; between them they will then preserve a reservoir of genetic
variation which will be tapped only when they pass back into breeding
contact.
In governing the flow of variability in a population, the genetic system necessarily affects the potentialities for response to selection; it is
thus one of the major factors to be taken into account in seeking to
understand the origin of adaptive variation in plant species. Baker
(1953, 1959) has considered the effects of differences in reproductive
methods on race formation, but he acknowledges that it would be
equally possible to “examine the consequences of the formation of
certain kinds of race upon breeding behaviour of the plants”. Indeed it is
barely justifiable to separate intrinsic and extrinsic aspects, for they are
always in interaction. The genetic system in the short run may determine the pattern of response to the environment, but environmental
selection acting in a retrospective mode will determine what kinds of
genetic system will prevail in the ultimGte )mrvivors in any particular
ecological situation. Nevertheless, however tangled may be the chain of
cause and effect, patterns of infraspecific variation will inevitably show
some relationship with genetic systems and the extrinsic factors impinging upon them.
B. MODES O F SELECTION
Mather (1953) has termed the three basic modes of selection stabilizing, directional and disruptive. Under stabilizing selection the mean
of the phenotypic distribution is favoured at the expense of the extremes; with directional selection, one extreme is favoured; and with
disruptive selection both extremes. It is obvious that in one and the