FORTY YEARS OF GENECOLOGY
196
material for variation is genic change ; the devices through which variability is regulated in populations constitute collectively the genetic system (Darlington, 1939). The three principal components of the genetic
system may be identified as : (a) the chromosomal system, which establishes the rate of gene segregation and recombination; (6) the breeding
system, which governs the level of hybridity; and (c) the intrinsic and
extrinsic factors determining in interaction the sizes of breeding groups.
We will consider the first two components here ; the third is discussed in
a later section (p. 204).
Darlington (1939)’has referred to the “sum of the haploid number of
chromosomes and of the average chiasma frequency of all the chromosomes in a meiotic cell” as the recombination index. A high index must
determine a rapid rate of gene segregation and recombination, and so a
rapid flow of variation from the concealed to the overt form and back
again ; a low index must in contrast establish a slow rate of turn-over of
variation. In terms of the fitness-flexibility compromise, a high index
gives flexibility, at the expense of the continuous destruction of any
gene combinations giving fitness in the prevailing milieu, while a low
index preserves any existing state of fitness at the sacrifice of future
adaptive potential.
The balance between homozygosity and heterozygosity in a population is established by the prevailing system of breeding; in higher plants,
that is to say, by the various properties determining the habitual mode
of pollination. Cross-pollination may be promoted by sex differences,
dichogamy, heterostyly, and incompatibility mechanisms including
certation phenomena; and self-pollination may be imposed by cleistogamy or mechanical adaptation of the flower ensuring direct transfer of
pollen from anther to stigma. In many plants mixed breeding is the rule,
either, as Mather (1943) has pointed out, because of the inefficiency of a
cross-pollinating mechanism, or because of the existence of a genetically
determined system enforcing a regulated ratio of self- to cross-pollination. Such regulated systems must be subject to selection, but retrospectively, like other components of the genetic system.
Heterozygosity is a necessary prior condition for recombination and
for the release of concealed genetic variation ; cross-pollination thus acts
like a high recombination index to promote variability. Self-pollination
by raising the level of homozygosity restricts recombination and checks
the flow of variation, in the extreme case leaving it eventually distributed among homozygous pure lines. Cross-pollination and high recombination index stre complementary in promoting the flow of variability, but low recombination index and selftng constitute alternative
means to one and the same end, namely the enhancement of genetic
stability over a succession of generations.
196
material for variation is genic change ; the devices through which variability is regulated in populations constitute collectively the genetic system (Darlington, 1939). The three principal components of the genetic
system may be identified as : (a) the chromosomal system, which establishes the rate of gene segregation and recombination; (6) the breeding
system, which governs the level of hybridity; and (c) the intrinsic and
extrinsic factors determining in interaction the sizes of breeding groups.
We will consider the first two components here ; the third is discussed in
a later section (p. 204).
Darlington (1939)’has referred to the “sum of the haploid number of
chromosomes and of the average chiasma frequency of all the chromosomes in a meiotic cell” as the recombination index. A high index must
determine a rapid rate of gene segregation and recombination, and so a
rapid flow of variation from the concealed to the overt form and back
again ; a low index must in contrast establish a slow rate of turn-over of
variation. In terms of the fitness-flexibility compromise, a high index
gives flexibility, at the expense of the continuous destruction of any
gene combinations giving fitness in the prevailing milieu, while a low
index preserves any existing state of fitness at the sacrifice of future
adaptive potential.
The balance between homozygosity and heterozygosity in a population is established by the prevailing system of breeding; in higher plants,
that is to say, by the various properties determining the habitual mode
of pollination. Cross-pollination may be promoted by sex differences,
dichogamy, heterostyly, and incompatibility mechanisms including
certation phenomena; and self-pollination may be imposed by cleistogamy or mechanical adaptation of the flower ensuring direct transfer of
pollen from anther to stigma. In many plants mixed breeding is the rule,
either, as Mather (1943) has pointed out, because of the inefficiency of a
cross-pollinating mechanism, or because of the existence of a genetically
determined system enforcing a regulated ratio of self- to cross-pollination. Such regulated systems must be subject to selection, but retrospectively, like other components of the genetic system.
Heterozygosity is a necessary prior condition for recombination and
for the release of concealed genetic variation ; cross-pollination thus acts
like a high recombination index to promote variability. Self-pollination
by raising the level of homozygosity restricts recombination and checks
the flow of variation, in the extreme case leaving it eventually distributed among homozygous pure lines. Cross-pollination and high recombination index stre complementary in promoting the flow of variability, but low recombination index and selftng constitute alternative
means to one and the same end, namely the enhancement of genetic
stability over a succession of generations.