204
J. HESLOP-HARRISON
It is apparent that a combination of sexuality and apomixis could be
specially advantageous in some ecological circumstances through offering simultaneously the ability to attain a high level of recombination
and the capacity to reproduce en mmse any well adapted genotypes that
may emerge (Heslop-Harrison, 1959~). In allowing the immediate fixation of an adapted genotype whatever its level of heterozygosity the
system has advantages over cyclical autogamy, where genetic stability
is only achieved with an approach to homozygosity.
A closely comparable situation exists where a species is capable of
efficient vegetative reproduction. A successful genotype, in achieving
vigorous growth, is immediately at a competitive advantage irrespective
of its reproductive performance through seed. If propagation is through
stolonifery or other direct vegetative means the dispersal capacity may
be limited, but an aggressive clone may be expected to spread within the
physical limits of the site to produce a highly homogeneous stand
(Harberd, 1961a).
It is significant that these versatile reproductive systems are commonly found in species - notably the perennial grasses - occupying
habitats of moderate permanency but of some ecological diversity. In
successional terms, they are neither ephemeral pioneers, nor yet necessarily components of climax vegetation. The fact accords well with the
theoretical advantages to be expected from the special properties of
their reproductive systems.
D. ISOLATION AND GENECOLOGICAL DIFFERENTIATION
The relationships set out in Table I V are obviously of great importance
for the interpretation of patterns of genecological differentiation in
plant species, particularly in connection with the much-debated matter
of ecoclinal as contrasted with ecotypic variation. We see that, in
general, the group of circumstances set out in the left-hand column of
the table will tend to favour continuity and so an ecoclinal type of
variation, while those in the right-hand column will favour discontinuity and so a pattern of variation interpretable in terms of discrete
ecotypes.
Table IV does not, however, account for all the agencies which may
act to generate variational discontinuity within a species, because it
does not cover those factors which impinge upon the operation of the
genetic system through their effect on the extent of breeding groups. I n
the obligate self-pollinator, the breeding groups constitute individuals.
In cross-pollinators, the size of the breeding group may be affected by
the ecological habit of the species, which establishes the density of individuals and the potential geographical range; by the nature of the
terrain, which controls the continuity of distribution; by the dispersal
J. HESLOP-HARRISON
It is apparent that a combination of sexuality and apomixis could be
specially advantageous in some ecological circumstances through offering simultaneously the ability to attain a high level of recombination
and the capacity to reproduce en mmse any well adapted genotypes that
may emerge (Heslop-Harrison, 1959~). In allowing the immediate fixation of an adapted genotype whatever its level of heterozygosity the
system has advantages over cyclical autogamy, where genetic stability
is only achieved with an approach to homozygosity.
A closely comparable situation exists where a species is capable of
efficient vegetative reproduction. A successful genotype, in achieving
vigorous growth, is immediately at a competitive advantage irrespective
of its reproductive performance through seed. If propagation is through
stolonifery or other direct vegetative means the dispersal capacity may
be limited, but an aggressive clone may be expected to spread within the
physical limits of the site to produce a highly homogeneous stand
(Harberd, 1961a).
It is significant that these versatile reproductive systems are commonly found in species - notably the perennial grasses - occupying
habitats of moderate permanency but of some ecological diversity. In
successional terms, they are neither ephemeral pioneers, nor yet necessarily components of climax vegetation. The fact accords well with the
theoretical advantages to be expected from the special properties of
their reproductive systems.
D. ISOLATION AND GENECOLOGICAL DIFFERENTIATION
The relationships set out in Table I V are obviously of great importance
for the interpretation of patterns of genecological differentiation in
plant species, particularly in connection with the much-debated matter
of ecoclinal as contrasted with ecotypic variation. We see that, in
general, the group of circumstances set out in the left-hand column of
the table will tend to favour continuity and so an ecoclinal type of
variation, while those in the right-hand column will favour discontinuity and so a pattern of variation interpretable in terms of discrete
ecotypes.
Table IV does not, however, account for all the agencies which may
act to generate variational discontinuity within a species, because it
does not cover those factors which impinge upon the operation of the
genetic system through their effect on the extent of breeding groups. I n
the obligate self-pollinator, the breeding groups constitute individuals.
In cross-pollinators, the size of the breeding group may be affected by
the ecological habit of the species, which establishes the density of individuals and the potential geographical range; by the nature of the
terrain, which controls the continuity of distribution; by the dispersal