FORTY YEARS OF OENECOLOGY
201
reveals, a long-maintained, favourable environment with no marked
trend of secular change does not mean evolutionary stagnation. This is
presumably because there must always be ways in which an organism
can steal a march on others even, as it were, in Utopia. Stabilizing
selection must favour the “optimum” phenotype from the phenotypes
available in each generation, but given a continued release of genotypic
variation in phenotypic form some recombinants may attain new and
better optima in respect to the capacity for exploiting the very same
environment. A trend of change may thus be initiated within the community ; and in so far as a change in one species produces a new environment for others it may be supposed that the pattern of selective forces
will be in continuous flux.
A final matter meriting consideration in the context of this section
concerns selection in the course of migration. There can be little doubt
that the genecological differentiation observable in north temperate
species is the product of evolutionary change during or following immediately upon fairly recent species migrations of a major scale. In
general terms, it is possible to homologize the progressive change in the
character of the environment that a species is likely to meet at the
periphery of an expanding area with a temporal environmental trend
experienced in one locality. Indeed, for the short-lived species of temporary or fluctuating habitats there would seem to be no essential difference between the two situations ; the problems of matching an existing
environment with enough fit genotypes, while at the same time generating sufficient diversity to permit colonization of slightly different ones,
remains the same. With the perennial pre-adapted for the stable, closed
community there are some significant differences. The principal one is
that in the van of a migration centripetal selection is eased to the extent
that dispersal is into an open community. The case is analogous to that
in which a population is in a phase of rapid increase of numbers : variation may be expected to rise (Mather, 1953). Moreover, the “optimum”
phenotype of the open marginal community may not be that of the main
closed community, so the phenotypic distribution of the colonists may
not only reveal a higher variance but a different mean. The exact implications of this situation have not hitherto been worked out, although
it may be supposed that the effect will mainly be transient since the
progressive closing of the community may be expected to restore the
original balance of selective forces. The matter is considered again in a
later section (p. 209).
c. VERSATILE REPRODUCTIVE SYSTEMS
Obligate self-pollination, by putting an end to recombination, must in
the long run jeopardize the survival of a species; it might therefore be
201
reveals, a long-maintained, favourable environment with no marked
trend of secular change does not mean evolutionary stagnation. This is
presumably because there must always be ways in which an organism
can steal a march on others even, as it were, in Utopia. Stabilizing
selection must favour the “optimum” phenotype from the phenotypes
available in each generation, but given a continued release of genotypic
variation in phenotypic form some recombinants may attain new and
better optima in respect to the capacity for exploiting the very same
environment. A trend of change may thus be initiated within the community ; and in so far as a change in one species produces a new environment for others it may be supposed that the pattern of selective forces
will be in continuous flux.
A final matter meriting consideration in the context of this section
concerns selection in the course of migration. There can be little doubt
that the genecological differentiation observable in north temperate
species is the product of evolutionary change during or following immediately upon fairly recent species migrations of a major scale. In
general terms, it is possible to homologize the progressive change in the
character of the environment that a species is likely to meet at the
periphery of an expanding area with a temporal environmental trend
experienced in one locality. Indeed, for the short-lived species of temporary or fluctuating habitats there would seem to be no essential difference between the two situations ; the problems of matching an existing
environment with enough fit genotypes, while at the same time generating sufficient diversity to permit colonization of slightly different ones,
remains the same. With the perennial pre-adapted for the stable, closed
community there are some significant differences. The principal one is
that in the van of a migration centripetal selection is eased to the extent
that dispersal is into an open community. The case is analogous to that
in which a population is in a phase of rapid increase of numbers : variation may be expected to rise (Mather, 1953). Moreover, the “optimum”
phenotype of the open marginal community may not be that of the main
closed community, so the phenotypic distribution of the colonists may
not only reveal a higher variance but a different mean. The exact implications of this situation have not hitherto been worked out, although
it may be supposed that the effect will mainly be transient since the
progressive closing of the community may be expected to restore the
original balance of selective forces. The matter is considered again in a
later section (p. 209).
c. VERSATILE REPRODUCTIVE SYSTEMS
Obligate self-pollination, by putting an end to recombination, must in
the long run jeopardize the survival of a species; it might therefore be