FORTY YEARS OF GENECOLOGY
197
same population selective forces could act in a stabilizing way on
some phenotypic features and, simultaneously, in a directional or disruptive way on others. Moreover, multipolar disruptive selection could
be imagined in which more t,han two expressions are simultaneously
favoured.
In considering how these selective situations are realized in populations of higher plants, some sense of the kinds of compromise involved in
long and short term adaptation is necessary. A plant population which
survives in a given habitat must do so in the face of various inimical
secular factors and against the predation and competition of other
organisms. Since survival depends upon a variety of properties (some,
like the needs to maintain gas exchanges and yet control water loss, even
antagonistic) a degree of compromise on the physiological level must
always be involved.
The success of that achieved by any given phenotype could be
measured by various criteria, but so far as the persistence of the population is concerned reproductive performance is obviously a principal one.
In the relative sense in which the concept of adaptation is commonly
applied, we might say that a population is well adapted to its habitat
when it succeeds a t least in maintaining its numbers through successive
generations.
If this is achieved over an interval of time in an outbreeding population without phenotypic change it may be supposed that the selective
forces at work are acting in a stabilizing way to eliminate phenotypes
deviating from what is evidently an adaptive mode. The extent of restriction of the phenotypic distribution provides a measure of the intensity of selection. Stringent selection must involve the loss of genetic
variation, but as we have seen where polygenic systems are concerned
some will always be conserved in a cryptic state.
Now a further element of compromise is involved in relation to environmental change. Plant habitats are subject to regular cyclical
changes of different periods ; to random fluctuations, again of different
periods; and to long term trends of change. Plant populations may be
accommodated to these changes in various ways. Taking first the nondirectional changes, a relationship of the first importance is between the
duration of the life span and the period of the cycle or fluctuation. If the
life span is such as to exceed the period of cyclical change, or the average
period of some habitually fluctuating environmental variable, it must be
supposed that a level of physiological adaptation has been achieved
which permits the effects to be absorbed. The patterns of developmental
periodicity seen in perennials, discussed more fully in a later section
(p. 227 et seq.), represent physiological solutions to the problem of accomodating annual variation in climate. Forest flora must also preserve
197
same population selective forces could act in a stabilizing way on
some phenotypic features and, simultaneously, in a directional or disruptive way on others. Moreover, multipolar disruptive selection could
be imagined in which more t,han two expressions are simultaneously
favoured.
In considering how these selective situations are realized in populations of higher plants, some sense of the kinds of compromise involved in
long and short term adaptation is necessary. A plant population which
survives in a given habitat must do so in the face of various inimical
secular factors and against the predation and competition of other
organisms. Since survival depends upon a variety of properties (some,
like the needs to maintain gas exchanges and yet control water loss, even
antagonistic) a degree of compromise on the physiological level must
always be involved.
The success of that achieved by any given phenotype could be
measured by various criteria, but so far as the persistence of the population is concerned reproductive performance is obviously a principal one.
In the relative sense in which the concept of adaptation is commonly
applied, we might say that a population is well adapted to its habitat
when it succeeds a t least in maintaining its numbers through successive
generations.
If this is achieved over an interval of time in an outbreeding population without phenotypic change it may be supposed that the selective
forces at work are acting in a stabilizing way to eliminate phenotypes
deviating from what is evidently an adaptive mode. The extent of restriction of the phenotypic distribution provides a measure of the intensity of selection. Stringent selection must involve the loss of genetic
variation, but as we have seen where polygenic systems are concerned
some will always be conserved in a cryptic state.
Now a further element of compromise is involved in relation to environmental change. Plant habitats are subject to regular cyclical
changes of different periods ; to random fluctuations, again of different
periods; and to long term trends of change. Plant populations may be
accommodated to these changes in various ways. Taking first the nondirectional changes, a relationship of the first importance is between the
duration of the life span and the period of the cycle or fluctuation. If the
life span is such as to exceed the period of cyclical change, or the average
period of some habitually fluctuating environmental variable, it must be
supposed that a level of physiological adaptation has been achieved
which permits the effects to be absorbed. The patterns of developmental
periodicity seen in perennials, discussed more fully in a later section
(p. 227 et seq.), represent physiological solutions to the problem of accomodating annual variation in climate. Forest flora must also preserve