FORTY YEARS OF GENECOLOGY
199
while the ephemeral habit, representing as it does mere opportunism,
must be associated with the most labile of all kinds of community.
These relationships between life-span, non-directional environmental
change and community type have important implications for the operation of selection. I n the stable community, it is obvious that selection
will tend to act rather consistently in a stabilizing mode. In the less
stable communities of fluctuating environments selective pressures are,
in contrast, likely to be erratic -with the annual, because of year to
year climatic and other variations, and with the ephemeral because of
the seasonal changes themselves and other more catastrophic events.
What is to be considered the “optimum” will depend on the period of
time over which the selective influences are integrated.
The time scale of change in relation to longevity is again of paramount
significance where longer-term, directional, environmental change is
concerned. A long-lived perennial will be exposed during its life-time to
various environmental fluctuations. If the habitat is truly stable, the
average experience of each generation will be the same. If there is a
trend of some kind, the change will emerge as a progressive shift of the
average. The effect of selection will be to favour those phenotypes conforming in each generation to a gradually changing optimum. A model
situation may be seen in forest migrations of the post-glacial. Climatic
changes meant the shift of climatic belts latitudinally. During the
quaternary in the northern hemisphere the changes were slow enough to
allow the major forest communities of long-lived species to migrate in
step. The migrations took place through the replacement of one species
by another in consequence of climatic selection, but, simultaneously,
intraspecific selection achieved the necessary adjustments of developmental periodicity to fit the forest dominants to their new latitudes.
The situation of short-lived species, with their diverse patterns of
physiological adaptation to short-term environmental fluctuations,
differs in that the generation time is not sufficient to ensure an automatic averaging over several cycles of change. Yet the average condition
may be changing progressively; and survival of the population must
depend upon the ability not only to accommodate the fluctuations but
to shift in step with the average. If this is to occur, the response to
selection must be regulated in such a way that the transient influences
are without effect in producing short term changes in genetical structure
- which would involve a sort of evolutionary “hunting” like that of a
cybernetic system adjusted so as to give excessive feed-back - while
the long-term trends are met by progressive, smooth adjustment of the
average phenotype.
All of the foregoing considerations involve aspects of Mather’s fitnessflexibility compromise, and we may now examine how the different
199
while the ephemeral habit, representing as it does mere opportunism,
must be associated with the most labile of all kinds of community.
These relationships between life-span, non-directional environmental
change and community type have important implications for the operation of selection. I n the stable community, it is obvious that selection
will tend to act rather consistently in a stabilizing mode. In the less
stable communities of fluctuating environments selective pressures are,
in contrast, likely to be erratic -with the annual, because of year to
year climatic and other variations, and with the ephemeral because of
the seasonal changes themselves and other more catastrophic events.
What is to be considered the “optimum” will depend on the period of
time over which the selective influences are integrated.
The time scale of change in relation to longevity is again of paramount
significance where longer-term, directional, environmental change is
concerned. A long-lived perennial will be exposed during its life-time to
various environmental fluctuations. If the habitat is truly stable, the
average experience of each generation will be the same. If there is a
trend of some kind, the change will emerge as a progressive shift of the
average. The effect of selection will be to favour those phenotypes conforming in each generation to a gradually changing optimum. A model
situation may be seen in forest migrations of the post-glacial. Climatic
changes meant the shift of climatic belts latitudinally. During the
quaternary in the northern hemisphere the changes were slow enough to
allow the major forest communities of long-lived species to migrate in
step. The migrations took place through the replacement of one species
by another in consequence of climatic selection, but, simultaneously,
intraspecific selection achieved the necessary adjustments of developmental periodicity to fit the forest dominants to their new latitudes.
The situation of short-lived species, with their diverse patterns of
physiological adaptation to short-term environmental fluctuations,
differs in that the generation time is not sufficient to ensure an automatic averaging over several cycles of change. Yet the average condition
may be changing progressively; and survival of the population must
depend upon the ability not only to accommodate the fluctuations but
to shift in step with the average. If this is to occur, the response to
selection must be regulated in such a way that the transient influences
are without effect in producing short term changes in genetical structure
- which would involve a sort of evolutionary “hunting” like that of a
cybernetic system adjusted so as to give excessive feed-back - while
the long-term trends are met by progressive, smooth adjustment of the
average phenotype.
All of the foregoing considerations involve aspects of Mather’s fitnessflexibility compromise, and we may now examine how the different