386
CHARLES J. KREBS AND JUDITH H. MYERS
maximize individual fitness. They concluded that the observed patterns
of reproductive changes in cyclic rodents were consistent with the
hypothesis that individuals attempt to maximize fitness in all phases
of the cycle.
We can thus see how a polymorphism in spacing behavior might be
maintained in a population by means of time lags in adjusting to
density changes. This, however, leads us to a further question of
whether the behavioral and reproductive changes need be genetically
determined or could be the plastic properties of a flexible phenotype.
We do not know the relative advantages and disadvantages of adopting
a phenotypic self-regulating mechanism or a genotypic mechanism.
We see no way of determining a priori which way evolution should move
within the general scope of self-regulatory mechanisms.
Why does natural selection not damp out population fluctuations in
rodents? There are some genetic advantages to population fluctuations
that might answer this question. Carson (1968) has discussed the
genetic consequences of the “population flush” in insects. The low
density after the decline allows both the testing of new genotypes and
the possibility of rapid spread of new genetic combinations because of
founder effects. If this is true, we would predict a more rapid rate of
evolutionary change in species of voles and lemmings than in species
of rodents which maintain stable numbers. If we can postulate that
there are genetic advantages to population fluctuations, we could then
see why evolutionary changes have reinforced the fluctuations rather
than damping them away.
In summary, population regulation mechanisms must be under some
evolutionary control, if self-regulatory hypotheses are correct. Periodic
fluctuations in microtine rodents could be generated by individual
selection toward two extreme morphs: a low-density, docile, reproductive form; and a high-density, aggressive, less reproductive form.
Time-lags in responding to density changes could generate a cycle.
Such a self-regulatory system would be preserved by natural selection
if there are some genetic advantages to population fluctuations.
V I I I . SUMMARY
Population cycles in voles and lemmings are accompanied by a
1. The increase phase is the least variable phase and may be over
very quickly.
2. The decline phase is the most variable phase; it may be very
rapid or may be slow and prolonged over two years. ((Crash”
declines over one or two weeks are not typical.
series of changes which we summarize here in point form:
CHARLES J. KREBS AND JUDITH H. MYERS
maximize individual fitness. They concluded that the observed patterns
of reproductive changes in cyclic rodents were consistent with the
hypothesis that individuals attempt to maximize fitness in all phases
of the cycle.
We can thus see how a polymorphism in spacing behavior might be
maintained in a population by means of time lags in adjusting to
density changes. This, however, leads us to a further question of
whether the behavioral and reproductive changes need be genetically
determined or could be the plastic properties of a flexible phenotype.
We do not know the relative advantages and disadvantages of adopting
a phenotypic self-regulating mechanism or a genotypic mechanism.
We see no way of determining a priori which way evolution should move
within the general scope of self-regulatory mechanisms.
Why does natural selection not damp out population fluctuations in
rodents? There are some genetic advantages to population fluctuations
that might answer this question. Carson (1968) has discussed the
genetic consequences of the “population flush” in insects. The low
density after the decline allows both the testing of new genotypes and
the possibility of rapid spread of new genetic combinations because of
founder effects. If this is true, we would predict a more rapid rate of
evolutionary change in species of voles and lemmings than in species
of rodents which maintain stable numbers. If we can postulate that
there are genetic advantages to population fluctuations, we could then
see why evolutionary changes have reinforced the fluctuations rather
than damping them away.
In summary, population regulation mechanisms must be under some
evolutionary control, if self-regulatory hypotheses are correct. Periodic
fluctuations in microtine rodents could be generated by individual
selection toward two extreme morphs: a low-density, docile, reproductive form; and a high-density, aggressive, less reproductive form.
Time-lags in responding to density changes could generate a cycle.
Such a self-regulatory system would be preserved by natural selection
if there are some genetic advantages to population fluctuations.
V I I I . SUMMARY
Population cycles in voles and lemmings are accompanied by a
1. The increase phase is the least variable phase and may be over
very quickly.
2. The decline phase is the most variable phase; it may be very
rapid or may be slow and prolonged over two years. ((Crash”
declines over one or two weeks are not typical.
series of changes which we summarize here in point form: