374
CHARLES J. RREBS AND JUDITH H. MYERS
suggest genetical hypotheses regarding fluctuations as long as more
directly ecological explanations can be invoked and tested”. The
second criticism was that selection could not be sufficiently strong
to bring about such dramatic genetic change over several generations
as to account for population fluctuations (Christian and Davis, 1964).
Both of these criticisms are examples of how only a short time ago, it
was the rule for the population ecologist to view populations largely as
genetically homogenous aggregates of individuals. While R.A. Fisher
(1930) considered that selective advantages of approximately 1 % per
generation were acting on natural populations, Ford (1964) reviews
cases demonstrating selective advantages of 20-30% per generation.
Ford concluded in his summary of “Ecological Genetics” that “unexpectedly great selective forces are normally operating” (p. 296). If
strong selection is possible in natural populations, we cannot automatically disregard the role of selection in population regulation.
Chitty’s genetic behavioral hypothesis to explain the cycling of
rodents began from a negative basis. All of the “simple” hypotheses
proposed to explain microtine cycles had been unsatisfactory (Chitty,
1960). The positive basis for the behavioral-genetic polymorphism idea
was the observation that a t the time of the population decline, Microtus
agrestis populations are composed of some individuals with high growth
potentials and others with low growth rates (Newson and Chitty, 1962).
Therefore, the population is a composite of individuals of different
phenotypes. The presence of large voles in peak populations suggests
that the composition of the population at the peak is different than that
during the decline. Krebs (1964b) observed a change in the relationship
of skull measurements with body measurements associated with density
changes in brown and varying lemmings (see Fig. 28). This again
suggests the possibility of selection varying the phenotypic composition
of microtine populations during different phases of the population cycle.
Whether the basis for the phenotypic change is genetic could only be
suggested.
A starting point for investigation of the genetic-behavioral hypothesis
is to inquire if genetic changes do occur as a microtine population
undergoes fluctuations in density. Gershenson (1945) and Voipio (1969)
have suggested that shifts in genetically controlled coat-color morph
frequencies occur in association with density changes in rodents, but
in microtines individuals with coat-color variations are scarce
(Semeonoff, 1972), Another technique for elucidating genetic systems
is to monitor changes in the frequency of alleles at polymorphic loci,
coding proteins with differing structure (Semeonoff and Robertson,
1968; Canham, 1969; Tamarin and Krebs, 1969; Gaines and Krebs,
1971). The technique of electrophoresis enables one t o demonstrate
CHARLES J. RREBS AND JUDITH H. MYERS
suggest genetical hypotheses regarding fluctuations as long as more
directly ecological explanations can be invoked and tested”. The
second criticism was that selection could not be sufficiently strong
to bring about such dramatic genetic change over several generations
as to account for population fluctuations (Christian and Davis, 1964).
Both of these criticisms are examples of how only a short time ago, it
was the rule for the population ecologist to view populations largely as
genetically homogenous aggregates of individuals. While R.A. Fisher
(1930) considered that selective advantages of approximately 1 % per
generation were acting on natural populations, Ford (1964) reviews
cases demonstrating selective advantages of 20-30% per generation.
Ford concluded in his summary of “Ecological Genetics” that “unexpectedly great selective forces are normally operating” (p. 296). If
strong selection is possible in natural populations, we cannot automatically disregard the role of selection in population regulation.
Chitty’s genetic behavioral hypothesis to explain the cycling of
rodents began from a negative basis. All of the “simple” hypotheses
proposed to explain microtine cycles had been unsatisfactory (Chitty,
1960). The positive basis for the behavioral-genetic polymorphism idea
was the observation that a t the time of the population decline, Microtus
agrestis populations are composed of some individuals with high growth
potentials and others with low growth rates (Newson and Chitty, 1962).
Therefore, the population is a composite of individuals of different
phenotypes. The presence of large voles in peak populations suggests
that the composition of the population at the peak is different than that
during the decline. Krebs (1964b) observed a change in the relationship
of skull measurements with body measurements associated with density
changes in brown and varying lemmings (see Fig. 28). This again
suggests the possibility of selection varying the phenotypic composition
of microtine populations during different phases of the population cycle.
Whether the basis for the phenotypic change is genetic could only be
suggested.
A starting point for investigation of the genetic-behavioral hypothesis
is to inquire if genetic changes do occur as a microtine population
undergoes fluctuations in density. Gershenson (1945) and Voipio (1969)
have suggested that shifts in genetically controlled coat-color morph
frequencies occur in association with density changes in rodents, but
in microtines individuals with coat-color variations are scarce
(Semeonoff, 1972), Another technique for elucidating genetic systems
is to monitor changes in the frequency of alleles at polymorphic loci,
coding proteins with differing structure (Semeonoff and Robertson,
1968; Canham, 1969; Tamarin and Krebs, 1969; Gaines and Krebs,
1971). The technique of electrophoresis enables one t o demonstrate