POPULATION CYCLES IN SMALL MAMMALS
383
observed in oscillating vole populations” (Charlesworth and Giesel,
1972) are very misleading.
One of the criticisms made by Charlesworth and Giesel (1972) of
the results showing genetic changes in fluctuating populations is that
the several systems arbitrarily chosen as genetic markers have all
shown a relation to population density. This criticism is not a serious
one if in fact periods of dramatic density changes are associated with
strong selection. In this case the whole genome would be expected to be
under selection, and a large proportion of individual loci would most
likely change in frequency. The females of both Microtus ochrogaster
and M . pennsylvanicus demonstrated significant correlations between
change in gene frequency and change in density for one of the two
systems studied by Gaines and Krebs (1971) but not for the other.
Significant correlations between gene frequency and density occurred
for both loci in males of the two species. We interpret the general trend
for all genetic systems which have been studied to show a relation
between genetic change and density change, to indicate strong selection
occurring with population fluctuations. However, we cannot explain
the exceptions which occurred in female M . ochrogaster and M .
pennsylvanicus which failed to show a relationship for one of the
two loci studied (Gaines and Krebs, 1971).
In conclusion, data have not yet contradicted the Chitty geneticbehavioral hypothesis of microtine cycles, but we have only accomplished
the preliminary steps in testing the hypothesis. Genetic changes seem
to be a part of population fluctuations in microtines, and this aspect
of genetic heterogeneity will have to be taken into consideration in
future studies of microtine cycles. Our studies have indicated that
dispersal, particularly during the phase of increase, is more important
than originally thought by Chitty. We propose a modified version of
the Chitty Hypothesis in Fig. 45. Two variations are included in this
version which were not included in the model diagrammed by Krebs
(1964a). The first of these is the loop which allows for emigration
and the colonization of new habitats and the possible establishment
of refuge populations. The second change is that rather than emphasizing
aggressive behavior, we feel that other forms of spacing behavior
could work in a similar way. For example, very docile individuals
might be at a selective advantage as the population increases. Selection
for aggressive behavior could also lead to susceptibility to other
selective factors. Originally, it was thought that aggressive behavior
among individuals might be the direct cause of the population decline.
However, until we know the nature of the mortality factors acting
during the decline we will not; be able to interpret this aspect of the
hypothesis.
383
observed in oscillating vole populations” (Charlesworth and Giesel,
1972) are very misleading.
One of the criticisms made by Charlesworth and Giesel (1972) of
the results showing genetic changes in fluctuating populations is that
the several systems arbitrarily chosen as genetic markers have all
shown a relation to population density. This criticism is not a serious
one if in fact periods of dramatic density changes are associated with
strong selection. In this case the whole genome would be expected to be
under selection, and a large proportion of individual loci would most
likely change in frequency. The females of both Microtus ochrogaster
and M . pennsylvanicus demonstrated significant correlations between
change in gene frequency and change in density for one of the two
systems studied by Gaines and Krebs (1971) but not for the other.
Significant correlations between gene frequency and density occurred
for both loci in males of the two species. We interpret the general trend
for all genetic systems which have been studied to show a relation
between genetic change and density change, to indicate strong selection
occurring with population fluctuations. However, we cannot explain
the exceptions which occurred in female M . ochrogaster and M .
pennsylvanicus which failed to show a relationship for one of the
two loci studied (Gaines and Krebs, 1971).
In conclusion, data have not yet contradicted the Chitty geneticbehavioral hypothesis of microtine cycles, but we have only accomplished
the preliminary steps in testing the hypothesis. Genetic changes seem
to be a part of population fluctuations in microtines, and this aspect
of genetic heterogeneity will have to be taken into consideration in
future studies of microtine cycles. Our studies have indicated that
dispersal, particularly during the phase of increase, is more important
than originally thought by Chitty. We propose a modified version of
the Chitty Hypothesis in Fig. 45. Two variations are included in this
version which were not included in the model diagrammed by Krebs
(1964a). The first of these is the loop which allows for emigration
and the colonization of new habitats and the possible establishment
of refuge populations. The second change is that rather than emphasizing
aggressive behavior, we feel that other forms of spacing behavior
could work in a similar way. For example, very docile individuals
might be at a selective advantage as the population increases. Selection
for aggressive behavior could also lead to susceptibility to other
selective factors. Originally, it was thought that aggressive behavior
among individuals might be the direct cause of the population decline.
However, until we know the nature of the mortality factors acting
during the decline we will not; be able to interpret this aspect of the
hypothesis.