POPULATION CYCLES IN SMALL MAMMALS
375
structurally-varying proteins. Genetic polymorphisms in albumins and
transferrins (both serum proteins) and esterases and leucine aminopeptidase (both enzymes occurring in serum) have now been investigated
in microtines.
The h s t study of this type was that of Semeonoff and Robertson
(1968) who discovered a change in the gene frequency of an esterase
locus during a population decline of Microtus agrestis in Scotland.
Canham (1969) monitored albumins and transferrins in Clethrionornys
rutilus and C. gapperi populations in the Northwest Territories and
Qlberta and found a correlation between density and heterozygote
fitness. The studies of Tamarin and Krebs (1969) and Gaines and Krebs
(1971) covered five years’ observation of gene frequencies of Microtus
ochrogaster and M . pennsylvanicus. Periods of very strong selection
occurred on the two loci under study, leucine aminopeptidase and
transferrin (Fig. 42). Therefore, dramatic genetic changes can occur
in association with density changes; but how repeatable are these
observations? Many of the findings of Tamarin and Krebs (1969) were
verified in the continuing study of Gaines and Krebs (1971), which
included more populations over another population cycle. In both
studies of M . ochrogaster there was a positive correlation between the
frequency of the TfE allele and changes in population density (Fig. 43).
This relationship was found to be largely the result of better survival
of the heterozygote (TfE/TfF) during the population decline (Table
XXI). In the six populations studied, male M . ochrogaster heterozygous
at the transferrin locus had either better or equal survival than the
TfE/TfE homozygote.
In M . pennsylvanicus, Tamarin and Krebs (1969) observed a negative
correlation between density and frequency of the TfE allele in females,
hile Gaines and Krebs (1971) found this relationship for males
P Table XXII). As shown in Table XXIII, the survival differences
among genotypes in the two studies are not always consistent but tend
to show the same trends. Certainly the relationship between gene
frequency and density change is not a perfect one (Fig. 43) but there is a
definite statistical trend.
Not all local populations are genetically similar, however. One
population studied by Gaines and Krebs (1971) (Grid I) had frequencies
of the TfE allele which were much lower than the other populations.
It was most common for gene frequencies for the TfE allele to range
between 0.40 and 0.60, but in this one population the frequencies rarely
exceeded 0.40 and were most often between 0.20 and 0.40. Unlike the
other populations in which the TfE/TfE homozygote and the Tfc/TfE
heterozygote had superior survival rates, in this population the Tfc/Tfc
homozygote had a survival advantage. However, this population still
375
structurally-varying proteins. Genetic polymorphisms in albumins and
transferrins (both serum proteins) and esterases and leucine aminopeptidase (both enzymes occurring in serum) have now been investigated
in microtines.
The h s t study of this type was that of Semeonoff and Robertson
(1968) who discovered a change in the gene frequency of an esterase
locus during a population decline of Microtus agrestis in Scotland.
Canham (1969) monitored albumins and transferrins in Clethrionornys
rutilus and C. gapperi populations in the Northwest Territories and
Qlberta and found a correlation between density and heterozygote
fitness. The studies of Tamarin and Krebs (1969) and Gaines and Krebs
(1971) covered five years’ observation of gene frequencies of Microtus
ochrogaster and M . pennsylvanicus. Periods of very strong selection
occurred on the two loci under study, leucine aminopeptidase and
transferrin (Fig. 42). Therefore, dramatic genetic changes can occur
in association with density changes; but how repeatable are these
observations? Many of the findings of Tamarin and Krebs (1969) were
verified in the continuing study of Gaines and Krebs (1971), which
included more populations over another population cycle. In both
studies of M . ochrogaster there was a positive correlation between the
frequency of the TfE allele and changes in population density (Fig. 43).
This relationship was found to be largely the result of better survival
of the heterozygote (TfE/TfF) during the population decline (Table
XXI). In the six populations studied, male M . ochrogaster heterozygous
at the transferrin locus had either better or equal survival than the
TfE/TfE homozygote.
In M . pennsylvanicus, Tamarin and Krebs (1969) observed a negative
correlation between density and frequency of the TfE allele in females,
hile Gaines and Krebs (1971) found this relationship for males
P Table XXII). As shown in Table XXIII, the survival differences
among genotypes in the two studies are not always consistent but tend
to show the same trends. Certainly the relationship between gene
frequency and density change is not a perfect one (Fig. 43) but there is a
definite statistical trend.
Not all local populations are genetically similar, however. One
population studied by Gaines and Krebs (1971) (Grid I) had frequencies
of the TfE allele which were much lower than the other populations.
It was most common for gene frequencies for the TfE allele to range
between 0.40 and 0.60, but in this one population the frequencies rarely
exceeded 0.40 and were most often between 0.20 and 0.40. Unlike the
other populations in which the TfE/TfE homozygote and the Tfc/TfE
heterozygote had superior survival rates, in this population the Tfc/Tfc
homozygote had a survival advantage. However, this population still