66
RICHARD S. MILLER
regions. Klopfer maintains that passerines have a wider behavior range
than non-passerines and are therefore capable of occupying a greater
range and less stable set of conditions, and that this is the reason that
members of this group are relatively more common in temperate
habitats.
Simpson (1964) analysed the species density of mammals in terms
of the number of species in areas of North America and found two major
trends: the most dominant trend is an increase in number of species
from north to south, with a lesser trend toward increase in species with
altitude. He decided that niche size is not an important contributing
factor, although no particular reason was given for this conclusion.
Van Allen (1965) analysed morphological variation in six species of
birds in relation to niche size. The niches of these species are known to
be broader on some islands than on the mainland and in each case there
is also greater variation in bill measurements in the island birds, with
the exception of one species in which the niche on the Canary Islands
is known to be smaller than on the mainland. Van Allen concludes that
niches on the zoogeographic mainland are relatively tightly packed
together by the action of stabilizing selection imposed, presumably
through competition, by ecologically adjacent species. On the islands
the available environment is partitioned into wider niches with weaker
stabilizing selection. Thus, the wider niches on the mainland would
permit greater phenotypic variation if phenotype is controlled to a
significant extent by the adaptive diversity of the niche (Van Allen,
1964).
I n examples of included niches, in which the niche size of the included
species is smaller by definition, there is a corresponding trend toward
greater specialization. This is especially evident among pocket gophers
in which Geomys bursarius, the species with the smallest niche, is the
most highly evolved in fossorial behavior and morphology (Miller,
1964a). It has also been noted that in these and other species in which
interference probably takes the form of overt aggression or threat,
there is a trend toward greater body size. It would be interesting to
compare examples of this sort with the exceptions that are known to
exist (e.g. burrowing rodents) to Bergmann’s Rule. As the realized niche
of an animal is the phenotypic expression of its responses to both its
biotic and physical environments, specialization for competition may
to some extent explain exceptions t o rules based on physical factors.
It is also interesting to note that, while the fossorial development of
Geomys bursarius may make it more efficient in sandy soils, this
specialization apparently makes it less well adapted than less specialized
forms (e.g. Thomomys talpoides) to a wide range of soil types.
Caution must be used, however, in forming generalizations about the
RICHARD S. MILLER
regions. Klopfer maintains that passerines have a wider behavior range
than non-passerines and are therefore capable of occupying a greater
range and less stable set of conditions, and that this is the reason that
members of this group are relatively more common in temperate
habitats.
Simpson (1964) analysed the species density of mammals in terms
of the number of species in areas of North America and found two major
trends: the most dominant trend is an increase in number of species
from north to south, with a lesser trend toward increase in species with
altitude. He decided that niche size is not an important contributing
factor, although no particular reason was given for this conclusion.
Van Allen (1965) analysed morphological variation in six species of
birds in relation to niche size. The niches of these species are known to
be broader on some islands than on the mainland and in each case there
is also greater variation in bill measurements in the island birds, with
the exception of one species in which the niche on the Canary Islands
is known to be smaller than on the mainland. Van Allen concludes that
niches on the zoogeographic mainland are relatively tightly packed
together by the action of stabilizing selection imposed, presumably
through competition, by ecologically adjacent species. On the islands
the available environment is partitioned into wider niches with weaker
stabilizing selection. Thus, the wider niches on the mainland would
permit greater phenotypic variation if phenotype is controlled to a
significant extent by the adaptive diversity of the niche (Van Allen,
1964).
I n examples of included niches, in which the niche size of the included
species is smaller by definition, there is a corresponding trend toward
greater specialization. This is especially evident among pocket gophers
in which Geomys bursarius, the species with the smallest niche, is the
most highly evolved in fossorial behavior and morphology (Miller,
1964a). It has also been noted that in these and other species in which
interference probably takes the form of overt aggression or threat,
there is a trend toward greater body size. It would be interesting to
compare examples of this sort with the exceptions that are known to
exist (e.g. burrowing rodents) to Bergmann’s Rule. As the realized niche
of an animal is the phenotypic expression of its responses to both its
biotic and physical environments, specialization for competition may
to some extent explain exceptions t o rules based on physical factors.
It is also interesting to note that, while the fossorial development of
Geomys bursarius may make it more efficient in sandy soils, this
specialization apparently makes it less well adapted than less specialized
forms (e.g. Thomomys talpoides) to a wide range of soil types.
Caution must be used, however, in forming generalizations about the