CHROMOSOMAL EVOLUTION AND SPECIATION
141
The same situation seems to exist in Orosophila montana, i.e., the race
which inhabits the Pacific Northwest has far more structural heterozygosity than the Rocky Mountain populations (Moorhead, 1954).
Dobzhansky (see Dobzhansky et al. 1950) has put forward the view
that a multiplicity of chromosomal rearrangements, each carrying its own
adaptive complex of genes, enables a species with much cytological
polymorphism to exploit a variety of habitats more efficiently. However,
this is probably not the only explanation for the development of these
complex systems of cytological polymorphism. Thus, to take what is
perhaps an extreme case, in the European Orosophila subobscura it is
extremely difficult to obtain laboratory stocks free from inversion polymorphism, since the viability of most of the homozygous genotypes is
too low, even in the ecologically simple environment of the culture
bottle.
In the grasshoppers there are a number of species which are very rare
or restricted in distribution, but which nevertheless possess mechanisms
of cytological polymorphism (White and Nickerson, 1951; White, 1954).
Genetic homeostasis (Lerner, 1954) resulting from cytological polymorphism may be vitally important to insects which are in most cases
liable to a large and variable mortality in the egg stage, dependent on
seasonal fluctuations in temperature and amount of precipitation (Richards and Waloff, 1954). But in the case of long-established polymorphisms the higher adaptive values of the heterozygotes are likely to be
due to the additive effects of increased viability and fecundity at all
stages of the life cycle, rather than to any single effect operating at
one stage or in one sex only. It is obvious that where extremely complex polymorphisms are present, involving structural rearrangements in
several or many chromosome pairs, the coadaptation of the individual
heterotic mechanisms must likewise be very complicated. But even
cytological polymorphism in ten out of the twelve chromosome pairs
(i.e., at least 3
10 cytologically different kinds of individuals) has not
enabled the grasshopper Τ rimer otr opts thalassica to colonize any environment other than the California Adenostoma chaparral which is its
sole habitat; while T. pallidipennis
seems to have successfully invaded
a very large number of habitats in western North America, from Vancouver Island to central Mexico and from sea level to a considerable
elevation in the Rocky Mountains, without the aid of any mechanism
of cytological polymorphism. Many such instances could be cited, and
it does not seem that there is any very close correlation between the
extent to which mechanisms of cytological polymorphism are developed
in species and the variety of habitats occupied. The whole subject of
the interrelationships between the cytogenetic composition of popula-
141
The same situation seems to exist in Orosophila montana, i.e., the race
which inhabits the Pacific Northwest has far more structural heterozygosity than the Rocky Mountain populations (Moorhead, 1954).
Dobzhansky (see Dobzhansky et al. 1950) has put forward the view
that a multiplicity of chromosomal rearrangements, each carrying its own
adaptive complex of genes, enables a species with much cytological
polymorphism to exploit a variety of habitats more efficiently. However,
this is probably not the only explanation for the development of these
complex systems of cytological polymorphism. Thus, to take what is
perhaps an extreme case, in the European Orosophila subobscura it is
extremely difficult to obtain laboratory stocks free from inversion polymorphism, since the viability of most of the homozygous genotypes is
too low, even in the ecologically simple environment of the culture
bottle.
In the grasshoppers there are a number of species which are very rare
or restricted in distribution, but which nevertheless possess mechanisms
of cytological polymorphism (White and Nickerson, 1951; White, 1954).
Genetic homeostasis (Lerner, 1954) resulting from cytological polymorphism may be vitally important to insects which are in most cases
liable to a large and variable mortality in the egg stage, dependent on
seasonal fluctuations in temperature and amount of precipitation (Richards and Waloff, 1954). But in the case of long-established polymorphisms the higher adaptive values of the heterozygotes are likely to be
due to the additive effects of increased viability and fecundity at all
stages of the life cycle, rather than to any single effect operating at
one stage or in one sex only. It is obvious that where extremely complex polymorphisms are present, involving structural rearrangements in
several or many chromosome pairs, the coadaptation of the individual
heterotic mechanisms must likewise be very complicated. But even
cytological polymorphism in ten out of the twelve chromosome pairs
(i.e., at least 3
10 cytologically different kinds of individuals) has not
enabled the grasshopper Τ rimer otr opts thalassica to colonize any environment other than the California Adenostoma chaparral which is its
sole habitat; while T. pallidipennis
seems to have successfully invaded
a very large number of habitats in western North America, from Vancouver Island to central Mexico and from sea level to a considerable
elevation in the Rocky Mountains, without the aid of any mechanism
of cytological polymorphism. Many such instances could be cited, and
it does not seem that there is any very close correlation between the
extent to which mechanisms of cytological polymorphism are developed
in species and the variety of habitats occupied. The whole subject of
the interrelationships between the cytogenetic composition of popula-