CHROMOSOMAL EVOLUTION AND SPECIATION
121
inverted segments is suppressed so that strands with deficiencies and
duplications are not formed at meiosis (White and Morley, 1955).
Similar rearrangements have reached fixation in many other grasshopper
species whose ancestors must have passed through a cytologically polymorphic stage. On the other hand, it is necessary to emphasize the fact
that some hundreds of grasshopper species have been examined cytologically without any pericentric inversions being found.
There have been no reports of inversion heterozygosity in species with
multiple centromeres, nor is it probable that they could be detected at
all readily in groups such as the Homoptera and Heteroptera.
IV. EVOLUTIONARY INCREASES AND DECREASES IN
CHROMOSOME NUMBER
The analysis of cytotaxonomic differences is easier in groups where
they are not too numerous. Paradoxically, it is in organisms which
show a considerable degree of evolutionary stability of the karyotype
that processes of chromosomal evolution can be understood best. It is
partly for this reason that the "grasshoppers" (a popular name which
may cover the families Acrididae, Eumastacidae, Tetrigidae, and sometimes the Tettigoniidae or "katydids" as well) have been so extensively
used in studies of this kind.
In the acridid grasshoppers, many hundreds of whose species have
been examined cytologically, there are two taxonomic subdivisions: (1)
the Chasmosacci, the overwhelming majority of which have 19 acrocentric chromosomes in the male; and (2) the Cryptosacci, in which
most species have 23 acrocentrics in the male. These undoubtedly represent the ancestral karyotypes of the two groups, the few exceptions
being later derivatives. Even peculiar aberrant genera and tribes which
have probably been confined to continents such as Australia and South
America for long periods of geologic history exhibit the typical chromosome number of the subdivision to which they belong. And no member
of either subdivision shows a chromosome number in excess of the
modal one, which is hence a maximum one as well. This is tantamount
to saying that "fragmentations" of the acrocentric chromosomes either
do not arise or cannot become established in this group. How the difference in chromosome number between the two subdivisions arose, is,
however, a mystery.
In both Chasmosacci and Cryptosacci there are species in which the
chromosome number is less than the typical one, but they are relatively
few. In most of them the reduction in number is evidently due to one
or more centric fusions. The most extreme example of this process is
the Mexican Philocleon anomalus (Helwig, 1941) in which 6 fusions
121
inverted segments is suppressed so that strands with deficiencies and
duplications are not formed at meiosis (White and Morley, 1955).
Similar rearrangements have reached fixation in many other grasshopper
species whose ancestors must have passed through a cytologically polymorphic stage. On the other hand, it is necessary to emphasize the fact
that some hundreds of grasshopper species have been examined cytologically without any pericentric inversions being found.
There have been no reports of inversion heterozygosity in species with
multiple centromeres, nor is it probable that they could be detected at
all readily in groups such as the Homoptera and Heteroptera.
IV. EVOLUTIONARY INCREASES AND DECREASES IN
CHROMOSOME NUMBER
The analysis of cytotaxonomic differences is easier in groups where
they are not too numerous. Paradoxically, it is in organisms which
show a considerable degree of evolutionary stability of the karyotype
that processes of chromosomal evolution can be understood best. It is
partly for this reason that the "grasshoppers" (a popular name which
may cover the families Acrididae, Eumastacidae, Tetrigidae, and sometimes the Tettigoniidae or "katydids" as well) have been so extensively
used in studies of this kind.
In the acridid grasshoppers, many hundreds of whose species have
been examined cytologically, there are two taxonomic subdivisions: (1)
the Chasmosacci, the overwhelming majority of which have 19 acrocentric chromosomes in the male; and (2) the Cryptosacci, in which
most species have 23 acrocentrics in the male. These undoubtedly represent the ancestral karyotypes of the two groups, the few exceptions
being later derivatives. Even peculiar aberrant genera and tribes which
have probably been confined to continents such as Australia and South
America for long periods of geologic history exhibit the typical chromosome number of the subdivision to which they belong. And no member
of either subdivision shows a chromosome number in excess of the
modal one, which is hence a maximum one as well. This is tantamount
to saying that "fragmentations" of the acrocentric chromosomes either
do not arise or cannot become established in this group. How the difference in chromosome number between the two subdivisions arose, is,
however, a mystery.
In both Chasmosacci and Cryptosacci there are species in which the
chromosome number is less than the typical one, but they are relatively
few. In most of them the reduction in number is evidently due to one
or more centric fusions. The most extreme example of this process is
the Mexican Philocleon anomalus (Helwig, 1941) in which 6 fusions