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Μ. J. D. WHITE
tions"; while the very closely related E. tyndarus with η = 8, has presumably accumulated about 20 "fusions."
There are other genera of Lepidoptera in which great evolutionary
changes of chromosome number have taken place and in a few instances
(e.g., some species of Leptidea, studied by Lorkovic, 1941; and of Vieris,
investigated by Maeki, 1953) the chromosome number seems to be quite
variable from individual to individual, probably due to polymorphism
for fusions or fragmentations. Unfortunately, the Lepidoptera are not
very favorable material for detailed cytological studies, and it may be
impossible, for example, to distinguish between bivalents and trivalents
at the first metaphase of meiosis.
There is a general tendency for the species of Lepidoptera in which
many "fragmentations" have occurred to show one chromosome which is
considerably larger than the rest. We have suggested that this may be
the X chromosome, which has remained immune to the fragmentation
process because any breaks in it would create multiple sex chromosome
mechanisms (which may be mechanically unstable or inefficient in this
group). The existence of such a large chromosome in the karyotypes of
the species with very high chromosome numbers is a reason against regarding them as polyploids.
The inevitable conclusion to be drawn from the whole of the above
discussion is that, in the evolution of such animal groups as
Orosophila,
grasshoppers, lizards, and many others, "fusions" have occurred more
often than "fragmentations." If this were the whole story, the evolution
of chromosome numbers would have followed a progressively downward
course, which is hardly reasonable, unless the chromosome numbers of
paleozoic animals were far higher than those of their modern descendants. Thus it seems almost certain that an opposing tendency to this
accumulation of fusions exists. Such a tendency could consist in the
very occasional occurrence of an "epidemic of fragmentations" in a
species. We have seen that such anomalous species in which multiple
"fragmentation" has occurred do exist in a few groups such as the
Lepidoptera. But there is no evidence for them in the Díptera or in any
of the orthopteroid groups, on which enough cytological work has been
carried out to demonstrate them if they did occur. We are consequently
left with only one plausible explanation for the continued existence in
these and other groups of species with high numbers of acrocentric
chromosomes (in face of an evolutionary pressure towards fusion of
such elements). However unexpected it may appear, we seem forced to
the conclusion that species in which multiple fusions have taken place
must in general have a lower "evolutionary potential" (in the sense of
the probability of leaving descendant species) than related forms in
Μ. J. D. WHITE
tions"; while the very closely related E. tyndarus with η = 8, has presumably accumulated about 20 "fusions."
There are other genera of Lepidoptera in which great evolutionary
changes of chromosome number have taken place and in a few instances
(e.g., some species of Leptidea, studied by Lorkovic, 1941; and of Vieris,
investigated by Maeki, 1953) the chromosome number seems to be quite
variable from individual to individual, probably due to polymorphism
for fusions or fragmentations. Unfortunately, the Lepidoptera are not
very favorable material for detailed cytological studies, and it may be
impossible, for example, to distinguish between bivalents and trivalents
at the first metaphase of meiosis.
There is a general tendency for the species of Lepidoptera in which
many "fragmentations" have occurred to show one chromosome which is
considerably larger than the rest. We have suggested that this may be
the X chromosome, which has remained immune to the fragmentation
process because any breaks in it would create multiple sex chromosome
mechanisms (which may be mechanically unstable or inefficient in this
group). The existence of such a large chromosome in the karyotypes of
the species with very high chromosome numbers is a reason against regarding them as polyploids.
The inevitable conclusion to be drawn from the whole of the above
discussion is that, in the evolution of such animal groups as
Orosophila,
grasshoppers, lizards, and many others, "fusions" have occurred more
often than "fragmentations." If this were the whole story, the evolution
of chromosome numbers would have followed a progressively downward
course, which is hardly reasonable, unless the chromosome numbers of
paleozoic animals were far higher than those of their modern descendants. Thus it seems almost certain that an opposing tendency to this
accumulation of fusions exists. Such a tendency could consist in the
very occasional occurrence of an "epidemic of fragmentations" in a
species. We have seen that such anomalous species in which multiple
"fragmentation" has occurred do exist in a few groups such as the
Lepidoptera. But there is no evidence for them in the Díptera or in any
of the orthopteroid groups, on which enough cytological work has been
carried out to demonstrate them if they did occur. We are consequently
left with only one plausible explanation for the continued existence in
these and other groups of species with high numbers of acrocentric
chromosomes (in face of an evolutionary pressure towards fusion of
such elements). However unexpected it may appear, we seem forced to
the conclusion that species in which multiple fusions have taken place
must in general have a lower "evolutionary potential" (in the sense of
the probability of leaving descendant species) than related forms in