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There is no group of organisms in which the analysis of chromosome variation in relation to geographical distribution has been carried further than in the Insecta. This kind of variation has tended to be neglected by many and ignored by some. Yet, in reality, it is more spectacular and, though less understood, at least as important as either the external variation or indeed the remaining genetic variation that exists in natural populations.The chromosome complement is not just another character. To regard it as such is to misunderstand the nature of phylogenetic change and thus the whole basis of biological evolution. The material of the genotype itself forms part of the structure of the chromosomes. For this reason the chromosome phenotype is far less influenced by external factors than is the morphological or the physiological phenotype. Moreover, the chromosome phenotype is often a much more sensitive indicator of bio-There is no group of organisms in which the analysis of chromosome variation in relation to geographical distribution has been carried further than in the Insecta. This kind of variation has tended to be neglected by many and ignored by some. Yet, in reality, it is more spectacular and, though less understood, at least as important as either the external variation or indeed the remaining genetic variation that exists in natural populations.The chromosome complement is not just another character. To regard it as such is to misunderstand the nature of phylogenetic change and thus the whole basis of biological evolution. The material of the genotype itself forms part of the structure of the chromosomes. For this reason the chromosome phenotype is far less influenced by external factors than is the morphological or the physiological phenotype. Moreover, the chromosome phenotype is often a much more sensitive indicator of bio-Dr. John is reader in cytology in the department of genetics at the University of Birmingham, England. Dr. Lewis is university lecturer in botany and Fellow of Exeter College at the University of Oxford, England. MAY 1966Dr. John is reader in cytology in the department of genetics at the University of Birmingham, England. Dr. Lewis is university lecturer in botany and Fellow of Exeter College at the University of Oxford, England. MAY 1966logical change and biological distinctiveness.Chromosome variation of different kinds has been described within or between many natural insect populations (Table 1). In most cases, however, no analysis of this variation has been undertaken. The detailed study of chromosome variation in different geographical areas was initiated in the United States by the pioneer investigation of Dobzhansky and Sturtevant (1). They studied the polymorphism obtaining in the banding sequence of the giant polytene chromosomes of Drosophila. Their approach, that of comparing patterns of polymorphism within and between populations, has since been used by many workers in many lands and with many species. The Drosophila studies, however, still represent the most compreh...

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Comparative fluorescence studies on the chromosome of ten species of acridid grasshoppers, with varying amounts and locations of C-band positive heterochromatin, indicate that the only regions to fluoresce differentially are those that C-band. Within a given species there is a marked tendency for groups of chromosomes to accumulate heterochromatin with similar fluorescence behaviour at similar sites. This applies to all three major categories of heterochromatin - centric, interstitial and telomeric. Different sites within the same complement, however, tend to have different fluorescence properties. In particular, centric C-bands within a given species are regularly distinguishable in their behaviour from telomeric C-bands. Different species on the other hand, may show distinct forms of differential fluorescence at equilocal sites. These varying patterns of heterochromatin heterogeneity, both within and between species, indicate that whatever determines the differential response to fluorochromes has tended to operate both on an equilocal basis and in a concerted fashion. This is reinforced by the fact that structural rearrangements that lead to the relocation of centric C-bands, either within or between species, may also be accompanied by a change in fluorescence behaviour.

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