CHROMOSOMAL EVOLUTION AND SPECIATION
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chromosomal regions they set limits to the free recombination of genes
within the population and so permit the development of stable combinations of genes which are coadapted to one another. Most of these genes
and gene complexes are likely to be more concerned with subtle quantitative characters and physiological qualities than with ones that are
phenotypically conspicuous in a taxonomic sense. Some
Drosophih
species such as D. pseudoobscura,
D. subobscura, and D. willistoni possess highly developed systems of cytological polymorphism without
showing any visible polymorphism of the color pattern or other external
features. Such color pattern polymorphism is known in two species of
Drosophih,
D. montium and D. polymorphs,
both of which are also
polymorphic for chromosomal inversions. Some grasshopper species
that have highly developed systems of cytological polymorphism are
also characterized by conspicuous polymorphism of the visible cuticular
color pattern. But color pattern polymorphism also occurs in many
species that seem to lack cytological polymorphism. Thus in such grasshopper genera as Ίrimerotropis,
Austroicetes,
and Moraba the same
kinds of color pattern polymorphism exist in species with much chromosomal polymorphism and with no chromosomal polymorphism (as far as
known). The grouse-locusts (family Tetrigidae), some species of which
are famous for their color polymorphism, are not known to exhibit
chromosomal polymorphism.
No instance of a direct correlation between a chromosomal rearrangement and an externally visible character is known, and although further
work on this point is very much needed, it seems probable that where
chromosomal polymorphism coexists with genie polymorphisms determining externally conspicuous characters, these are largely independent
adaptive systems.
III. CHROMOSOME STRUCTURE AND REARRANGEMENTS
In most groups of animals each chromosome contains, at some point
along its length, a localized region by which it is attached to the spindle
at mitosis. We need not be concerned here with the exact appearance
or chemical nature of the centromere,
as this region is now generally
known, nor with the question whether it is to be regarded as a special
gene or group of genetic loci. The essential facts for our present purpose are that in such groups as the Díptera, the orthopteroid insects,
and the vertebrates each chromosome contains only one centromere or
perhaps a short region containing several centromeric units; that this
always seems to occupy an interstitial position and never a terminal
one; that chromosomes lacking a centromere (produced by radiationinduced rearrangement) are unable to attach themselves to the spindle;
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chromosomal regions they set limits to the free recombination of genes
within the population and so permit the development of stable combinations of genes which are coadapted to one another. Most of these genes
and gene complexes are likely to be more concerned with subtle quantitative characters and physiological qualities than with ones that are
phenotypically conspicuous in a taxonomic sense. Some
Drosophih
species such as D. pseudoobscura,
D. subobscura, and D. willistoni possess highly developed systems of cytological polymorphism without
showing any visible polymorphism of the color pattern or other external
features. Such color pattern polymorphism is known in two species of
Drosophih,
D. montium and D. polymorphs,
both of which are also
polymorphic for chromosomal inversions. Some grasshopper species
that have highly developed systems of cytological polymorphism are
also characterized by conspicuous polymorphism of the visible cuticular
color pattern. But color pattern polymorphism also occurs in many
species that seem to lack cytological polymorphism. Thus in such grasshopper genera as Ίrimerotropis,
Austroicetes,
and Moraba the same
kinds of color pattern polymorphism exist in species with much chromosomal polymorphism and with no chromosomal polymorphism (as far as
known). The grouse-locusts (family Tetrigidae), some species of which
are famous for their color polymorphism, are not known to exhibit
chromosomal polymorphism.
No instance of a direct correlation between a chromosomal rearrangement and an externally visible character is known, and although further
work on this point is very much needed, it seems probable that where
chromosomal polymorphism coexists with genie polymorphisms determining externally conspicuous characters, these are largely independent
adaptive systems.
III. CHROMOSOME STRUCTURE AND REARRANGEMENTS
In most groups of animals each chromosome contains, at some point
along its length, a localized region by which it is attached to the spindle
at mitosis. We need not be concerned here with the exact appearance
or chemical nature of the centromere,
as this region is now generally
known, nor with the question whether it is to be regarded as a special
gene or group of genetic loci. The essential facts for our present purpose are that in such groups as the Díptera, the orthopteroid insects,
and the vertebrates each chromosome contains only one centromere or
perhaps a short region containing several centromeric units; that this
always seems to occupy an interstitial position and never a terminal
one; that chromosomes lacking a centromere (produced by radiationinduced rearrangement) are unable to attach themselves to the spindle;