138
Μ. J. D. WHITE
which the Y-autosome fusion (or Yi-Y 2 fusion) has not yet occurred;
whereas D. miranda, which belongs to the obscura group in which an
X-autosome fusion (and probably a Y-autosome fusion as well) occurred a long way back in the phylogeny, has later acquired what is
probably a second Y-autosome fusion. Multiple sex chromosome mechanisms in Drosophüa thus seem to be relatively transitory intermediate
stages in the evolutionary transformation of one stable XY mechanism
into another. In the morabine grasshoppers, on the other hand, XY
mechanisms seem to have been transitory stages in the transformation
of XO mechanisms into Χ Χ Χ 2 Υ ones, since approximately 80 XO species,
8 XY ones, and 9 XiX 2 Y ones are known (White, unpublished data).
Since in a phylum like the Arthropoda, with well-developed mechanisms of genetical sex determination, the sex chromosomes must have
been repeatedly "capturing" autosomal material since paleozoic times,
it is clear that the modern species would lack autosomes altogether unless some opposing processes counteracted this continual incorporation
of autosomal genes in the X and Y. The transfer, by translocation, of
the proximal part of the X to the fourth chromosome in
Drosophila
ananassae and perhaps also in the related species D. bipectinata
and
D. montium (Kaufmann, 1937; Kikkawa, 1938) is the only known instance of a sex chromosome giving up anything to an autosome. Thus
the evolutionary growth of the sex chromosomes at the expense of the
autosomes is, in all probability, counteracted by occasional deletions of
parts of the X and Y and duplications of autosomal material (White,
1954).
VI. CHROMOSOMAL POLYMORPHISM AND SPECIATION
Up till now we have considered mainly the continuous aspect of the
evolutionary process. However, from time to time the changes which
are going on all the while in the genetic composition of populations give
rise to discontinuity (speciation). We recently stated (White, 1954)
that "the possibility that structural rearrangements such as translocations
might play a special role in speciation, as the primary cause of specific
divergence . . . never received much support. . . . Later work has failed
entirely to confirm this hypothesis. . . . " One reviewer completely misunderstood this passage, which he interpreted as a denial that structural
rearrangements played any role at all in speciation. However, the concept which we must now reject is that speciation may automatically follow the establishment of a rearrangement in a population because the
two homozygous classes are well adapted while the heterozygotes are
selectively eliminated. This picture does not correspond to reality because a rearrangement would stand little or no chance of establishment
Μ. J. D. WHITE
which the Y-autosome fusion (or Yi-Y 2 fusion) has not yet occurred;
whereas D. miranda, which belongs to the obscura group in which an
X-autosome fusion (and probably a Y-autosome fusion as well) occurred a long way back in the phylogeny, has later acquired what is
probably a second Y-autosome fusion. Multiple sex chromosome mechanisms in Drosophüa thus seem to be relatively transitory intermediate
stages in the evolutionary transformation of one stable XY mechanism
into another. In the morabine grasshoppers, on the other hand, XY
mechanisms seem to have been transitory stages in the transformation
of XO mechanisms into Χ Χ Χ 2 Υ ones, since approximately 80 XO species,
8 XY ones, and 9 XiX 2 Y ones are known (White, unpublished data).
Since in a phylum like the Arthropoda, with well-developed mechanisms of genetical sex determination, the sex chromosomes must have
been repeatedly "capturing" autosomal material since paleozoic times,
it is clear that the modern species would lack autosomes altogether unless some opposing processes counteracted this continual incorporation
of autosomal genes in the X and Y. The transfer, by translocation, of
the proximal part of the X to the fourth chromosome in
Drosophila
ananassae and perhaps also in the related species D. bipectinata
and
D. montium (Kaufmann, 1937; Kikkawa, 1938) is the only known instance of a sex chromosome giving up anything to an autosome. Thus
the evolutionary growth of the sex chromosomes at the expense of the
autosomes is, in all probability, counteracted by occasional deletions of
parts of the X and Y and duplications of autosomal material (White,
1954).
VI. CHROMOSOMAL POLYMORPHISM AND SPECIATION
Up till now we have considered mainly the continuous aspect of the
evolutionary process. However, from time to time the changes which
are going on all the while in the genetic composition of populations give
rise to discontinuity (speciation). We recently stated (White, 1954)
that "the possibility that structural rearrangements such as translocations
might play a special role in speciation, as the primary cause of specific
divergence . . . never received much support. . . . Later work has failed
entirely to confirm this hypothesis. . . . " One reviewer completely misunderstood this passage, which he interpreted as a denial that structural
rearrangements played any role at all in speciation. However, the concept which we must now reject is that speciation may automatically follow the establishment of a rearrangement in a population because the
two homozygous classes are well adapted while the heterozygotes are
selectively eliminated. This picture does not correspond to reality because a rearrangement would stand little or no chance of establishment