110
Μ. J. D. WHITE
by exaggerating our ignorance. Seriously analytical and experimental
studies have, however, only been carried out on a very small fraction of
the 4000 species mentioned above.
In the following discussion we shall refer to any cytologically detectable difference between the chromosome sets of related species, whether
involving a difference in chromosome number or merely a change in the
relative sequence of parts within a chromosome, as a cytotaxonomic
difference.
A special category of cytotaxonomic differences is concerned
with the frequency and distribution of the chiasmata along the chromosomes. It is still very uncertain whether such differences in the pattern
of chiasma distribution, which frequently exist between individuals,
populations, and related species, are mainly due to "point mutations"
or to structural rearrangements. However, it is now clear that, apart
from polyploidy (whose role in plants has been considerable but which
has played a minor and in the author's opinion almost negligible part
in the multicellular animals), all cytotaxonomic differences in chromosome number and in sequence of genes have come about through chromosomal rearrangement involving breakage and reunion. We shall consider later (Section III) the principles governing the occurrence of such
rearrangements.
II. CYTOTAXONOMY AND POPULATION GENETICS
Exactly as in the case of a "point mutation," any chromosomal rearrangement that undergoes fixation in a population or a species must
be capable of existing in the heterozygous condition for at least one
generation and usually for a number of generations. In very rare instances fixation of a rearrangement (i.e., attainment of 100% frequency
and extinction of the original type of chromosome) may occur through
"drift" in a small isolated population. And if other colonies of the species
become extinct or irrevocably isolated by distance or geographic barriers, a taxonomic entity may arise from the small population in which
the rearrangement has undergone fixation. This is essentially what
Callan and Spurway (1951) believe occurred in the differentiation of
the members of the Rassenkreis Triturus cristatus which differ in respect of translocations.
Far more common will be the type of evolutionary process in which
fixation of the rearrangement is preceded by a condition of chromosomal
polymorphism which persists for several or many generations because
it is adaptive (either through giving rise to heterosis or for other reasons). Thus the great majority of the cytotaxonomic differences which
we observe between present-day species must represent, as it were, the
remains of the adaptive polymorphisms which existed in the past history
Μ. J. D. WHITE
by exaggerating our ignorance. Seriously analytical and experimental
studies have, however, only been carried out on a very small fraction of
the 4000 species mentioned above.
In the following discussion we shall refer to any cytologically detectable difference between the chromosome sets of related species, whether
involving a difference in chromosome number or merely a change in the
relative sequence of parts within a chromosome, as a cytotaxonomic
difference.
A special category of cytotaxonomic differences is concerned
with the frequency and distribution of the chiasmata along the chromosomes. It is still very uncertain whether such differences in the pattern
of chiasma distribution, which frequently exist between individuals,
populations, and related species, are mainly due to "point mutations"
or to structural rearrangements. However, it is now clear that, apart
from polyploidy (whose role in plants has been considerable but which
has played a minor and in the author's opinion almost negligible part
in the multicellular animals), all cytotaxonomic differences in chromosome number and in sequence of genes have come about through chromosomal rearrangement involving breakage and reunion. We shall consider later (Section III) the principles governing the occurrence of such
rearrangements.
II. CYTOTAXONOMY AND POPULATION GENETICS
Exactly as in the case of a "point mutation," any chromosomal rearrangement that undergoes fixation in a population or a species must
be capable of existing in the heterozygous condition for at least one
generation and usually for a number of generations. In very rare instances fixation of a rearrangement (i.e., attainment of 100% frequency
and extinction of the original type of chromosome) may occur through
"drift" in a small isolated population. And if other colonies of the species
become extinct or irrevocably isolated by distance or geographic barriers, a taxonomic entity may arise from the small population in which
the rearrangement has undergone fixation. This is essentially what
Callan and Spurway (1951) believe occurred in the differentiation of
the members of the Rassenkreis Triturus cristatus which differ in respect of translocations.
Far more common will be the type of evolutionary process in which
fixation of the rearrangement is preceded by a condition of chromosomal
polymorphism which persists for several or many generations because
it is adaptive (either through giving rise to heterosis or for other reasons). Thus the great majority of the cytotaxonomic differences which
we observe between present-day species must represent, as it were, the
remains of the adaptive polymorphisms which existed in the past history