Some General Problems of Chromosomal Evolution
and Speciation in Animals
M. J. D. WHITE
Division of Plant Industry, C.S.I.R.O., Canberra, Australia*
Page
I. Introduction
109
II. Cytotaxonomy and Population Genetics
110
III. Chromosome Structure and Rearrangements
113
IV. Evolutionary Increases and Decreases in
Chromosome Number
121
V. Fusions of Sex Chromosomes
133
VI. Chromosomal Polymorphism and Speciation
138
References
142
1. INTRODUCTION
The number of described species of animals is believed to be somewhat over a million (Mayr in 1946 estimated it in round figures at a
million, since which date there has probably been a net increase of at
least 50,000 species). Of this total, about 4000 have been studied by
nuclear cytologists to some extent (Makino, 1951, listed 3317 species,
but his compilation was incomplete at the time of publication and many
additions have been made since). This sample of approximately 0.4%
is not randomly distributed over the whole, so that, for example, we
know far more about the cytology of mammals than of fishes. Most of
the marine groups of animals have been very inadequately studied, and
several, such as the Cephalopoda and Brachiopoda, are represented by
blank spaces in our catalogues of cytological knowledge, while the
grasshoppers, beetles, Lepidoptera, and spiders fill many pages. Darlington (1953), lamenting the poverty of our knowledge of comparative
animal cytology, states that the chromosome numbers "are accurately
known for much less than one per cent of all the species of mammals,"
but this is a considerable underestimate; the total number of species of
mammals being approximately 3500 and the number whose chromosomes
have been counted (omitting early work of doubtful accuracy), about
200 (5.7%). In the marsupials approximately 20% have now been studied
cytologically—an astonishingly high proportion. That the basic data
for understanding the evolution of the hereditary mechanism in animals
are still very incomplete is evident, but there is nothing to be gained
* Present address: Dept. of Zoology, University of Missouri, Columbia, Missouri.
109
and Speciation in Animals
M. J. D. WHITE
Division of Plant Industry, C.S.I.R.O., Canberra, Australia*
Page
I. Introduction
109
II. Cytotaxonomy and Population Genetics
110
III. Chromosome Structure and Rearrangements
113
IV. Evolutionary Increases and Decreases in
Chromosome Number
121
V. Fusions of Sex Chromosomes
133
VI. Chromosomal Polymorphism and Speciation
138
References
142
1. INTRODUCTION
The number of described species of animals is believed to be somewhat over a million (Mayr in 1946 estimated it in round figures at a
million, since which date there has probably been a net increase of at
least 50,000 species). Of this total, about 4000 have been studied by
nuclear cytologists to some extent (Makino, 1951, listed 3317 species,
but his compilation was incomplete at the time of publication and many
additions have been made since). This sample of approximately 0.4%
is not randomly distributed over the whole, so that, for example, we
know far more about the cytology of mammals than of fishes. Most of
the marine groups of animals have been very inadequately studied, and
several, such as the Cephalopoda and Brachiopoda, are represented by
blank spaces in our catalogues of cytological knowledge, while the
grasshoppers, beetles, Lepidoptera, and spiders fill many pages. Darlington (1953), lamenting the poverty of our knowledge of comparative
animal cytology, states that the chromosome numbers "are accurately
known for much less than one per cent of all the species of mammals,"
but this is a considerable underestimate; the total number of species of
mammals being approximately 3500 and the number whose chromosomes
have been counted (omitting early work of doubtful accuracy), about
200 (5.7%). In the marsupials approximately 20% have now been studied
cytologically—an astonishingly high proportion. That the basic data
for understanding the evolution of the hereditary mechanism in animals
are still very incomplete is evident, but there is nothing to be gained
* Present address: Dept. of Zoology, University of Missouri, Columbia, Missouri.
109