CHROMOSOMAL EVOLUTION AND SPECIATION
129
which fewer or no fusions have occurred. Such grasshopper species as
Philocleon anomalus, Orosophila species with η ~ 3, and the midges of
the genera Eusimulium and Cryptochironomus
with η — 2 (Rothfels
and Dunbar, 1953; Bauer, 1945) may represent evolutionary dead ends.
Mather (1953) has expressed the belief that very low chromosome
numbers, such as those found in Drosophih,
represent an adaptation to
a type of life cycle with several or many generations in the course of a
year. The idea is that if recombination were not reduced to a relatively
low level in such forms the hereditary mechanism would suffer too much
from the disruptive influence of natural selection acting in quite different directions on successive generations during the changing environment of the seasons.
There may be some basis for this contention, but it is far from proven.
The mites, with their very short life cycles and low chromosome numbers, certainly support it. Whether some other groups with low chromosome numbers, such as Turbellaria and iceryine coccids, do is more
doubtful. And as far as Drosophih is concerned, low chromosome numbers seem to be characteristic of the whole order Díptera, including the
primitive crane flies and many forms whose life cycles are comparable in
length with those of the Lepidoptera, which have far higher chromosome numbers.
The view that polyploidy has occurred in the evolution of some
strictly bisexual groups of animals and that it accounts for some of the
unusually high chromosome numbers which have been recorded in certain species has recently been put forward by Darlington (1953) and
Gates (1953). Such a process seems inherently improbable, even in a
group, like the Amphibia, in which the Y is the preponderant sexdetermining element in some or all of the species; but in view of the
vast time scale of evolution we must give consideration to the possibility of even the most improbable events having occurred. However, a
careful scrutiny of the evidence that has been produced in support of
the hypothesis does not inspire confidence in it. As far as the vertebrates
are concerned, the main instances in which polyploidy has been claimed
are the following:
I. CRICETINE RODENTS
2n
Cricetulus griseus
22
Cricetulus migratorius
22
Cricetus cricetus
22
Cricetulus tritón
30
Mesocricetus hrandti
42
Mesocricetus
auratus
44
129
which fewer or no fusions have occurred. Such grasshopper species as
Philocleon anomalus, Orosophila species with η ~ 3, and the midges of
the genera Eusimulium and Cryptochironomus
with η — 2 (Rothfels
and Dunbar, 1953; Bauer, 1945) may represent evolutionary dead ends.
Mather (1953) has expressed the belief that very low chromosome
numbers, such as those found in Drosophih,
represent an adaptation to
a type of life cycle with several or many generations in the course of a
year. The idea is that if recombination were not reduced to a relatively
low level in such forms the hereditary mechanism would suffer too much
from the disruptive influence of natural selection acting in quite different directions on successive generations during the changing environment of the seasons.
There may be some basis for this contention, but it is far from proven.
The mites, with their very short life cycles and low chromosome numbers, certainly support it. Whether some other groups with low chromosome numbers, such as Turbellaria and iceryine coccids, do is more
doubtful. And as far as Drosophih is concerned, low chromosome numbers seem to be characteristic of the whole order Díptera, including the
primitive crane flies and many forms whose life cycles are comparable in
length with those of the Lepidoptera, which have far higher chromosome numbers.
The view that polyploidy has occurred in the evolution of some
strictly bisexual groups of animals and that it accounts for some of the
unusually high chromosome numbers which have been recorded in certain species has recently been put forward by Darlington (1953) and
Gates (1953). Such a process seems inherently improbable, even in a
group, like the Amphibia, in which the Y is the preponderant sexdetermining element in some or all of the species; but in view of the
vast time scale of evolution we must give consideration to the possibility of even the most improbable events having occurred. However, a
careful scrutiny of the evidence that has been produced in support of
the hypothesis does not inspire confidence in it. As far as the vertebrates
are concerned, the main instances in which polyploidy has been claimed
are the following:
I. CRICETINE RODENTS
2n
Cricetulus griseus
22
Cricetulus migratorius
22
Cricetus cricetus
22
Cricetulus tritón
30
Mesocricetus hrandti
42
Mesocricetus
auratus
44