CHROMOSOMAL EVOLUTION AND SPECIATION
123
the latter case we must surely be dealing with a widespread adaptive
polymorphism, presumably based on heterosis. Where, however, as in
Trimerotropis sparsa, fusion heterozygosity is confined to a narrow belt
of country, this may be a "zone of tension" between areas in which the
two homozygous classes of individuals are adaptively superior. Even
in such a situation, however, the polymorphism is probably adaptive
where it exists, although it may not necessarily owe its adaptiveness to
heterosis.
In the gastropod Purpura lapillus Staiger found two kinds of cytologically monomorphic populations off the Brittany coast, one with η = 18,
the other with η — 13. Ten pairs of acrocentric chromosomes in the
former are represented by 5 pairs of metacentrics in the latter, i.e., 5
centric fusions have established themselves. Populations with η = 13
occur in intertidal habitats with an abundant food supply and exposed
to strong wave action; whereas those with η = 18 are found in localities
where food is relatively scarce and which are sheltered from the breakers.
Areas which are ecologically intermediate between these extremes are
inhabited by cytologically polymorphic populations in which the mean
haploid number lies between 13 and 18. The structurally heterozygous
individuals in these polymorphic populations show 1 to 5 trivalents at
meiosis. A pericentric rearrangement and some "unequal bivalents" are
also present in some individuals. Staiger believes that the structural
heterozygotes are adaptively superior in the ecologically intermediate
habitats, the homozygous combinations being superior in the extreme
habitats (0 fusions in sheltered localities, all 5 fusions in exposed ones).
This most remarkable case of cytological polymorphism seems to be
unique in several respects.
In order that any chromosomal fusion may exist as a polymorphism,
without seriously impairing the fertility of the heterozygotes, it is necessary that the fused chromosome should be capable of forming at least
two chiasmata at meiosis with complete regularity—one with each of
the two unfused chromosomes. And it is further necessary that the
trivalent so formed should regularly orient itself on the meiotic spindle
so that the two unfused elements pass to one pole and the fused one
to the opposite end of the spindle. These conditions seem in fact to
exist in the instances referred to above where this type of polymorphism
has been found in natural populations. Exactly the same considerations
will apply where the heterozygosity has been produced by a "fragmentation" rather than by a fusion.
It seems likely that these two somewhat stringent conditions (and
especially the first one) furnish an explanation for the previously rather
mysterious fact that in the grasshoppers and some of the vertebrates the
123
the latter case we must surely be dealing with a widespread adaptive
polymorphism, presumably based on heterosis. Where, however, as in
Trimerotropis sparsa, fusion heterozygosity is confined to a narrow belt
of country, this may be a "zone of tension" between areas in which the
two homozygous classes of individuals are adaptively superior. Even
in such a situation, however, the polymorphism is probably adaptive
where it exists, although it may not necessarily owe its adaptiveness to
heterosis.
In the gastropod Purpura lapillus Staiger found two kinds of cytologically monomorphic populations off the Brittany coast, one with η = 18,
the other with η — 13. Ten pairs of acrocentric chromosomes in the
former are represented by 5 pairs of metacentrics in the latter, i.e., 5
centric fusions have established themselves. Populations with η = 13
occur in intertidal habitats with an abundant food supply and exposed
to strong wave action; whereas those with η = 18 are found in localities
where food is relatively scarce and which are sheltered from the breakers.
Areas which are ecologically intermediate between these extremes are
inhabited by cytologically polymorphic populations in which the mean
haploid number lies between 13 and 18. The structurally heterozygous
individuals in these polymorphic populations show 1 to 5 trivalents at
meiosis. A pericentric rearrangement and some "unequal bivalents" are
also present in some individuals. Staiger believes that the structural
heterozygotes are adaptively superior in the ecologically intermediate
habitats, the homozygous combinations being superior in the extreme
habitats (0 fusions in sheltered localities, all 5 fusions in exposed ones).
This most remarkable case of cytological polymorphism seems to be
unique in several respects.
In order that any chromosomal fusion may exist as a polymorphism,
without seriously impairing the fertility of the heterozygotes, it is necessary that the fused chromosome should be capable of forming at least
two chiasmata at meiosis with complete regularity—one with each of
the two unfused chromosomes. And it is further necessary that the
trivalent so formed should regularly orient itself on the meiotic spindle
so that the two unfused elements pass to one pole and the fused one
to the opposite end of the spindle. These conditions seem in fact to
exist in the instances referred to above where this type of polymorphism
has been found in natural populations. Exactly the same considerations
will apply where the heterozygosity has been produced by a "fragmentation" rather than by a fusion.
It seems likely that these two somewhat stringent conditions (and
especially the first one) furnish an explanation for the previously rather
mysterious fact that in the grasshoppers and some of the vertebrates the