fission”. Paricentric inversions and supernumerary chromosomes occur in some species. Deletions have been found only in the X chromosome of some species. Small variations in the size and shape of chromosomes have been observed in several species, but their interpreta- tion is often embarrassing.

With populations of small terrestrial mammals it seems to be very interesting to understand the patterns of chromosomal polymorphism, but researches must be extended to large samples involving analysis of diploid metaphases, meiosis and hybrids.

The study of chromosomal polymorphism in mammalian populations allows to point a finger at one aspect of the evolution. In many cases, polymorphic forms can be regarded as “species in statu nascendi” (Matthey 1969) and it is often difficult to distinguish the intra from the interspecific variation. New models of speciation will be needed to explain the observed situation (White 1968).

BIOCHEMICAL POLYMORPHISMS AND GENETIC STRUCTURE IN POPULATIONS OF PEROMYSCUS D. I. RASMUSSEN Department of Zoology, Arizona State University, Tempe, Arizona, U.S.A.

The reality of intraspecific localization of the genetic contents in species of small mammals became apparent as systematists developed concepts of subspecies and geographical races. Ecologists became aware of the spatial organization of individual small mammals within local populations as behaviourally defined spatial units such as home ranges and temtories were investigated.

Currently, numerous cryptic biochemical polymorphisms are being defined within popula- tions of small mammals. Investigations of biochemical polymorphisms in natural populations of deer mice of the genus Peromyscur present an opportunity to further assess spatial organiza- tion and genetic structure between and within local populations. While the use of data on biochemical polymorphisms for the assessment of mammalian movement is at best inferential, data suggest restricted gene flow between and within populations of the deer mouse. Restriction of gene flow and the existence of demic structure are indicated by macro- geographic and microgeographic patterns of polymorphisms, parallelism in the degree of heterogeneity at functionally unrelated loci within populations, temporal changes in gene frequencies, and zygotic frequencies in local populations. Such data add to the evidence the genetic structure of certain mammalian species is a mosaic of small demic units patterned into constellations or themes of macrogeographical variation.

BIOCHEMICAL POLYMORPHISM IN POPULATIONS OF THE HOUSE MOUSE

ROBERT K. SELANDER Department of Zoology, University of Texas, Austin, Texas, U.S.A.

Studies of electrophoretically demonstrable polymorphism in enzymes and non-enzymatic proteins in the house mouse (Mus musculus) and the old-field mouse (Peromyscus) indicate that individuals are heterozygous at approximately 10% of their structural gene loci. At most polymorphic loci, one or two common alleles occur throughout the range of a species, but rare alleles with very localized distributions have been detected at many loci. Patterns

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AND OLD-FIELD MOUSE