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Bipolarity, the presence of a species in the high latitudes separated by a gap in distribution across the tropics, is a well-known pattern of global species distribution. But the question of whether bipolar species have evolved independently at the poles since the establishment of the cold-water provinces 16–8 million years ago, or if genes have been transferred across the tropics since that time, has not been addressed. Here we examine genetic variation in the small subunit ribosomal RNA gene of three bipolar planktonic foraminiferal morphospecies. We identify at least one identical genotype in all three morphospecies in both the Arctic and Antarctic subpolar provinces, indicating that trans-tropical gene flow must have occurred. Our genetic analysis also reveals that foraminiferal morphospecies can consist of a complex of genetic types. Such occurrences of genetically distinct populations within one morphospecies may affect the use of planktonic foraminifers as a palaeoceanographic proxy for climate change and necessitate a reassessment of the species concept for the group.

Several species related to Drosophila melanogaster have two loci containing sequences which code for the major heat-shock protein of molecular weight 70,000 (hsp70). These sequences are arranged as two pairs of inverted repeats. The structure of the 87C locus in D. melanogaster seems to have arisen through DNA insertion. Hsp70-coding sequences at the two loci are not evolving independently. Gene conversion between hsp70 genes has apparently occurred both within and between loci.

GENIC heterozygosity at enzyme loci has been examined for correlation with parameters as diverse as substrate specificity1, physiological function2 and quaternary structure3. However, none of these has provided an adequate explanation for allozymic variation in general. One report4 suggested a positive relationship for Drosophila enzymes between heterozygosity and subunit molecular weight (MW), which can be equated to the size of the structural gene element. Various models, both neutral and selective, would predict a correlation between gene size and variability5–7. Recently, a similar correlation has been found within three classes of vertebrates8,31, and between the number of rare alleles and subunit size in human populations9,10. We report here that we have tested the relationship between heterozygosity and subunit size in Drosophila species with a new selection of enzymes. Our results indicate that the correlation is indeed general. The relationship is quasi-linear and does not differ significantly from several models. The results support the hypothesis that there are constraints on the number of sites available to charge substitution within an enzyme molecule.

ATTEMPTS to demonstrate the action of natural selection at enzyme loci have relied mainly on in vitro biochemical techniques to detect differences between allelic forms of the enzymes1–3. It is usually assumed that differences detected in this way will be relevant in vivo. The data on human inborn errors of metabolism do not support this view, however; in the majority of cases these are recessive. Heterozygotes, which usually have about 50% normal enzyme activity, rarely show clinical symptoms4. As any difference in activity between allozymes is likely to be much smaller, it cannot be assumed that such differences will necessarily be physiologically important. The properties of an enzyme molecule can only be biologically significant to the extent to which they affect the concentrations and flow of intermediates in its metabolic pathways. It was recently shown that changes in the concentration of most enzymes in a pathway at steady state will have little effect on flux5,6. Therefore, to demonstrate that differences in fitness between strains possessing different genotypes at an enzyme locus are caused by that gene substitution, it is necessary not only to show that differences in kinetic parameters can be detected in vitro, but also that changes in the activity of the enzyme will affect flux through that pathway. I previously described7 significant changes in genotype frequency at a phosphoglucomutase locus in a wild population of field mice (Apodemus sylvaticus) which occurred at a time of food shortage and declining population numbers. I report here the results of some in vivo experiments on glycogen metabolism in animals of different genotypes at this locus, inducing glycogenolysis by fasting. The results parallel survival data from the wild population and suggest that the genotype of an individual at this locus is of physiological significance and may therefore be subject to natural selection.