GARSTANG'S HYPOTHESIS AND GASTROPOD TORSION

Recent experiments purporting to show that torsion in gastropodveligers does not function defensively against attacks by planktonicpredators are inconclusive. There is still much to be said forGarstang's suggestion that torsion is advantageous to the veligerin allowing it to withdraw head first in to the mantle cavity,sealing the shell aperture with the foot and operculum, andthat the ability to do so may indeed have originated with asingle gene mutation in evolution. (Received 25 April 1986;     

Why are some snails visibly polymorphic,and others not?

Work on Cepaea land-snails since 1950 is surveyed, and various explanations for their visible polymorphism, including predator selection, the influence of sunlight and temperature, co-adaptation and linkage disequilibrium, and "area effects", are discussed. All of these can modify the genetic make-up of natural populations in particular circumstances, but none provide a satisfactory answer to the question of why some species are visibly polymorphic whereas others present a uniform external appearance. The likely explanation is that probably all species are genetically heterogeneous at numerous loci, as a result of heterozygote advantage, co-adaptation and other selective factors maintaining the different alleles in equilibrium, which may sometimes have visible effects on the phenotype. If these are positively disadvantageous, selection for epistatic genes will suppress the visible polymorphism, without affecting the underlying genetical heterogeneity, preserved by selection for other non-visible pleiotropic effects of the alleles involved. But this will not happen if the visibly distinct effects of these different allelomorphs are selectively more or less neutral. Many examples of polymorphism, including the so-called 'pseudo'-polymorphism, are therefore essentially non-adaptive so far as their visible manifestation is concerned, being maintained as balanced polymorphism by selection for non-visible pleiotropic effects of the genes involved.     

Does the aardwolf mimic a hyena?

A recent suggestion that the visual resemblance of the Aardwolf to the Striped Hyena may have been evolved as Batesian mimicry is critically examined. But the fact that the close similarity in appearance (apart from size) between these two species appears to be just as good in southern Africa, where the Striped Hyena does not occur, being replaced by the Brown Hyena which does not look much like an aardwolf, argues against the mimicry hypothesis. It seems more likely that the resemblance is due simply to close phylogenetic relationship, both species having retained the cryptic coloration of a common ancestor, together with its erectile dorsal crest, used for threat display as in many other mammals.