multiple alleles. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see epistasis). In England the snail is regularly preyed upon by the song thrush Turdus philomelos, which breaks them open on thrush anvils (large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat least well. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony. A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. Thirdly, apostatic selection is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available. The polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a supergene with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants. In this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogeneous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous advantage in the supergene. Recent work has included the effect of shell colour on thermoregulation, and a wider selection of possible genetic influences is also considered. Chromosome polymorphism in Drosophila In the 1930s Theodosius Dobzhansky and his co-workers collected Drosophila pseudoobscura and D. persimilis from wild populations in California and neighbouring states. Using Painter's technique, they studied the polytene chromosomes and discovered that all the wild populations were polymorphic for chromosomal inversions. All the flies look alike whatever inversions they carry, so this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible: Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to Ns (number in sample) = p2+2pq+q2 where 2pq is the number of heterozygotes (see Hardy–Weinberg equilibrium). Using a method invented by L'Heretier and Teissier, Dobzhansky bred populations