PARTITIONING A MORPHOLOGY AMONG ITS CONTROLLING FACTORS

In the first known attempt to use a sympatric, non-mimetic convergence ('a naturally controlled experiment in evolutionary morphology') to q uantitatively partition a morphology among its controlling factors, sh ells of the polygyrid land snails Neohelix major (Binney) and Mesodon normalis (Pilsbry) were analysed in a factorially designed rearing exp eriment. The complete absence of character displacement or character r elease confirmed previous field studies. Growth rate, a determinant of adult shell size, was controlled 50% by direct environmental inductio n (with temperature twice as effective as humidity), 10% by proximate natural selection (with a shorter growing season selecting for faster growth), 10% by genotype-environment interaction (with moisture genera lly inducing a greater growth surge in a drier climate), and 30% by wi thin-cohort variation (apparently heritable). Spire height confirmed p redictions that flatter shells make better estivators (shelterers from drought) and that taller shells make better climbers and tall/flat ni ches are readily filled: high within-cohort variation (70% of total) p rovides the raw material for niche-filling (but is reduced in severe c limates by stabilizing selection); environmental induction (10%) is st rictly by humidity, which makes shells taller for foraging more widely under more favourable conditions; proximate natural selection (10%) p roduces taller shells in moister climates (with intensity of effect de pendent on evolutionary time); and genotype-environment interaction (1 0%) makes the drought-induction of flat shells stronger in more drough t-prone areas. Coiling tightness, allied to whorl-expansion rate, show s distinct evidence of adaptive neutrality: 60% of total variance is e xplained by phylogenetic constraints despite an estimated 120 million years of genetic isolation and despite the demonstrated ability of rel ated clades to converge; the 10% due to genotype-environment interacti on is generally random and nonadaptive in its response; and the remain ing 30% appears random, non-induced, and non-selected. This study's pr otocol of experimentally partitioning a 'closed universe' of morpholog y among a full set of controlling factors is recommended as an alterna tive to the usual practice of analysing the effects of one or a subset of factors on an 'open universe' of morphology. One advantage of this new protocol is that it obviates the 'nature-nurture debate' by quant ifying the relative contributions of each, which in this snail-shell c ase differ drastically among the various components of a single morpho logy.