Data from: Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

Hotter conditions favour effectively unbanded (EUB) shells in the snail Theba pisana. T. pisana is also polymorphic for colour of the shell's apex, determined by a pair of alleles at a locus linked to the banding locus. Apex colour is epistatic to shell banding, such that banded snails with a dark apex have darker bands. Annual censuses over 22 years across an ecotone between a sheltered Acacia thicket and open dune vegetation showed a persistent association of both EUB shells and pale apex with the Open habitat. The parallel variation was due partly to strong phenotypic disequilibrium, as the combination of EUB with dark apex was rare. Nevertheless, in fully banded shells the frequency of pale apex was also higher in the Open habitat, confirming independent, parallel associations of the two contributors to paleness. Within the Acacia habitat, temporal variation of the frequencies of banding morphs was much greater than for apex colour, and EUB shells were associated with hotter summers. Consistent with its primary effect only on the very small snails, apex colour did not vary with summer conditions, but instead, higher frequencies of pale apices were associated with sunnier winters. The intensity of selection was lower on apex colour than shell banding, due partly to the constraint of phenotypic disequilibrium. The shell traits in T. pisana are an example of complex responses to climatic variation, in which phenotypic disequilibrium constrains evolution of apex colour, but separate mechanisms of selection are evident.

高温环境更有利于庭园蜗牛（Theba pisana）的有效无纹壳（effectively unbanded, EUB）。该物种同时存在壳顶端颜色的多态性，这一性状由与纹位点（banding locus）连锁的一个位点（locus）上的一对等位基因（alleles）决定。壳顶端颜色对壳纹具有上位性（epistatic）效应：带有深色顶端的带纹蜗牛，其壳纹颜色也更深。研究人员在一处位于遮蔽型金合欢灌丛（Acacia thicket）与开放沙丘植被之间的生态样带（ecotone）中开展了为期22年的年度普查，结果发现有效无纹壳与浅色顶端均持续与开放生境存在稳定关联。这种平行变异部分源于显著的表型不平衡（phenotypic disequilibrium）——有效无纹壳与深色顶端的组合极为罕见。尽管如此，在完全带纹的个体中，浅色顶端的频率在开放生境中同样更高，这证实了两种浅色相关性状各自独立且平行的生境关联。在金合欢灌丛生境内，壳纹形态（morphs）的频率随时间的波动远大于顶端颜色，且有效无纹壳与更炎热的夏季相关联。与仅对极小个体产生主要影响的结论一致，顶端颜色并未随夏季环境条件发生变化；相反，浅色顶端的高频率与更晴朗的冬季相关联。由于表型不平衡的约束作用，顶端颜色所受的选择强度（selection intensity）低于壳纹性状。庭园蜗牛的壳性状是生物对气候变化产生复杂响应的典型案例：表型不平衡约束了顶端颜色的演化，但同时也显现出独立的选择作用机制。