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.

高温生境更利于带螺（Theba pisana）形成有效无纹壳（effectively unbanded, EUB）。带螺同时存在壳顶颜色多态性，该性状由与壳纹基因座（banding locus）连锁的一个基因座（locus）上的一对等位基因（allele）决定。壳顶颜色对壳纹具有上位性（epistatic）效应：带有深色壳顶的个体，其壳纹颜色也更深。研究人员在遮蔽型金合欢灌丛（Acacia thicket）与开放沙丘植被（open dune vegetation）之间的生态交错带（ecotone）开展了为期22年的年度普查，结果发现有效无纹壳与浅色壳顶均持续与开放生境呈现稳定关联。这种平行变异部分源于显著的表型不平衡（phenotypic disequilibrium）：有效无纹壳与深色壳顶的组合极为罕见。即便如此，在完全带纹的个体中，浅色壳顶的频率在开放生境中仍更高，这证实了两种促使壳色偏浅的性状各自与开放生境存在独立且平行的关联。在金合欢灌丛生境内，壳纹形态（morph）的频率随时间的波动幅度远大于壳顶颜色；且有效无纹壳与更炎热的夏季存在关联。鉴于壳顶颜色仅对幼龄螺产生主要影响，其频率并未随夏季环境条件发生变化；相反，浅色壳顶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，壳顶颜色所受的选择强度低于壳纹性状。带螺的壳性状是生物对气候变异产生复杂响应的典型案例：表型不平衡限制了壳顶颜色的进化，但同时也显现出独立的选择作用机制。