What explains rare and conspicuous colours in a snail? A test of time-series data against models of drift, migration or selection

It is intriguing that conspicuous colour morphs of a prey species may be maintained at low frequencies alongside cryptic morphs. Negative frequency-dependent selection by predators using search images (‘apostatic selection’) is often suggested without rejecting alternative explanations. Using a maximum likelihood approach we fitted predictions from models of genetic drift, migration, constant selection, heterozygote advantage or negative frequency-dependent selection to time-series data of colour frequencies in isolated populations of a marine snail (Littorina saxatilis), re-established with perturbed colour morph frequencies and followed for >20 generations. Snails of conspicuous colours (white, red, banded) are naturally rare in the study area (usually <10%) but frequencies were manipulated to levels of ~50% (one colour per population) in 8 populations at the start of the experiment in 1992. In 2013, frequencies had declined to ~15–45%. Drift alone could not explain these change...

令人颇感兴趣的是，猎物物种的醒目色型（conspicuous colour morphs）与隐蔽色型（cryptic morphs）能够以较低频率共存。学界常将利用搜索图像的捕食者所介导的负频率依赖选择（apostatic selection）作为该现象的解释，却未对其他备选假说予以排除。本研究采用最大似然法，将遗传漂变、迁移、恒定选择、杂合子优势以及负频率依赖选择等模型的预测结果，拟合至某海洋螺类（Littorina saxatilis）孤立种群的体色频率时间序列数据；该种群初始体色频率已被人为扰动，并被连续追踪超过20个世代。在研究区域内，具有醒目体色的螺类（白色、红色、带纹型）天然种群频率通常低于10%；但本实验于1992年启动时，我们将8个种群的对应体色频率分别调整至约50%（每个种群仅调整一种体色）。至2013年时，这些体色频率已回落至约15%至45%之间。仅靠遗传漂变无法解释上述变化……