Data from: Evolution of body shape in sympatric versus non-sympatric Tropheus populations of Lake Tanganyika

Allopatric speciation often yields ecologically equivalent sister species, so that their secondary admixis enforces competition. The shores of Lake Tanganyika harbor about 120 distinct populations of the cichlid genus Tropheus, but only some are sympatric. When alone, Tropheus occupies a relatively broad depth zone, but in sympatry, fish segregate by depth. To assess the effects of competition, we studied the partial co-occurrence of Tropheus moorii ‘Kaiser’ and ‘Kirschfleck’ with Tropheus polli. A previous study demonstrated via standardized breeding experiments that some observed differences between Tropheus ‘Kaiser’ living alone and in sympatry with T. polli have a genetic basis despite large-scale phenotypic plasticity. Using geometric morphometrics and neutral genetic markers, we now investigated whether sympatric populations differ consistently in body shape from populations living alone and if the differences are adaptive. We found significant differences in mean shape between non-sympatric and sympatric populations, whereas all sympatric populations of both color morphs clustered together in shape space. Sympatric populations had a relatively smaller head, smaller eyes and a more anterior insertion of the pectoral fin than non-sympatric populations. Genetically, however, non-sympatric and sympatric ‘Kaiser’ populations clustered together to the exclusion of ‘Kirschfleck’. Genetic distances, but not morphological distances, were correlated with geographic distances. Within- and between-population covariance matrices for T. moorii populations deviated from proportionality. It is thus likely that natural selection acts on both phenotypic plasticity and heritable traits and that both factors contribute to the observed shape differences. The consistency of the pattern in five populations suggests ecological character displacement.

异域物种形成（allopatric speciation）通常会产生生态等价的姊妹物种，二者的次级基因混合会引发种间竞争。坦噶尼喀湖沿岸栖息着约120个不同的饰鳍丽鱼属（Tropheus）种群，但仅有部分种群呈现同域分布状态。当单独栖息时，饰鳍丽鱼属鱼类的分布水深范围相对较广；而在同域共存的情况下，它们会按水深进行生态位分隔。为评估竞争的影响，我们针对“凯撒”莫瑞饰鳍丽鱼（Tropheus moorii ‘Kaiser’）、“Kirschfleck”饰鳍丽鱼与波氏饰鳍丽鱼（Tropheus polli）的部分共现情况展开了研究。此前一项研究通过标准化繁殖实验证实，尽管存在大范围的表型可塑性（phenotypic plasticity），单独栖息的“凯撒”莫瑞饰鳍丽鱼与同域分布的波氏饰鳍丽鱼之间的部分差异具有遗传基础。本研究借助几何形态测量法（geometric morphometrics）与中性遗传标记（neutral genetic markers），探究了同域种群与单独栖息种群在体型上是否存在稳定差异，以及这些差异是否具有适应性。我们发现非共域种群与同域种群的平均体型存在显著差异；而两种色型的所有同域种群在形态空间中均聚为一类。相较于非共域种群，同域种群的头部相对更小、眼部尺寸更短，且胸鳍附着点更靠前。但在遗传层面，“凯撒”莫瑞饰鳍丽鱼的非共域与同域种群聚为一类，且与“Kirschfleck”饰鳍丽鱼区分开来。遗传距离（而非形态距离）与地理距离存在相关性。莫瑞饰鳍丽鱼种群的种群内与种群间协方差矩阵（covariance matrices）偏离比例一致性。因此，自然选择（natural selection）很可能同时作用于表型可塑性与可遗传性状，二者共同促成了观测到的体型差异。五个种群中该模式的一致性表明，这一过程属于生态特征替代（ecological character displacement）。