Day Julia J, Cotton James A, Barraclough Timothy G
Department of Biology, University College London, London, United Kingdom.
PLoS One. 2008 Mar 5;3(3):e1730. doi: 10.1371/journal.pone.0001730.
Understanding the causes of disparities in species diversity across taxonomic groups and regions is a fundamental aim in evolutionary biology. Addressing these questions is difficult because of the need for densely sampled phylogenies and suitable empirical systems.
METHODOLOGY/PRINCIPAL FINDINGS: Here we investigate the cichlid fish radiation of Lake Tanganyika and show that per lineage diversification rates have been more than six times slower than in the species flocks of Lakes Victoria and Malawi. The result holds even at peak periods of diversification in Lake Tanganyika, ruling out the age of the lake as an explanation for slow average rates, and is robust to uncertainties over the calibration of cichlid radiations in geological time. Moreover, Lake Tanganyika lineages, irrespective of different biological characteristics (e.g. sexually dichromatic versus sexually monochromatic clades), have diversified at similar rates, falling within typical estimates across a range of plant and animal clades. For example, the mostly sexually dichromatic haplochromines, which have speciated explosively in Lakes Victoria and Malawi, have displayed modest rates in Lake Tanganyika (where they are called Tropheini).
CONCLUSION/SIGNIFICANCE: Our results show that either the Lake Tanganyika environment is less conducive for cichlid speciation or the remarkable diversifying abilities of the haplochromines were inhibited by the prior occupancy of older radiations. Although the results indicate a dominant role for the environment in shaping cichlid diversification, differences in the timing of diversification among the Tanganyikan tribes indicate that biological differences were still important for the dynamics of species build-up in the lake. While we cannot resolve the timing of the radiation relative to the origin of the lake, because of the lack of robust geological date calibrations for cichlids, our results are consistent with a scenario that the different clades reflect independent adaptive radiations into different broad niches in the lake.
了解不同分类群和地区物种多样性差异的原因是进化生物学的一个基本目标。由于需要密集采样的系统发育树和合适的实证系统，解决这些问题很困难。
方法/主要发现：在这里，我们研究了坦噶尼喀湖的丽鱼辐射，结果表明每个谱系的多样化速率比维多利亚湖和马拉维湖的物种群慢六倍以上。即使在坦噶尼喀湖多样化的高峰期，这一结果依然成立，排除了湖泊年龄作为平均速率缓慢的解释，并且对于地质时期丽鱼辐射校准的不确定性具有稳健性。此外，坦噶尼喀湖的谱系，无论其生物学特征如何不同（例如，具有两性异色与两性单色的分支），都以相似的速率多样化，处于一系列动植物分支的典型估计范围内。例如，在维多利亚湖和马拉维湖迅速形成物种的大多具有两性异色的haplochromines，在坦噶尼喀湖（在那里它们被称为Tropheini）的多样化速率则较为适中。
结论/意义：我们的结果表明，要么坦噶尼喀湖的环境不利于丽鱼物种形成，要么haplochromines显著的多样化能力受到了较早辐射的先前占据的抑制。尽管结果表明环境在塑造丽鱼多样化中起主导作用，但坦噶尼喀湖各部落多样化时间的差异表明，生物学差异对于湖中物种积累的动态变化仍然很重要。虽然由于缺乏丽鱼可靠的地质年代校准，我们无法确定辐射相对于湖泊形成时间的早晚，但我们的结果与这样一种情景一致，即不同的分支反映了独立适应辐射进入湖中不同的广泛生态位。
