The microcrustacean Daphnia is arguably one of the most studied zooplankton species, having a well understood ecology, life history, and a relatively well studied evolutionary history. Despite this wealth of knowledge, species boundaries within closely related species in this genus often remain elusive and the major evolutionary forces driving the diversity of daphniids remain controversial. This genus contains more than 80 species with multiple cryptic species complexes, with many closely related species able to hybridize. Here, we review speciation research in Daphnia within the framework of current speciation theory. We evaluate the role of geography, ecology, and biology in restricting gene flow and promoting diversification. Of the 253 speciation studies on Daphnia, the majority of studies examine geographic barriers (55%). While evidence shows that geographic barriers play a role in species divergence, ecological barriers are also probably prominent in Daphnia speciation. We assess the contribution of ecological and nonecological reproductive isolating barriers between closely related species of Daphnia and found that none of the reproductive isolating barriers are restricting gene flow completely. Research on reproductive isolating barriers has disproportionally focused on two species complexes, the Daphnia pulex and Daphnia longispina species complexes. Finally, we identify areas of research that remain relatively unexplored and discuss future research directions that build our understanding of speciation in daphniids.

中文翻译：

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水蚤的物种形成

微甲壳类水蚤可以说是研究最多的浮游动物之一，其生态，生活史和进化史都比较了解。尽管有如此丰富的知识，但该属中密切相关的物种内的物种界限常常仍然难以捉摸，而驱动蚤形虫多样性的主要进化力仍然存在争议。该属包含80多个物种，具有多种隐性物种复合体，并且许多紧密相关的物种能够杂交。在这里，我们在当前物种形成理论的框架内回顾了水蚤的物种形成研究。我们评估了地理，生态和生物学在限制基因流动和促进多样化方面的作用。水蚤的253种物种研究中，大多数研究都检查了地理障碍（55％）。尽管有证据表明地理障碍在物种差异中起作用，但生态障碍在水蚤物种中也可能很突出。我们评估密切相关的水蚤物种之间的生态和非生态生殖隔离障碍的贡献，发现没有任何生殖隔离障碍完全限制基因流动。生殖隔离障碍的研究不成比例地集中在两种物种复合体：水蚤（Daphnia pulex）和长水蚤（Daphnia longispina）物种复合体。最后，我们确定尚待开发的研究领域，并讨论未来的研究方向，以加深我们对蚤形目动物物种形成的了解。