Data from: Extraordinarily rapid speciation in a marine fish

Divergent selection may initiate ecological speciation extremely rapidly. How often and at what pace ecological speciation proceeds to yield strong reproductive isolation is more uncertain. Here, we document a case of extraordinarily rapid speciation associated with ecological selection in the postglacial Baltic Sea. European flounders (Platichthys flesus) in the Baltic exhibit two contrasting reproductive behaviors: pelagic and demersal spawning. Demersal spawning enables flounders to thrive in the low salinity of the Northern Baltic, where eggs cannot achieve neutral buoyancy. We show that demersal and pelagic flounders are a species pair arising from a recent event of speciation. Despite having a parapatric distribution with extensive overlap, the two species are reciprocally monophyletic and show strongly bimodal genotypic clustering and no evidence of contemporary migration, suggesting strong reproductive isolation. Divergence across the genome is weak but shows strong signatures of selection, a pattern suggestive of a recent ecological speciation event. We propose that spawning behavior in Baltic flounders is the trait under ecologically based selection causing reproductive isolation, directly implicating a process of ecological speciation. We evaluated different possible evolutionary scenarios under the approximate Bayesian computation framework and estimate that the speciation process started in allopatry ∼2,400 generations ago, following the colonization of the Baltic by the demersal lineage. This is faster than most known cases of ecological speciation and represents the most rapid event of speciation ever reported for any marine vertebrate.歧化选择可能以极快的速度启动生态物种形成 (ecological speciation)。然而，生态物种形成以何种频率、何种速率演化出强生殖隔离 (reproductive isolation)，仍尚不明确。本研究记录了冰后期波罗的海中与生态选择相关的一例极快速物种形成事件。波罗的海的欧洲鲽（Platichthys flesus）存在两种截然不同的繁殖行为：浮游产卵与底栖产卵。底栖产卵使鲽能够在北波罗的海的低盐度水域存活繁衍——该水域内鱼卵无法达到中性浮力。本研究证实，底栖产卵与浮游产卵的鲽为近期物种形成事件所产生的一对物种。尽管二者呈邻域分布 (parapatric distribution) 且存在广泛的重叠区域，但这两个物种彼此互为单系群，呈现出显著的双峰基因型聚类 (genotypic clustering) 特征，且未检测到当代基因流，这表明二者间存在极强的生殖隔离。全基因组间的分化程度较弱，但却呈现出强烈的选择信号，这一模式暗示这是一起近期发生的生态物种形成事件。本研究提出，波罗的海鲽的产卵行为是受生态选择作用、进而引发生殖隔离的关键性状，直接印证了生态物种形成过程的存在。本研究基于近似贝叶斯计算 (approximate Bayesian computation) 框架评估了多种可能的演化场景，估算得到该物种形成过程始于约2400代前的异域分布阶段，彼时底栖产卵支系刚刚定居波罗的海海域。这一事件的发生速度快于多数已知的生态物种形成案例，同时也是目前有记录以来所有海洋脊椎动物 (marine vertebrate) 中最快的物种形成事件。