The Macrhybopsis aestivalis complex (nine recognized species of small-bodied, riverine cyprinids) is predicted to have diversified allopatrically in response to prehistoric reorganizations of rivers tributary to the Gulf of México. To flesh out details of this hypothesis, we derived, from previous work, a mitochondrial (ND2) chronogram for interpretation against the background of a published nuclear gene phylogeny (S7 intron 1), paleo-geomorphology, and other species with congruent biogeographies. The complex arose in the Early Miocene and potentially occupied most of its present range by Late Miocene-Early Pliocene. Biogeography is largely consistent with allopatric speciation and secondary contact driven by Neogene river-course rearrangements. One exceptional case of ecologically driven speciation is indicated for an undescribed form in the Ohio River drainage. Heterospecific mtDNA fixation is indicated in the Arkansas and Red rivers as well as in a case of ancient exchange involving M. meeki or M. gelida from outside the complex. A novel result is the suggestion that interspecific mtDNA sharing can exist over millions of years before donor and recipient lineages start to diverge mitochondrially. Additional study is needed to fully resolve the phylogeography of the complex, especially in the Wabash River, Upper Tennessee River, and the southern Mississippi Embayment. Finally, biogeographic considerations and critique of a prior, morphologically based revision of the M. aestivalis complex indicate a need to revisit the synonymy of presently unrecognized M. sterletus with M. aestivalis in the Río Grande drainage. Hypotheses presented are potentially applicable to biogeographies of other riverine fishes.

小麦草复合体（九种公认的小体型河流鲤科鱼类）预计将随着墨西哥湾支流河流的史前重组而异域多样化。为了充实这一假设的细节，我们从以前的工作中得出了一个线粒体 (ND2) 计时图，用于在已发表的核基因系统发育 (S7 内含子 1)、古地貌学和其他具有一致生物地理学的物种的背景下进行解释。该复合体产生于早中新世，并可能在晚中新世-早上新世占据其目前的大部分范围。生物地理学与由新近纪河道重排驱动的异域物种形成和次生接触在很大程度上是一致的。生态驱动的物种形成的一个例外情况是俄亥俄河排水系统中未描述的形式。来自综合体外部的M. meeki或M. gelida。一个新的结果是，在供体和受体谱系开始线粒体分化之前，种间 mtDNA 共享可以存在数百万年。需要进一步的研究来完全解决复杂的系统地理学，特别是在瓦巴什河、田纳西河上游和密西西比河湾南部。最后，生物地理学的考虑和对先前基于形态学的M. aestivalis复合体的修订的批评表明，有必要重新审视目前未被识别的M. sterletus与M. aestivalis的同义词在里奥格兰德排水系统。提出的假设可能适用于其他河流鱼类的生物地理学。