Subgenus Fundulus

First branching within subgenus Fundulus might reflect both ecological and geographical speciation. Fundulus majalis (Walbaum, 1792) – Fundulus persimilis Miller, 1955 – Fundulus similis (Baird & Girard, 1853) inhabit unvegetated coastal habitats where they dive into soft sediments for cover rather than retreating to vegetation like typical Fundulus (Martin & Finucane, 1968; Harvey, 1998; Miller et al. 2005). They are adapted for continuous swimming in the surf zone (Yetsko and Sancho 2015) and spawn in unvegetated habitats (Greeley et al. 1986). Fundulus majalis segregates from congeners in unvegetated, high-salinity waters (Weisberg 1986, Wagner and Austin 1999). We propose that this ecologically divergent lineage descends from an ancestor adapted for open, wave-swept coasts. Divergence 21.0–15.2 Mya (Fig. 3, node 16) was concurrent with sea-level rise that culminated in the MMCO (Table 2) and could have facilitated range expansion (Dolby et al. 2016, 2018). Given that coastlines adjacent to Early Miocene deltas were wave swept and sand dominated (Snedden and Galloway 2019), this could account for ecological specialization in this species group. In the earliest Middle Miocene (15.6 Mya), a new Guadalupe River delta emerged to the west (Fig. 8). Wave-dominated conditions there (Snedden and Galloway 2019) suggest that the MRCA of the F. similis species group evolved there.