2: Origins, Species Diversity, and Ecological Diversification in Trout and Char
Ernest R. Keeley
Salmonid fishes (salmon, trout, char, and their relatives) are found on every continent except Antarctica, and many archipelagos in both the Northern and Southern hemispheres; however, the natural or native distribution of salmonids is Holarctic, covering significant areas of the Northern Hemisphere (Figure 1A; Berra 2001; Nelson et al. 2016). Along with the subfamily of trout, char, salmon, huchen, and lenox (Salmoninae), salmonids also include the whitefishes and ciscoes (Coregoninae) and graylings (Thymallinae). While the early evolutionary history of salmonids is not well described, the first known evidence of salmonids comes from Eocene fossil specimens of Eosalmo driftwoodensis (approximately 45–50 million years before present [Ma]), collected from British Columbia, Canada and Washington, USA (Wilson 1977; Wilson and Li 1999). As McPhail (1997) notes, a large gap in the fossil record for salmonids occurs between the Eosalmo specimens and records of more derived salmonid species from the late to middle Miocene (7–12 Ma). During this latter period, salmonids appear in the fossil record of North America further south over time and included archaic species of trout (Rhabdofario), species related to the extant Eurasian Hucho, as well as species related to the genus Oncorhynchus (Smith and Miller 1985; Stearley and Smith 1993, 2016; Cavender 1998; Eiting and Smith 2007). Fewer sources of fossil evidence for the timing of salmonid fish evolution are known from sites in Europe or Asia, but fishes of the genera Hucho and Salmo are known from mid- to late Miocene sites in Russia, Germany, and Ukraine (Kovalchuk 2015). More recent examples of late Pleistocene salmonids can be found at fossil sites in both North America and Europe that are representative of extant Salmonid taxa present today (Bachhuber 1989; Smith et al. 2007; Borvon et al. 2018).