Sample collection and site characterization
In the Northern Range Mountains in Trinidad, guppies originating from high predation (HP) environments have been previously transplanted into guppy-free, low-predation (LP) environments, and monitored as they evolved towards the LP guppy ecotype, forming one of the best examples of replicated parallel evolution in the wild to date (Reznick et al. 1996; Magurran 2005; Gee et al. 2009). Translocations occurred in six instances (1957, 1976, 1981, 2008, and for two streams in 2009) across five drainages (Aripo, Campo, El Cedro, Guanapo, Turure; Fig. 1). In February 2014, we field-collected guppies from each of these six transplant sites, which we call “Introductions” (LP environments where HP populations were previously introduced). We also collected fish from the three original HP populations that served as sources for the introductions, and from three native LP populations as presumed locally adapted references (Fig. 1). We divided sites into four categories: HP source, LP native, old introduction (introduced 30-60 years prior to sampling), and recent introduction (5-6 years prior to sampling).
We collected 19-35 female guppies from each of these 12 sites. We sampled three distinct pools that were at least 30 m apart to capture within-site heterogeneity, and only females to control for possible sex differences (Bolnick et al. 2014b) and for comparability to Sullam et al. (2015). Sampling was lower at Naranjo (N= 19) because rain limited sampling time and Tumbasson (N=21) because of its small population size. We weighed and euthanized individuals with an overdose of MS-222 immediately after field capture, and preserved them in 95% ethanol. We measured stream pH and temperature using a handheld meter (Hanna Instruments, Smithfield, RI, USA), and recorded GPS coordinates of each pool. Fish condition was calculated as weight divided by length.