Abstract
Populations that are asymmetrically isolated, such as above waterfalls, can sometimes export emigrants in a direction from which they do not receive immigrants, and thus provide an excellent opportunity to study the evolution of dispersal traits. We investigated the rheotaxis of guppies above barrier waterfalls in the Aripo and Turure rivers in Trinidad—the later having been introduced in 1957 from a below-waterfall population in another drainage. We predicted that, as a result of strong selection against downstream emigration, both of these above-waterfall populations should show strong positive rheotaxis. Matching these expectations, both populations expressed high levels of positive rheotaxis, possibly reflecting contemporary (rapid) evolution in the introduced Turure population. However, the two populations used different behaviors to achieve the same performance of strong positive rheotaxis, as has been predicted in the case of multiple potential evolutionary solutions to the same functional challenge (i.e., “many-to-one mapping”). By contrast, we did not find any difference in rheotactic behavior above versus below waterfalls on a small scale within either river, suggesting constraints on adaptive divergence on such scales.
Highlights
Populations that become newly established in isolated places, such as on islands or above waterfalls or dams, provide excellent opportunities to study evolutionary processes [1,2,3]
Given that contemporary evolution has been documented in many organisms experiencing strong selection [2,26,27], especially guppies [28], we suggest that positive rheotaxis will evolve quickly in guppies introduced above waterfalls
(ii) Turure and Aripo fish achieved this positive rheotaxis in different ways: fish from Turure first occupied low-flow areas, and moved to high-flow areas, whereas fish from Aripo occupied intermediate-flow areas throughout the trial. (iii) Rheotactic behavior within each river did not differ between fish from above the upstream waterfall and those from below the downstream waterfall
Summary
Populations that become newly established in isolated places, such as on islands or above waterfalls or dams, provide excellent opportunities to study evolutionary processes [1,2,3]. Rheotactic behavior is genetically based in many fishes [21,22,23], and so we hypothesize that asymmetrically isolated riverine fish populations (those above waterfalls) should evolve positive rheotaxis. Consistent with this hypothesis, several studies have shown that fish populations above waterfalls have higher positive rheotaxis than those below waterfalls [22,23,24]. (We acknowledge here that this speculation was discouraged by anonymous reviewers.) given the impossibility of evaluating initial rheotaxis in the introduced population, and given ambiguity as to the precise ancestral source (details below), our inferences on this point will remain speculative
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