The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. III. correlated traits and experimental evidence on predation

Abstract

SUMMARYApeltes quadracus is polymorphic for the number of dorsal spines, the variation is heritable, and it is subject to natural selection. Here we investigate selective predation favouring the higher-spined morphs. We predict that populations with greater mean spine number will have longer spines, and we so find. However, the morphs do not differ for spine length within populations, and we conclude that spine number and spine length are selected independently. Populations with greater mean spine number also have longer and deeper bodies, thus having a greater defensive circumference. Again, the morphs do not differ within populations.Experimental tests demonstrate selective predation on the morphs for four fish species in the laboratory, and for a fish and a bird species in the field.However the experiments also show that: (a) the higher-spined morphs are not always favoured (b) vegetation plays a role in selective predation (c) factors other than spine number are involved (d) these factors operate before Apeltes is captured (behavioural/ecological) (e) the factors are correlated with spine number. We conclude that predators are selective agents on the polymorphism, but that the relationship between fitness of the morphs and predators is complex.

The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. IV. phenotypic covariation with closely related species

“…This is strong evidence that selection is acting on the dorsal spines of both species. Since both species have more and longer spines where predators are present and where cover is reduced, and since predators are known to prey selectively on Apeltes (Blouw and Hagen, 1984c), the results are consistent with the hypothesis that predators are a common selective agent.…”

“…If the covariation between Apeltes and Pungitius is due to selective predation, we predict that spine length in Pungitius should be greater where spine number is higher, as is found in Apeltes (Blouw and Hagen, 1984c). We scored 43 samples of Pungitius for spine length (table 2).…”

“…The correlations suggest that predatory fishes act as selective agents favouring the higher-spined morphs. Indeed, populations with higher mean spine number also have longer spines and larger mean body size than populations with fewer spines (Blouw and Hagen, 1984c). Laboratory and field experiments with predators (four fish and one bird species) show that the spine morphs are differentially susceptible to predation.…”

“…Laboratory and field experiments with predators (four fish and one bird species) show that the spine morphs are differentially susceptible to predation. However the higherspined morphs are not always favoured, and vegetation plays a modifying role (Blouw and Hagen, 1984c). Thus the relationship between spine number and predation is complex.…”

SELECTIVE PREDATION FOR VERTEBRAL PHENOTYPE INGASTEROSTEUS ACULEATUS: REVERSAL IN THE DIRECTION OF SELECTION AT DIFFERENT LARVAL SIZES

“…I varied cover treatment in this way to test the possibility that the target of selection was a behavioral difference in the use of cover. This possibility was tested because (1) behavioral traits have been implicated as the targets of selection in previous demonstrations ofselective predation for morphological characters of sticklebacks (e.g., Moodie et aI., 1973;Reist, 1980aReist, , 1980bBlouw and Hagen, 1984a); and (2) the outcome of previous predation experiments has depended on the presence or absence of cover vegetation (Blouw and Hagen, 1984a). Larvae were allowed 15 hr of acclimation to experimental conditions before the introduction of sunfish to experimental tanks.…”