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Natural Selection in Experimental Populations of Reef-Building Corals (Scleractinia)

D. C. Potts
Evolution
Vol. 38, No. 5 (Sep., 1984), pp. 1059-1078
DOI: 10.2307/2408440
Stable URL: http://www.jstor.org/stable/2408440
Page Count: 20
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Natural Selection in Experimental Populations of Reef-Building Corals (Scleractinia)
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Abstract

Environmental conditions likely to exert selective pressures on the isoporan corals Acropora palifera and A. cuneata were examined in five habitats on the Heron Island reef, using clonal fragments that simulated juveniles 1-2 years old. By two criteria, growth and survival, the corals ranked the habitats in the same order of decreasing favorability (outer flat, crest, lagoon, inner flat, slope). Environmental favorability and stability were not correlated; the worst habitat (slope) was the most stable, the second worst (inner flat) was the most unstable. The habitats differed qualitatively in the physical (e.g., cold, storms, sedimentation) and biological factors (e.g., predation, territorial fish) causing stress to corals, and the factors differed quantitatively in the frequency, predictability, duration and intensity of their action. Most deaths occurred in short discrete events superimposed on low "back-ground" probabilities of death. Even if the same factor caused heavy mortality in more than one habitat, its mode of action differed. No mortality events were detected after 3 years, possibly because juveniles achieve refuges in size at ages ranging from 1-2 years on the outer flat to > 5 years on the slope, or because most vulnerable phenotypes had been eliminated by selection. Non-random patterns of mortality and growth favoring some genotypes at particular times in certain habitats indicate that each habitat is capable of exerting a distinctive selective regime on isoporan corals. Assuming that gene flow occurs among habitats on the same reef, the data are interpreted as evidence of complex regimes of disruptive selection tending to maximize genetic variation within populations over the reef as a whole. Since this study covered only a small part of the potential life spans of these animals, the descriptions of selective regimes are necessarily incomplete. However, the results do indicate the kinds of factors, and the temporal and spatial scales of environmental heterogeneity that need to be considered in ecologically realistic models of disruptive selection affecting juvenile corals.

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