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Above: The predominant morph at St. Paul’s Rocks, the golden.

Below: The all white morph.

range as an intrinsic characteristic of the species in question.

Fixed and permanent changes in color patterns are attributed to genetic differentiation, and its short-term development must be associated with an isolated population or with a population that was isolated in the past. This differentiation tends to occur faster in small populations (Palumbi 1994).

The colour variations occurring in the H. ciliaris population of St. Paul’s Rocks were interpreted as evidence of inbreeding in an isolated population (Edwards & Lubbock 1983b). One of the most recognized effects of inbreeding in a population is the trend to higher frequencies of homozygous genes and the consequent manifestation of

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recessive traits (Futuyma 1986). The variations are strong evidence that the St. Paul’s population is not receiving, or is receiving a very few H. ciliaris recruits from the mainland or from the closest islands of Fernando de Noronha Archipelago.

The causes of the H. ciliaris isolation are not clear and contradictory with the observation that most of the reef fishes of St. Paul’s Rocks are brought from the Brazilian coast or Fernando de Noronha by the Equatorial Undercurrent (Bowen 1966; Edwards & Lubbock 1983). Despite their widespread distribution in the Tropical Western Atlantic and the many examples of the long dispersal capabilities of some tropical reef fishes (Joyeux et al. 2001), H. ciliaris are not found in other extremely isolated oceanic

January 2003

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