TEMPORAL VARIATION IN POPULATIONS OF THE MARINE ISOPOD EXCIROLANA - HOW STABLE ARE GENE-FREQUENCIES AND MORPHOLOGY

Excirolana braziliensis is a dioecious marine isopod that lives in the high intertidal zone on both sides of tropical America. It lacks a di spersal phase and displays a remarkable degree of genetic divergence e ven between localities less than 1 km apart. Nine populations of this nominal species from both sides of the Isthmus of Panama and one popul ation of the closely allied species, Excirolana chamensis, from the ea stern Pacific were studied for 2 yr for allozymic temporal variation i n 13 loci and for 3 to 4 yr for morphological variation in nine charac ters. The genetic and morphological constitution of 9 out of 10 popula tions remained stable. Allele frequencies at two loci and overall morp hology in a tenth beach occupied by E. braziliensis changed drasticall y and significantly between 1986 and 1988. The change in gene frequenc y is too great to explain by genetic drift occurring during a maximum of 14 generations regardless of assumed effective population size; dri ft is also unlikely to have caused observed changes in morphology. Sel ective survival of a previously rare genotype is more plausible but st ill not probable. The most credible explanation is that the resident p opulation at this locality became extinct and that the beach was recol onized by immigrants from another locality. Such infrequent episodes o f extinction and recolonization from a single source may account for t he large amount of genetic divergence between local populations off. b raziliensis. However, the low probability of large temporal genetic ch ange even in a species such as this, in which gene flow between local demes is limited and generation time is short, suggests that a single sample through time is usually adequate for reconstructing the genetic history of populations.