. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. POPULATION GENETICS OF A MEIOFAUNAL POLYCHAETE 217. Figure 1. Records (open circlesl and sampling sites (solid circles) for Hesionides gohari and sampling site for H. riegemrum (triangle). fresh water, H. riegerorum Westheide, 1979: it can be distinguished from H. gohari by various morphological features. The demonstrated almost cosmopolitan distribution of these Hesionides species, together with that of many other species of various meiofaunal taxa, gave rise to a still- controversial hypothesis of Sterrer (1973) (see also
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. POPULATION GENETICS OF A MEIOFAUNAL POLYCHAETE 217. Figure 1. Records (open circlesl and sampling sites (solid circles) for Hesionides gohari and sampling site for H. riegemrum (triangle). fresh water, H. riegerorum Westheide, 1979: it can be distinguished from H. gohari by various morphological features. The demonstrated almost cosmopolitan distribution of these Hesionides species, together with that of many other species of various meiofaunal taxa, gave rise to a still- controversial hypothesis of Sterrer (1973) (see also Rao, 1972) based on the notion that speciation of these forms is extraordinarily slow. It is postulated that they were already present on an old supercontinent and were distributed over their present vast range by the drifting apart of the conti- nental plates. This explanation could apply only if the genetic changes in the separated populations were relatively small over long periods of time, and mostly had no effect on the phenotype. Such minimal genetic variability can be explained by assuming a complete constancy of the ecolog- ical factors in the habitat of these animals, the sand beaches. Sterrer's hypothesis appeared necessary because long- range, transoceanic dispersal of these meiofaunal organisms seemed inconceivable: they are not capable of active swim- ming and, with few exceptions, have no larval dispersal stages. However, it gradually became evident that, even without dispersal stages, many of these species do succeed in colo- nizing geologically young islands far from any coast (West- heide. 1991): furthermore, dispersal of meiofaunal individ- uals in the seawater column along shores has been observed and demonstrated experimentally (Hagerman and Rieger, 1981; Palmer, 1988; Armonies. 1989). These observations led to the proposal that the cosmopolitan distribution pattern is ascribable not to geologically ancient processes but rather to occasional contemporary eve
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