Orbacz and Gaffney: Genetic stucture of Tautoga onilis 



341 



action of historical fluctuations in 

 population size, such as bottle- 

 necks caused by disease epidem- 

 ics or environmental change, as 

 well as a large variance in repro- 

 duction among females (Bowen 

 and Avise, 1990). Low levels of 

 nucleotide sequence diversity ob- 

 served in some teleost fish have 

 also been attributed to rates of 

 sequence evolution that are lower 

 than those estimated for birds and 

 mammals (Bernatchez and Danz- 

 mann, 1993). 



The Pleistocene glaciation is 

 believed to have decreased the 

 genetic diversity of many species 

 through bottleneck effects (Ber- 

 natchez et al., 1989; Avise, 1992). 

 During the Pleistocene era, ten or 

 more glacial advances and retreats 

 that caused changes in sea level 

 and climate are believed to have 

 had significant impacts on the dis- 

 tributions of coastal and marine 

 fauna (Avise, 1992 ). Tautog population size may have 

 been reduced through the reduction of suitable habi- 

 tats for both themselves and their food resources. 

 During the most recent glacial event, ice covered all 

 waters north of Long Island and would have forced 

 temperate populations southward into regions where 

 high relief, hard substrate habitats are patchier and 

 less abundant. Glacial advances also resulted in 

 drops in sea level that would have reduced the avail- 

 ability of estuarine environments. In addition, post- 

 glacial dispersal from southern refugia may have 

 created population bottlenecks if current populations 

 were founded by a limited number of propagules. 

 The same considerations may apply to the black sea 

 bass Centropristis striata, an estuarine-dependent 

 species with low nucleotide diversity (0.03%), corre- 

 sponding to an estimate of =5,000 for N^^^, the effec- 

 tive number of females (Bowen and Avise, 1990). 



In addition to the low levels of within-population 

 nucleotide diversity observed, this analysis also failed 

 to detect genetic heterogeneity among tautog sam- 

 ples from different geogi-aphical regions. The uncor- 

 rected mean nucleotide sequence divergences among 

 tautog samples were of the same magnitude as mean 

 nucleotide diversities within samples; the mean dif- 

 ference between haplotypes randomly drawn from a 

 single sample was equivalent to the mean difference 

 between haplotypes drawn from different samples. 



Low levels of intraspecific genetic variation do 

 not preclude the detection of genetic differentiation 

 among stocks. Shulman and Bermingham (1995) 

 detected significant population subdivision within 

 the Caribbean reef fish Stegastes leucostictus, for 

 which they reported a mean within-sample sequence 

 diversity of 0.077f. Bowen and Avise (1990) reported 

 a mean within-sample sequence diversity of 0.03% 



