FISHERY BULLETIN: VOL. 85, NO. 3 



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DIVERGENCE 



Figure 3. — Phenetic relationships among mtDNA composite cleavage 

 patterns of Atlantic herring. Estimates of sequence divergence were 

 clustered by UPGMA. 



12 > 1 14 3 ) ^ 



21 



Figure 4. — Cladistic relationships of 26 composite cleavage pat- 

 terns of Atlantic herring mtDNA. Composites are connected 

 parsimoniously to minimize the number of restriction site 

 changes required. Shaded numbers refer to composites observed 

 at all three spawning locations. Crossbars on connecting lines 

 indicate minimum number of site changes required to connect 

 adjacent composites; arrows indicate direction of site losses. Lo- 

 cality symbols: square - Trinity Ledge; triangle - Jeffries Ledge; 

 diamond - St. Lawrence. 



of mtDNA should have been able to differentiate 

 among such reproductively isolated populations. 

 However, the spawning groups studied were not 

 fully distinguishable by composite mtDNA diges- 

 tion patterns generated by six-base restriction en- 

 donucleases; no absolute stock markers were 

 present. Six of the twenty-six composite designa- 

 tions, representing more than 65% of all individu- 

 als, were shared by at least two geographically 

 distinct spawing groups. 



The occurrence of common composites in all 

 spawning populations could occur for at least two 

 reasons: First, commonality could reflect recent 

 and/or ongoing gene exchange among popula- 

 tions. Consistent with this idea, there is no associ- 

 ation between frequencies of common composites 

 (nos. 1-5) and spawning locality (G = 6.29, p 0.5, 

 Sokal and Rohlf 1981). As is generally acknowl- 

 edged, small numbers of individuals migrating 

 among populations are sufficient to homogenize 

 different groups (Allendorf 1983). The absence of 

 two common composites (1 and 3) in the 1984 

 Trinity Ledge sample might be due to the stochas- 

 tic effect of small sample size, though this obser- 

 vation could also imply some element of temporal 

 instability in composition. Second, and alterna- 

 tively, these common composites could represent 

 ancestral mtDNAs which were widespread prior 

 to any genetic isolation of populations. That is, 



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