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found in models considering two, three and four clusters. Here, the algorithm found the
same clustering solution across all runs, with an average pairwise similarity of H= 0.98
that was far beyond the threshold value of0.85 for highly similar clustering results
(Rosenberg et al. 2004). Conversely, several clustering solutions appeared for K > 4;
these results may occur when trying to identify more gene pools than there are sets of
allele frequencies in the model (Gamier et al. 2004).
The uppermost hierarchical structure present in the data, the ~K statistics (Evanno et al.
2005) evidenced a weak peak occurring at K = 2 (Fig. 2b); nevertheless, the two clusters
(A and B) did not identify groups of geographically isolated populations (Fig. 3). At K =
3 we found a cluster that was almost restricted to the Mediterranean sea (cluster C),
whereas the other showed a patchy distribution in the Atlantic, western Mediterranean
and eastern Mediterranean basins. However cluster C did not result from the split ofone

cluster found at K =2; rather individuals ami/or populations assigned to clusters A and
B were merged in this cluster (Fig. 3). Sirnilarly, the fourth cluster (D) found at K = 4

resulted from the re-allocation of individuals previously assigned to clusters A, B, C.
Using an individuai mernbership coefficient threshold of Q 2:: 0.51 56 out of 478
individuals (12 %) were re-allocated to cluster D; this fraction decreased to 9% (42
individuals) according to a threshold of Q 2:: 0.80 which denote pure ancestry to a
cluster (Vaha et al. 2007).

The two-level AMOVA evidenced a significant genetic structure arnong
populations which accounted for 27% oftotal genetic variation (<DsT = 0.271 , P<
0.001). The three-level AMOVA did not show a significant genetic differentiation
between Tenerife (Macaronesia) and the continental samples, as well as between
Atlantic and Mediterranean regions (Table 4). Conversely, we found a significant, albeit
slight genetic differentiation between western and eastern Mediterranean basins,

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Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PA TELLA FéRRUGINEA, PATEL-LA UL l'SSIPONENSIS (MOLLUSCA: GASTROPODA) E PINNA
NOBILIS (MOLLUSCA: BIVALVIA): lLCONTRJBUTO DEl DATI MOLECOLARl ALLA CONSERVAZIONE DI SPECIE MINACCIATE - Tesi d i douorato in Scienze
della natura e delle sue risorse • indiri7.zo BK>logi.a Ambientale, Università degli Studi di Sassarl