There are various studies that indicate varying degrees of connections between Sri Lankan Tamils, Sinhalese and Indian ethnic groups.
[2] Tamils and the Sinhalese also share similar cultures in terms of kinship classification, cousin marriage, dress and housing.
[5][6][7] Another study has also found "no significant genetic variation among the major ethnic groups in Sri Lanka".
Dr Sarabjit Mastanain finding states cophenetic correlation was 0.8956 and it indicates Sinhalese & Tamil as native population.
Also, it reflects on genetic distance among five populations of Sri Lanka as per given below eigenvector plot of the R-matrix.