The Glutamate Decarboxylase(GAD) Pathway (GDP) is a major acid resistance mechanism that allows Lactic acid bacteria (LABs) to survive in low pH food environments. In the thesis, we aimed to study the molecular evolution and population genetics of GDP genes in LABs to understand evolutionary processes shaping adaptation to high acid environments by contrasting species where the GDP genes are organized as an operon structure (Levilactobacillus brevis) versus lack of an operon structure (Lactiplantibacillus plantarum). Intraspecies molecular population genetic analyses with GDP genes of L. brevis and L. plantarum from various environments revealed that synonymous and non-synonymous nucleotide diversity is driven mainly by low-frequency changes. Neutrality tests revealed mostly negative values indicating negative selection against replacement changes. Similarly, molecular structure and amino acid characteristic analyses showed that none of the replacement changes on the GDP genes alter the important residues of the proteins supporting negative selection against non-conservative amino acid changes. Interspecies analyses were used to identify the closely related LABs. Moreover, phylogenetic analyses showed that the GDP gene tree topologies differed from the LAB species tree, indicating divergent evolutionary histories. The functionally preserved two gad copies of the L. brevis grouped separate phylogenetic clades, showing that the origin of the second gad gene might be via horizontal gene transfer from a phylogenetically distant LAB species rather than gene duplication. In conclusion, GDP in LABs exhibits a dynamic molecular evolutionary history that enables organisms to thrive in high acid environments.