A great number of phenomena ocurring in the living world can be explained through the symbiosis. Not only has the symbiosis given many advantages to the organisms which are parts of it, but also has accelerated the evolution of the living world. Horizontal gene transfer emerged as a consequence of endosymbiosis between the heterotrophic eukaryotes and the autotrophic prokaryotes. It is a process through which genes are exchanged between distantly related species. It is a key component of the genome evolution in the living organisms. Horizontal gene transfer gives the selective advantage to the organisms, providing the new metabolic pathways, antibiotic resistance, inhabitance of new ecological niches, speciation. Horizontal gene transfer ocurrs in all three groups of the organisms which can make a lichen. Fungi have acquired new metabolic synthetic pathways, pathogenecitiy and capability of inhabiting new ecological niches through horizontal gene transfer; cyanobacteria are capable of nitrogen fixation because of horizontally transfered genes; green algae have received genes that enable them to photosynthesize. Horizontal gene transfer between simbionts is still quite unknown and not existing. There is evidence though, which tells that there are certain genes being upregulated during the phase of forming symbiosis between algae and fungi. Vertical gene transfer is transfer of genes from parents to offspring by means of vegetative and sexual reproduction in lichens. Ecological conditions impact the genetic structure of lichen populations a lot. The speed of lichen evolution and their survival and prevalence of their existence depends on the genetic structure of populations. For lichens, both transfers have advantages. Horizontal gene transfer provides long-term adaptations and it has an impact on choosing the photobiont. Vertical gene transfer provides fast adaptations to the sudden changes in the enviroment.