VII GENE FLOW
Gene flow, which consists inthe transfer of genes between populations, most commonly via migrating individuals, is an important factor in evolution. Depending on its intensity and on the structure of the population, it can either speed up evolution, or, on the contrary, slow it down significantly. Gene flow becomes an important factor in mobile organisms as well as in organisms that never move during their lifetimes, i.e. also in sessile animals and in plants. This is because, for the purposes of gene flow, the most important parameter is not based on an individual’s mobility within the population of its species, but rather the ability to migrate, i.e., the usual distance between the place where a particular individual is born and where its offspring are born. Consequently, a pine-tree population whose pollen is spread over large distances by wind has a much greater migration ability, and thus also much more intense gene flow, than a bat population, whose members fly thousands of kilometers in their lifetime, but ultimately breed in the same cave as that in which they were born. It should be mentioned that, in single-cell organisms, especially prokaryotic organisms, the gene flow between populations can take the form of transfer of the genes themselves, such as in a viral transfection. Analogical processes of horizontal gene transfer between individuals of the same species, as well as between different species, can also occur in multicellular organisms. In this case, however, the mobility of individuals tends to be much higher than the mobility of the genes or viruses, rendering these processes practically negligible in gene flow. This chapter is concerned with the subject of migration in structured populations and the impact of gene flow on evolutionary processes inside populations and species.