The modern evolutionary synthesis (often referred to simply as the modern synthesis) brings together Charles Darwin's theory of the evolution of species by natural selection with Gregor Mendel's theory of genetics as the basis for biological inheritance. Major figures in the development of the modern synthesis include Theodosius Dobzhansky, Julian Huxley, Ernst Mayr, and George Gaylord Simpson.
According to the modern synthesis, genetic variation in populations arises by chance through mutation (mistakes in DNA replication) and recombination (crossing over of homologous chromosomes during meiosis). Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift, gene flow and natural selection. Speciation occurs gradually when populations are reproductively isolated by geographic barriers.
See also: Population genetics