What is the unit of evolution according to the synthetic theory of evolution?

this is a population.
in accordance with the synthetic theory of evolution, not individual characters or individuals are selected, but the genotype of the whole population, however, it is carried out through the phenotypic characters of individual individuals. This leads to the spread of beneficial changes throughout the population. The utility of variability will be determined by the natural selection of the group of individuals most adapted to life under certain conditions. Thus, the elementary unit of evolution is no longer considered to be an individual (as Lamarck believed), not a species (as Darwin believed), but a collection of individuals of one species that can interbreed, i.e. population.