Products of mutation, migration, and/or chromosomal rearrangement.

New alleles, that is, can arise as variations on alleles that are already present within an organism's genome (mutation), from other organisms (migration, that is, genetic migration, a.k.a., horizontal gene transfer), or by recombining together different loci (genes) already found within a given organism.

Together these processes supply what is often described as the 'raw material' of evolution, or more precisely, that genetic variation upon which natural selection can act. That is, what alleles that are present within a population can be modified in terms of underlying allele frequencies, including via the introduction of new alleles, which in turn are then subject to the same evolutionary forces.