<P> Crossing over (genetic recombination) and random segregation during meiosis can result in the production of new alleles or new combinations of alleles . Furthermore, random fertilization also contributes to variation . </P> <P> Variation and recombination can be facilitated by transposable genetic elements, endogenous retroviruses, LINEs, SINEs, etc . </P> <P> For a given genome of a multicellular organism, genetic variation may be acquired in somatic cells or inherited through the germline . </P> <P> Genetic variation can be divided into different forms according to the size and type of genomic variation underpinning genetic change . Small - scale sequence variation (<1Kbp) includes base - pair substitution and indels . Large - scale Structural variation (> 1Kbp) can be either copy number variation (loss or gain), or chromosomal rearrangement (translocation, inversion, or Segmental acquired uniparental disomy). Genetic variation and recombination by transposable elements and endogenous retroviruses sometimes is supplemented by a variety of persistent viruses and their defectives which generate genetic novelty in host genomes . Numerical variation in whole chromosomes or genomes can be either polyploidy or aneuploidy . </P>

Which two factors provide the genetic base for the variation within a species