How does (inheritable) variation arise in any population?

Variation arise in any population by mutation, random mating between organisms, random fertilization, and crossing over between chromatids of homologous chromosomes during meiosis.

Explanation:

Genetic variation or Inheritable variation is a general estimation that identifies the existing genetic differences over the population. These differences take place in the DNA or Genes and each variation is known as an allele.

Thus Genetic variation is also defined as the variation in alleles of DNA or genes that takes place among and within the population. Each individual is adopted to its nature and environment.

The Genetic Variation is achieved by mutation, mating and fertilizing between random organisms and cross over of chromatids during meiosis (cell division).