Answer the following question given this paragraph: 

Mutations and evolution There are often multiple alleles of a particular gene in the population and they all may be equally normal, that is have similar effects on reproductive success andin terms of the phenotypes they produce. If there is no significant selective advantage between them, their relative frequencies within a population will drift. At the same time, the phenotype associated with a particular allelecan be influenced by which alleles are present at other genetic loci, known as the genetic background. Since most traits are the results of hundreds or thousands of genes functioning together, and different combinations of alleles can produce different effects, the universe of variation is large. This can make identifying the genetic basis of a disease difficult, particularly when variation at any one locus may make only a minor contribution to the disease phenotype. On top of that, environmental and developmental differences can outweigh genetic influence on phenotype. Such genetic background effects can lead to a particular allele producing a disease in one person and not another.270 Mutations are the ultimate source of genetic variation – without them evolution would not occur. Mutations can lead to a number of effects, in particular, they can create new activities. At the same time these changes may reduce the original (and necessary) activity of an important gene. Left unresolved such molecular level conflicts would greatly limit the flexibility of evolutionary mechanisms. For example, it is common to think of a gene (or rather the particular gene product it encodes) as having one and only one function or activity, but in fact, when examined closely many catalytic gene products (typically proteins) can catalyze “off-target” reactions or carry out, even if rather inefficiently, other activities - they interact with other molecules within the cell and the organism. Assume for the moment that a gene encodes a gene product with an essential function as well as potentially useful (from a reproductive...


Q: What are the ultimate source of genetic variation?


A:
mutations