Increase in number and especially frequency of a specific genotype.

Clonal expansion is the rise of a clone particularly as a consequence of some form of selection. The consequence therefore tends to be an end point of a higher as well as more relevant frequency (i.e., not or no longer rare as well as more fit) but not necessarily resulting in total dominance of a population by that clone (as may occur as a consequence instead of severe genetic bottlenecking).

This selection can be either or negative (i.e., either for the expanding clone or against the other clones) and the resulting expansion can be in absolute terms (increase in ) or relative terms (increase in the frequency of the clone).

Clonal expansion as a term is used similarly to that of and selective sweep, except that it is less microbiological in application and less dependent on increasing allele frequency to fixation. Use of the term instead tends to be found in immunology, , and epidemiology.

The following, very short video is an illustration of clonal expansion within the context of immunology:

As cited from , note the use of "clonal expansion" as highlighted in bold:

Over the past few decades, an increase in the frequency of in has resulted in the preferential use of for disease therapy, with a concomitant increase in the numbers of -resistant strains. In some areas, such as southern , most are now nalidixic acid resistant . It was conceivable that this increase in resistance reflected the clonal expansion of cells derived from a single nalidixic acid-resistant , but this was not the case. Five independent mutations were detected within H58 [a strain] and at least 16 distinct were identified among 119 strains of Typhi that were resistant to nalidixic acid… These observations provide a striking demonstration of the strength of the exerted by for acute disease.