Co-option (also co-optation, sometimes spelled coöption or coöptation) has two common meanings:
Two common uses of co-option are:
Co-opted members may or may not have the same rights as the elected members of a group (such as the right to vote on motions), depending on the rules of the group.
If a group is elected or appointed based on the its members representing specific constituencies, co-option to fill vacancies is inappropriate, as a member selected by existing members will not necessarily represent the interests of the group represented by the vacating member. In this case, vacancies may be filled via a mechanism specified in its rules, such as a by-election. Examples are:
Sociologist Philip Selznick, in the context of the Tennessee Valley Authority (TVA), described this form as "formal co-optation" (he used the term "cooptation").
This is arguably a derivation from the first sense.
The outcome of such co-option will be specific to the individual case, and will depend on the relative strength of the co-opting and co-opted groups, the degree of alignment of their interests and the vigour with which their members are prepared to pursue those interests. For example, if a group concerned with the welfare of horses co-opted a group concerned with the welfare of mules, the resulting group might change its name, its publicity, or its methods of addressing cases of abuse; it might extend its operations to the welfare of donkeys or wild equines; etc.
Selznick, again in the context of the Tennessee Valley Authority, described this form as "informal co-optation", although the process he describes is almost indistinguishable from the corrupt sale of political influence.