Evolutionary Biology Blogι**=7/3ψ
by Lars Witting

Permanent interactions in sessile lifeforms

Competition between sessile individuals is a permanent interaction for an inflowing resource (like light for plants), and this selects against co-operating individuals. Photo L. Witting

Some of the major transitions are found only in mobile organisms. Co-operate breeding and eusocial colonies are absent in sessile organisms, and their sexual reproduction is of another kind where it often occurs between hermaphrodites, and not between male and female individuals.

These differences are selected by interactions that are permanent, instead of transient, between sessile individuals (Witting, 1997, 2002). Sessile organisms are generally competing by position and size for a resource that is provided by radiation or by a flowing medium. Any interaction is a permanent competition between neighbouring individuals for resources, and this is generating constant interference with selection of size whenever the density of individuals is sufficiently high.

The permanent nature of the interactions is destroying the opportunity for the usual forms of cooperation between individuals. In sessile organisms that compete for an inflowing resource, one individual cannot compete for another unless it is also competing against it, and this is constraining the interacting unit to a single individual only; with no cooperation between female and male individuals, no cooperative breeding, and no eusocial colonies.

Males can compete for females when interactions are transient, but not when they are permanent (Witting, 2002). It thus follows that sessile organisms do not have an interacting selection that can outbalance the two-fold cost of the male, and nor the two-fold cost of meiosis. It is therefore not surprising that sexual reproduction in sessile organisms tends to occur between hermaphrodites that avoid both the two-fold cost of the male and the two-fold cost of meiosis. And with no direct male choice there is also no selection to prevent the evolution of asexual reproduction in sessile organisms with sexual reproduction.


  • Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL http://mrLife.org.
  • Witting, L. 2002. From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61:171--195.