Malthusian Relativityι**=7/3ψ
The natural selection of lifeforms

Sessile organisms - the permanent interactor

Sessile organisms are selected by interactive encounters that are permanent

Malthusian Relativity is developed mainly for mobile organisms where interactive encounters between individuals are transient. In sessile organisms, the interactive encounters are permanent, and this is altering natural selection and the resulting life history.

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.

As males can compete for females when interactions are transient, but not when they are permanent, it 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 (Witting, 1997, 2002). 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 male choice there is 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
  • Witting, L. 2002. From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61:171--195.