  A general procedure to formulate asexual (unstructured, deterministic)
population dynamical models resulting from individual pairwise interactions is
proposed. Individuals are characterized by a continuous strategy that
represents all their behavioral, morphological, and functional traits.
Populations group conspecific individuals with identical strategy and are
measured by densities in space. Species can be monomorphic, if only one
strategy is present, or polymorphic otherwise. The procedure highlights the
structural properties fulfilled by the population per-capita growth rates. In
particular, the effect of perturbing a set of similar strategies is
proportional to the product of the corresponding densities, with a
proportionality coefficient that is density-dependent only through the total
density. This generalizes the law of mass action, which traditionally refers to
the case in which the per-capita growth rates are linearly density-dependent
and insensitive to joint strategy perturbations. Being underpinned with
individual strategies, the proposed procedure is most useful for evolutionary
considerations, in the case strategies are inheritable. The developed body of
theory is exemplified on a Holling-type-II many-prey-one-predator system and on
a model of a cannibalistic community.
