Population Models Cases With An Evolutionary Focus

The role of evolutionary forces in determining the niche of an ecosystem engineer has also received considerable attention (e.g., Odling-Smee et al. 2003). The implication is that ecosystem engineering (or, equivalently, niche construction) is both affecting and affected by natural selection. These models fall into the same general class of engineer-environment feedback models as the ecological models we have described.

In this volume, Wilson (Ch 3) relates the simple population model of Cuddington et al. (unpublished) to evolutionary consequences. He illustrates how a mutant could increase in a population by responding positively to environmental changes that are made by the wild-type population activities. Given the feedback he describes, it is possible for selection to favor enhanced environmental modification.

In the first modeling contributions on this topic, Laland et al. (1996, 1999) use a set of recursion equations to examine the dynamics of traits that code for environmental alteration, "recipient" traits whose fitness depends on such alteration, and the amount of resource in the environment. A simple two-locus engineering model shows that novel evolutionary dynamics can result from ecosystem engineering, with a key role played by temporal scales. Engineering can lead to the fixation of deleterious alleles, the elimination of stable polymorphisms, and a sta bilization of previously unstable polymorphisms. One characteristic of ecosystem engineering is to lengthen the ecological timescale, which thus means that the evolutionary and ecological processes are operating on more similar timescales. As a result periods of evolutionary inertia or momentum can emerge.

In a set of simulation models, Hui and coauthors extend the investigation of the evolutionary consequences of engineering to the spatial domain (Hui et al. 2004, Hui and Yue 2005, Han et al. 2006). For example, Hui et al. (2004) find that engineering can alter the genetic dynamics and diversity of metapopulations on a lattice environment, while concurrently altering environmental heterogeneity.

Several authors have examined the particular case of the evolution of flammability in plants (Bond and Midgley 1995, Kerr et al. 1999, Schwilk and Kerr 2002). Mutch (1970) speculates that higher levels of flammabil-ity could arise if plants had the ability to pass on their genes in spite of periodic fires. The question arises as to how exactly increased flammabil-ity could spread in a population, since the possessor of these alleles would be more likely to die. Kerr et al. (1999) develop a haploid model with separate loci for flammability and response to fire. They track flammable and less flammable alleles, as well as alleles that code for additional or reduced success in gaps created by fire. They describe gap frequency as a linear function of the frequency of highly flammable plants. These authors find that the presence of flammability enhancing traits can redirect the evolution of other traits, through the effect that the flammability alleles have on the environment. Stable polymorphisms of flammability are possible, as well as stable and unstable oscillations of genotypes. The magnitude of the engineering effect determines when such behavior is possible. Schwilk and Kerr (2002) extend this work to the spatial domain by tracking diploid genotype frequency on a lattice. They find that if mating, dispersal, and production of fire gaps are all strictly local processes, flammability may increase in frequency without any direct fitness benefit.

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