ADAPTIVE STRATEGIES OF COLONIZING ANIMAL SPECIES

1. An understanding of the adaptive strategies of colonizing animal species depends upon an integration of population genetics and ecology, but behavioural components should not be ignored especially higher in the phylogenetic series than insects. 2. An ecologically marginal habitat from which colonists are derived can be regarded as one in which physical stresses of climatic origin tend to be variable and extreme, so that resources are unpredictable and short lived. Prerequisites for genetic analysis are therefore phenotypes relatable to the r K continuum of adaptive strategies. These can be called ‘ecological phenotypes’. 3. Ecological phenotypes include tolerances to environmental stresses, development time, and resource utilization variability. Such phenotypes enable distinctions to be made between colonizing species and non-colonists. For example, colonizing species have ecological phenotypes incorporating high resistance to physical stress, rapid development time, and the exploitation of an array of food resources. They are ecologically versatile generalists. This includes the use of ethanol as a resource in Drosophila. 4. There is a substantial literature on variation in central and marginal populations based upon gene and chromosome polymorphisms. Most data show a reduction of chromosome polymorphisms and of lethals and semilethals towards the margins, but no equivalent reduction in enzyme polymorphisms. Widespread species tend to have low levels of chromosome polymorphisms as in marginal populations, but enzyme polymorphism levels vary too much among species for meaningful interpretations. Since these are genotypic assessments not directly relatable to the field situation, the somewhat unsatisfactory nature of these data from the interpretative point of view is understandable. In addition, the fundamental issue is not the variability of the genome, but the nature and role of loci controlling ecological phenotypes. 5. Ecological phenotypes can be analyzed at the population level with isofemale strains as the starting material. In theory, genetic activity can then be localized to the chromosomal and even genic levels in a species such as D. melanogaster. Isofemale strain studies in D. melanogaster are interpretable in terms of the r K continuum, and so reflect adaptive strategies in nature. 6. It is highly likely that the genetic architecture of ecological phenotypes of marginal populations mainly comprises a few additive genes of relatively large effect. This is an architecture permissive of rapid adaptation to new habitats, provided that the appropriate genes are present. Discussions of speciation via the founder principle, a colonization event in itself, have invoked a similar explanation.