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This chapter focuses on evolutionary implications that can be inferred from the existence of within-plant variation in organ traits and the associated ecological phenomena mediated by interactions with animals. The adaptive levels of environmental phenotypic variance in organ traits can be maintained by selection, regardless of the mechanisms producing it. Subindividual variability often accounts for nontrivial proportions of total environmental phenotypic variance of organ traits; selection by animals on variability has the potential to modify the magnitude of environmental variance and, in so doing, shift the balance between the genetic and environmental components. It is suggested that the environmental and genetic factors may be envisaged as “competing” to produce a given level of phenotypic variance. Thus, the spatial and temporal dynamics of such competition has manifold evolutionary implications, and animals can play a driving role by shifting the balance toward one side or the other.

It is assumed that dominance genetic variance contributes little to the prediction of evolutionary change in polygenic traits. This is based on the assumption that populations are large, panmictic, and randomly mating. However, the ecological contexts of most wild populations studied to date violate one, if not several, of these assumptions, and the widespread occurrence of inbreeding and inbreeding depression of phenotypic traits and fitness suggests dominance genetic effects are ubiquitous. This chapter reviews what genetic dominance represents at the level of a single locus and how this contributes to phenotypic variation and discusses how to estimate dominance variance with emphasis on the complications arising in wild populations and with inbreeding. Next, empirical estimates of dominance variance are reviewed. Since no estimates exist of dominance variance in the wild (except for humans), laboratory and agricultural populations are examined, and it is shown that dominance variance is a major contributor to phenotypic variation and in some cases contributes as much as additive genetic variance. This chapter also discusses how inbreeding and dominance affect predictions of evolutionary change, and ends with a review of some of the empirical questions for which genetic dominance is an important quantity in its own right. In this chapter, it is argued that dominance variance has been ignored for too long, may hamper the ability to predict evolutionary change, can be a major contributor to phenotypic variance, is interesting to study in its own right, and provides many avenues of research to be addressed by empirical study.

The paleontological and phylogenetic evidence have shown that the main features ultimately responsible for plant modularity were already present at a very early stage in the evolution of land plants, and are a property shared by the whole lineage. The ecological and evolutionary implications of plant modularity have frequently been highlighted following White's pioneering treatment of plant individuals as metapopulations of repeated modules. One of the consequences of plant modularity is the appearance of a distinctive source of phenotypic variance, that is, the within-plant or subindividual component. Another consequence of the multiplicity of modules is variation in the characteristics of the copies of the same organ produced on different modules of the same plant. A thesis is developed that the multiplicity of homologous structures arising from plant modularity gives rise to a subindividual level of phenotypic differences among organs of the same plant.