Annual Review of Ecology, Evolution, and Systematics - Volume 24, 1993

To achieve a coherent evolutionary theory, it is necessary to account for the effects of the environment on the process of development. Phenotypic plasticity is the change in the expressed phenotype of a genotype as a function of the environment. Various measures of plasticity exist, many of which can be united within the framework of a polynomial function. This function is the norm of reaction. For the special case of a linear reaction norm, genetic variation can be partitioned into portions that are independent and dependent on the environment. From this partition two heritability measures are derived which can be used, alternatively, to compare populations or make predictions about the response to selection. Genetically, plasticity is likely due both to differences in allelic expression across environments and to changes in interactions among loci; plasticity is not a function of heterozygosity. Plasticity responds to both artificial and natural selection. The evolution of plasticity is modeled in three ways: optimality models, quantitative genetic models, and gametic models. All models make similar predictions about the conditions that will favor plasticity. In need of further development are the extension of quantitative genetic models, and structured population models; also needed are data on the true shapes of reaction norms and genetic variation and covariation for nonlinear reaction norm parameters and multiple environments.

Although the potential genetic risks associated with rare or endangered plants and small populations have been discussed previously, the practical role of population genetics in plant conservation remains unclear. Using theory and the available data, we examine the effects of genetic drift, inbreeding, and gene flow on genetic diversity and fitness in rare plants and small populations. We identify those circumstances that are likely to put these plant species and populations at genetic risk. Warning signs that populations may be vulnerable include changes in factors such as population size, degree of isolation, and fitness. When possible, we suggest potential management strategies.

We assessed the current state of avian molecular systematics by (i) considering some of the historical factors that have shaped the field in the last 20 years, (ii) reviewing the most commonly used molecular methods, and (iii) comparing higher-level phylogenies via congruence analysis. This three-pronged approach permitted us to identify strongly supported aspects of avian phylogeny and to propose technological and methodological explanations when congruence was low. We found, in general, that few areas of higher-level avian phylogeny are well supported and, hence, well understood. One main reason for this is that, despite a great deal of effort, few studies of higher-level avian phylogenetic relationships have been well planned and executed. Some investigations, for example, have gone astray because of preconceptions about rates of molecular evolution and monophyly, and others suffer from such problems as failure to find the shortest tree, lack of an outgroup, use of a nonmetric distance measure, and simple mistakes. This is not to say, however, that available techniques are incapable of reconstructing avian phylogeny. The extent of congruence that we found among branching patterns estimated by different methods, including carefully designed cladistic morphological analyses, indicates that when applied appropriately, a variety of methods provide useful insight into phylogeny.

Within-population variability in plant reproductive traits can influence both male and female fitness, but research on the male function of flowers has been hindered by the difficulty of measuring male fertility. Here we evaluate studies that employ paternity analysis to examine how specific plant traits affect male reproductive success (RS) in both natural and artificial populations. These studies illustrate the risks of assuming that male RS is correlated with female RS or with components of male fitness, such as the amount of pollen produced per plant. In some studies, paternity was assigned by simple genetic exclusion using unique multilocus allozyme profiles. More powerful methods involve statistical procedures that assign paternity to the most likely father of each offspring. Lack of genetic markers is a common problem in paternity analysis, and we discuss the types of molecular markers that may soon become more widely used in small, natural populations.