A great deal of effort is spent protecting geographically peripheral populations of widespread species. We consider under what conditions it is appropriate to expend resources to protect these populations. The conservation value of peripheral populations depends upon their genetic divergence from other conspecific populations. Peripheral populations are expected to diverge from central populations as a result of the interwoven effects of isolation, genetic drift, and natural selection. Available empirical evidence suggests that peripheral populations are often genetically and morphologically divergent from central populations. The long-term conservation of species is likely to depend upon the protection of genetically distinct populations. In addition, peripheral populations are potentially important sites of future speciation events. Under some circumstances, conservation of peripheral populations may be beneficial to the protection of the evolutionary process and the environmental systems that are likely to generate future evolutionary diversity.



Se gasta mucho esfuerzo protegiendo poblaciones geograficamente perifericas de especies ampliamente distribuidas. En el presente estudio, consideramos bajo que condiciones es apropiado gastar recursos para proteger estas poblaciones. El valor de conservacion de las poblaciones perifericas depende de su divergencia genetica con respecto a otras poblaciones de la misma especie. Se espera que las poblaciones perifericas diverjan de poblaciones centrales como resultado de los efectos interconectados del aislamiento, la deriva genetica y la seleccion natural. Evidencia empirica disponible sugiere que las poblaciones perifericas son con frecuencia, divergentes genetica y morfologicamente de las poblaciones centrales. La conservacion a largo plazo de las especies depende probablemente de la proteccion de las poblaciones geneticamente distintas. Adicionalmente, las poblaciones perifericas son potencialmente importantes para eventos de especiacion futuros. Bajo algunas circunstancias, la conservacion de poblaciones perifericas podria ser beneficiosa para la proteccion de los procesos evolutivos y sistemas ambientales que probablemente generen la diversidad evolutiva futura.

When Are Peripheral Populations Valuable for Conservation

Genetic structuring of plant populations is strongly influenced by both common ancestry and current patterns of interpopulation genetic exchange. The interaction of these two forces is particularly confounding and hence interesting in plants. This complexity of plant genetic structures is due in part to a diversity of reproductive ecologies affecting genetic exchange and the fact that reproductive barriers are often weak between otherwise morphologically well-defined species. Phylogeographic methods provide a means of examining the history of genetic exchange among populations, with the potential to distinguish biogeographic patterns of genetic variation caused by gene flow from those caused by common ancestry. With regard to plants, phylogeography will be most useful when applied broadly across the entire spectrum of potential genetic exchange. Although current phylogeographic studies of plants show promise, widespread application of this approach has been hindered by a lack of appropriate molecular variation; this problem is discussed and possible solutions considered.

Phylogeographic studies in plants: problems and prospects

This survey emphasizes the possibilities of isozymes as marker genes in solving a variety of problems important to population geneticists, evolutionists, systematists, and plant breeders.

Isozymes in plant biology

Summary
Enormous strides have been made in the study of polyploidy over the last 20 yr. Here, we highlight some of these discoveries and note where our understanding of polyploid evolution has changed. Genetic and genomic studies have dramatically altered the polyploidy paradigm. The estimated frequency of polyploidy has increased, and it is now recognized that multiple origins are the rule for most polyploids. Likewise, autopolyploidy is much more common than traditionally maintained. Rapid genomic rearrangements, genomic downsizing, movement of genetic elements across genomes, and the movement of foreign genetic materials into the polyploid genome illustrate the complex dynamics of polyploid genomes. Following polyploidization, both genetic and epigenetic mechanisms may play an important role in altering gene expression. Ecological studies reveal that plant polyploidy can have profound effects on interactions with animal herbivores and pollinators and that polyploidy may trigger changes in the reproductive biology of a species. Despite the recent advances in our understanding of polyploid evolution, many exciting aspects remain under-investigated. Some of these include the consequences of genetic and genomic changes in natural polyploid populations, the physiological and ecological effects of polyploidy, and whether recurrent polyploidy prompts evolution to repeat itself.

Advances in the study of polyploidy since Plant speciation

Ecological opportunity – through entry into a new environment, the origin of a key innovation or extinction of antagonists – is widely thought to link ecological population dynamics to evolutionary diversification. The population-level processes arising from ecological opportunity are well documented under the concept of ecological release. However, there is little consensus as to how these processes promote phenotypic diversification, rapid speciation and adaptive radiation. We propose that ecological opportunity could promote adaptive radiation by generating specific changes to the selective regimes acting on natural populations, both by relaxing effective stabilizing selection and by creating conditions that ultimately generate diversifying selection. We assess theoretical and empirical evidence for these effects of ecological opportunity and review emerging phylogenetic approaches that attempt to detect the signature of ecological opportunity across geological time. Finally, we evaluate the evidence for the evolutionary effects of ecological opportunity in the diversification of Caribbean Anolis lizards. Some of the processes that could link ecological opportunity to adaptive radiation are well documented, but others remain unsupported. We suggest that more study is required to characterize the form of natural selection acting on natural populations and to better describe the relationship between ecological opportunity and speciation rates.

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Ecological opportunity and the origin of adaptive radiations

The Hawaiian silversword alliance consists of the three genera Dubautia, Argyroxiphium, and Wilkesia, and is a classic example of adaptive radiation in an insular setting. Genetic variation and interspecific genetic differentiation based on ten enzyme loci are described for Dubautia and Wilkesia. Genetic identities among species span the range of values expected from interpopulation comparisons within a single species (I = 0.90-1.00) to those typical of interspecific comparisons (I→=0.67). Genetic-identity values correspond to biogeographic distribution and morphological distinctiveness, supporting a correlation of increasing genetic distance associated with the time of separation among lineages. It may be inferred that the high genetic identities observed within the Hawaiian Madiinae and other island plant groups are due to limited time spans available for taxa to accumulate new genetic variation through mutation. It appears that species may remain genetically similar (I > 0.90) even after time spans on the order of magnitude of 1,000,000 years.

Martha S. Witter

Papers

Adaptive radiation and genetic differentiation in the hawaiian silversword alliance (compositae: madiinae)

Duplicate expression of biochemical gene markers in the Hawaiian silversword alliance (Madiinae:Compositae)