| Phylogeography and conservation genetics of the lesser white-fronted goose (Anser erythropus) | ||
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Variation in nature is partitioned into different levels of hierarchy: genes, individuals, populations, species and ecosystems, and together constitute the spatially and temporally variable biodiversity of nature. The heritable basis for this diversity is genetic variation, of which a large proportion is neutral or nearly so (Kimura 1968, 1983), and once introduced into a population by mutation, recombination and migration, its fate is determined largely by random genetic drift. Genetic variation affecting the fitness of an individual is the target for natural selection (Fisher 1930, Wright 1931, Haldane 1932).
Organisms live in an environment that is not constant over time. The present patterns of species’ diversity are affected by both past evolutionary history and current population processes and their relative significance are often difficult to distinguish based on contemporary observations. For example, genetic homogeneity among the populations of a species can be due to a recent common ancestry or contemporary gene flow. The ecological, life-history and behavioural characteristics of species, such as population size, population subdivision, dispersal and social structure, affect present diversity. Increasingly, environmental changes caused by man, such as habitat fragmentation, are involved.
Among the most influential factors in history affecting the structuring of present populations are the ice ages. During the past 2.5 My, the climatic and environmental fluctuations of the Pleistocene have forced species to adjust the distributional areas according to their adaptive ability. During cold periods, species responded by moving southwards to refugia and during milder periods they migrated northwards or to high altitudes, or expanded their distributional ranges without leaving the refugia (Roy et al. 1996, Blondel & Mourire-Chauviré 1998). The homogeneity and low level of diversification in the Northern Hemisphere have been explained as a consequence of repeated mixing of populations during the climatic oscillations preventing differentiation or limited availability of refugia and consequent high extinction rate (Webb & Bartlein 1992, Blondel & Mourer-Chauviré 1998, for empirical evidence see e.g. Klicka & Zink 1997).
By using a comparative approach genetic diversity can be organised into a meaningful estimation of the evolutionary relationships among lineages of organisms i.e. a phylogeny. Phylogenetic inference is an attempt to estimate the branching order of taxa and to define monophyletic groups of taxa (topology), and sometimes the process includes also the estimation of the evolutionary rates along the lineages (branch lengths). In practise a phylogenetic tree can be reconstructed by using different statistical methods, most often based on two kinds of criteria: by defining an algorithm which determines the tree (e.g. based on genetic distances: the estimated genetic distances between pairs of OTUs reflecting the degree of relatedness), or an optimality criterion used in selecting the best tree among all possible alternative trees (e.g. maximum parsimony: the number of evolutionary changes to explain the data is minimised) (Hillis et al. 1996). A variety of traits and characters, such as morphology, behavioural, physiological or life-history features and, nowadays, molecular characters have been increasingly employed to infer the phylogenetic relationships of taxa (for a review, see Hillis et al. 1996). A fundamental difference exists between a species tree, representing the true evolutionary pathways of a group of species, and a gene tree, often constructed based on one gene. The gene tree and the species tree are not necessarily congruent in terms of topology or branch lengths, owing to e.g. polymorphism at the time of divergence, reticulate evolution and sampling errors (Nei 1987).
The term phylogeography was first introduced by Avise et al. (1987). The developments in phylogeography mainly rely on the use of (animal) mtDNA: due to the maternal inheritance and nonrecombining nature of the mtDNA the haplotypes can be ordered phylogenetically, whereas most nuclear markers, e.g. microsatellites and allozymes, provide allele frequency-based data. Phylogeography deals with the genealogy of lineages and their geographical location, at the intra- and interspecific levels, and it can be considered as a bridge linking the study of micro- and macroevolutionary processes (Avise 2000). In the analysis and interpretation of lineage distributions, other fields of natural sciences such as population genetics, ecology, ethology, phylogenetics, paleontology and geological history are employed.
In comparative phylogeography, the phylogeographic patterns among the species are compared in order to find general patterns of evolutionary history and to reveal the evolutionary processes behind the patterns (Bermingham & Avise 1986, Zink 1996, Hewitt 2000). Comparative phylogeography can identify groups of species that have a common history of vicariance. For example, the post-glacial colonisation routes of mammals, amphibians, arthropods and plants in Europe showed that northern regions were generally colonised from refugia in the Iberian peninsula and the Balkans, whereas Italian lineages were often isolated by the Alpine barrier, although the distributions of the phylogroups were not spatially congruent among all the species (Taberlet et al. 1998). In studies of plant phylogeography, the cpDNA has been mostly employed (argan tree: el Mousadik & Petit 1996, oaks: Dumolin-Lapeque et al. 1997, Ferris et al. 1998). As a regional approach, comparative phylogeography can be used for conservation purposes to localise areas of high biodiversity and thus of high conservation value (Moritz & Faith 1998).