Department of Biology
University of Memphis
Ellington Hall
3700 Walker Ave.
Memphis, TN 38152
phone: 901.678.4304
fax: 901.678.4746
Phylogenetic Approaches to Studying the Evolution of Avian Migratory Behavior
What are explicit factors leading to the evolution of migration? My research is driven by this question – by integrating molecular systematics, character mapping and historical biogeography, I attempt to uncover broad patterns that can explain why migration evolves across many avian taxa. An exciting aspect of this type of work is its broad applicability. The techniques I use and describe here can be applied to virtually any group of species or even within-species, and to any migration system.
Hypotheses for the evolution of migration.- Many hypotheses have been proposed to explain the evolution of migration, some of which present specific geographical and/or environmental factors that induce migration (Cox 1985; Levey and Stiles 1992; Safriel 1995; Chesser and Levey 1998). Cox’s (1985) geographical hypothesis, developed for Neotropical migration systems, describes migration evolving in a step-wise fashion: sedentary species expand their ranges into semi-seasonal subtropical areas. This seasonality then drives the evolution of migratory species (with other mechanisms operating). Hence, there is a relationship between seasonality and migration, and the hypothesis predicts that migratory species will have a subtropical origin. In other words, migratory and sedentary sister pairs (on a phylogenetic tree) will share a southern, subtropical ancestral area. An additional prediction is that there should be a phylogenetic correlation between seasonality (once a proxy is identified) and migration.
An ecological model, such as Levey and Stiles (1992) and Chesser and Levey (1998), also known as the evolutionary precursor hypothesis, states that open habitat/edge species may be predisposed to evolve migratory behavior. Hence, there is a relationship between habitat and migration, and the hypothesis predicts that there should be a phylogenetic correlation between open/edge habitat (non-buffered) and migration. There is evidence that partial migrant/migrants within the Austral system do use open habitats, but does this drive the evolution of migration?
Testing hypotheses for the evolution of migration.- These theories are testable within an evolutionary, phylogenetic framework, a method which a few authors, notably Chesser and Levey (1998) have undertaken. With regard to my own research, I am interested in testing the relationships between specific factors (e.g., geographical and ecological) and migration, by using robust molecular phylogenies as a foundation.
It is clear that hypotheses have been tested without phylogenies (e.g., Cox 1985; Levey and Stiles 1992). However, when examining potential patterns of relationships between taxa, and the subsequent evolutionary patterns of traits within a phylogeny, it become obvious that we cannot adequately describe patterns of evolution without knowing how the taxa are related that possess the traits of interest. In the simplest case, assume we are interested in looking at the
Figure 1a. Uncertain phylogenetic relationships between three taxa. Figure 1b. Phylogenetic relationships that suggest a sedentary ancestor of the three taxa. Figure 1c. Phylogenetic relationships that render an equivocal ancestral state at the root of the tree.
evolution of migration in a lineage consisting of three taxa (Figure 1). The two possibilities of relationships (Figures 1 b and c) between these two sedentary and one migratory taxa give us very different pictures about the evolution of migration in this lineage. In the first case (Figure 1b), sedentary behavior is ancestral, and migratory behavior evolved once, but in the second case (Figure 1c), the character state at the base of the tree is ambiguous. To reiterate, not only is the pattern of evolution unclear without a phylogeny, but we also could not examine the phylogenetic correlations between migration and any other number of potentially related traits.
The phylogenetic approaches discussed in this paper can apply to multiple taxonomic levels. Within-species studies, where genetic relationships between migratory and sedentary populations are examined, are the most process-oriented or mechanistic (Joseph et al. 2003). Other levels address pattern rather than process, but include species-level, where my own work has focused (Outlaw et al. 2003; Voelker and Outlaw submitted), genus-level, and family-level (Chesser and Levey 1998). As studies move up in taxonomic level, however, resolving power becomes limited.
The phylogenetic approach is not without limitations, and while there are many, I will specifically address the microevolutionary studies of Berthold and others (Berthold 2001).
Migration, as a suite of traits each controlled by many genes, can change rapidly under selection. A question that often arises in my work, is that even if analyses establish that sedentary behavior (for example) is ancestral at a given node (Figure 2), given the rapidity with which migration state can change, is not it possible that along the branch from that node, migratory and sedentary behavior have switched back and forth so many times, that the ancestral character state is meaningless? The answer is both yes and no. My results must be interpreted with caution, but there are a few techniques that acknowledge this rapidity (see below), and I would argue that while the approach has limitations, it still provides a meaningful framework for investigating patterns. Furthermore, the potential for switching is a universal limitation of any character-mapping study.
Despite being pattern-driven, I believe my approach has merit. One of the issues with microevolutionary (mechanistic) studies, is that they often do not address specific factors driving migration, and particularly not in obligate migratory species (while evolution of migration hypotheses do not address specific genetic mechanisms; Berthold 2001). Artificial selection, while important for understanding the heritability of migratory traits, does not equate with natural selective pressures. In addition, the species-level phylogenies with which I have worked may provide a broader framework for guiding future, mechanism-based studies.
References
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