then past magnetic orientations should be coincident with present-day orientations. However, this is obviously not the case. Indeed, magnetic anomalies (the nonalignment of the magnetic field in rocks with the present-day magnetic field) are an important data source in the reconstruction of the earth’s surface (Torsvik et al. 2001). Today the earth’s surface is understood to comprise a series of semirigid plates that are moving relative to one another, driven by the powerful convection currents of the underlying mantle. Because the continents are riding on the plates, the term continental drift has been replaced by plate tectonics (Cattermole 2000; Cox and Moore 2005). Additional evidence for plate tectonics comes from modern studies of the alignment of continents and the paleodistribution of organisms as shown by fossils.
Ages of North American Fish Families
Understanding how long particular fish groups have inhabited North America is, at best, a difficult endeavor. The direct evidence is based on the fossil record, which is usually incomplete and requires access to rock layers of various ages (i.e., the rock formations of particular ages must be exposed by weathering or excavation such as during road construction). In addition, even if there is a rock outcrop of a particular age, it might only represent a particular kind of habitat from one particular region and thus show great ecological bias (Patterson 1981). This would be akin to trying to understand the entire modern North American fish fauna by sampling only a few low-gradient rivers where they enter into the sea or several isolated lakes. In addition, the ages represented by fossils are likely minimum ages because, from the standpoint of vicariance biogeography, groups are older than their oldest fossil (Box 2.2; Parenti 1981). However, in spite of its shortcomings, the fossil record is often the best evidence that is available.
BOX 2.2 • Biogeographic Theory
From the standpoint of interpreting broadscale distribution patterns of organisms, there are two major paradigms of biogeography: dispersal explanations and vicariance explanations.
DISPERSAL EXPLANATIONS
There is a natural tendency of organisms to disperse within areas of suitable habitat, and certainly many organisms, including fishes, have welldefined long-distance dispersal patterns, achieved either through adult movement or through dispersal of larval stages. In the dispersal model, organisms are assumed to have migrated across preexisting barriers and there are many examples where this has occurred. However, when we consider the present-day distribution of related taxa over widely separated areas with no intervening populations (e.g., a disjunct distribution), then the dispersal explanation becomes more difficult.
The dispersal model was espoused by numerous early biogeographers, such as Darwin’s contemporary, Alfred Russell Wallace (1876), and later William D. Matthew (1915) and Phillip J. Darlington (1957). Among ichthyologists, Briggs (1974, 1995, and included papers) has been a strong proponent for the importance of dispersal as a primary mechanism.
VICARIANCE EXPLANATIONS
The basic tenet is that organisms are passively transported by movement of tectonic plates or by other geological means. If this occurs, then several or more taxa should share common distribution patterns, where the distribution of each taxon is referred to as a track. As such, a major starting point in vicariance biogeography is the search for common patterns of distribution (i.e., generalized tracks) among different taxa. If a common pattern of distribution exists for two or more monophyletic taxa, then this suggests that the generalized track may be due to geologic events (Croizat et al. 1974; Wiley 1981; Grande 1990). The emphasis in vicariance biogeography is on patterns generated by many, and not necessarily closely related, taxa. (In contrast, while not ignoring the generality of patterns, dispersalists have, at least historically, focused more on individual taxa.) The range of a species can be disrupted by the formation of a barrier (a vicariant event) so that a formerly contiguous population is split into separate populations (termed vicariance).
Pioneering studies by Croizat (1958; Croizat et al. 1974) helped form the basis for vicariance biogeography. For instance, Croizat’s panbiogeography (1958) ultimately worked as “a major catalyst for change during the 1960s resurgence of interest in biogeographical thought” (Keast 1991). Croizat amassed distributional patterns of species (e.g., tracks) and stressed the importance of concordant patterns (e.g., generalized tracks), even though in his 1958 book he still discounted the role of continental drift. Strong ichthyological proponents of vicariance biogeography have included Gareth Nelson and Norman Platnick (e.g., Nelson and Platnick 1981), Edward Wiley (e.g., 1981), and the late Donn Rosen (e.g., Rosen 1978).
SYNOPSIS OF PARADIGMS IN BIOGEOGRAPHY
In a dispersal model, the barrier is older than at least one of the isolated populations and the age of the barrier is older than the disjunction in range. In the vicariance model, the populations predate the age of the barrier and ages of the barrier and the disjunction in the range are the same. While there has been considerable argument among biogeographers about the relative merits of each major explanation, a synthesis of views is, undoubtedly, required to understand the distribution of fishes (Wiley 1981; Briggs 1995; Moyle and Cech 2004)
FIGURE 2.2. The configuration of the major continental landmasses at the close of the Paleozoic (250 mya) as Pangea approached its maximum extent. Based on Torsvik and Van der Voo (2002) and Torsvik and Cocks (2004).
Ages can also be determined indirectly from a well-developed phylogeny if there are fossils or geologic events available for calibration of molecular divergence times (Box 2.3; Lieberman 2003). For example, based on a calibrated molecular clock analysis, species’ divergence times within logperches (a group of darters in the genus Percina) ranged from 4.20 to 0.42 mya, with most speciation events taking place in the Pleistocene (Near and Benard 2004). The divergence times were based on the assumption that there is a constant rate of gene substitutions over time, and that by comparing the degree of genetic divergence among darter lineages, it is possible to convert the degree of divergence to a time estimate—the “molecular clock.” Because it is known that rates of gene substitution vary among taxonomic groups (Britten 1986; Avise 2004) and, consequently, that molecular clocks need to be calibrated using related taxa, Near and Benard (2004) used rates of gene substitution from the family Centrarchidae (in the same order, Perciformes, as the darters) and applied this to their analysis of logperches to generate their divergence time estimates.
BOX 2.3 • Phylogenies and Cladistic Methodology
The study of evolutionary relationships of species or higher taxa is the field of systematic biology. The concepts and methodologies underlying how evolutionary relationships are studied, and how best to portray the resultant phylogenies, have been areas of considerable debate and advancement over the last several decades. (For an overview, see Mayden and Wiley [1992] and Mayden and Wood [1995].) Concurrent with the development of principles and procedures of systematic biology, molecular data, first as protein analyses and then broadened to include mitochondrial and genomic DNA information, have complemented morphological, ecological, and behavioral data that are used in developing phylogenies. Phylogenetic (i.e., evolutionary) relationships are shown hierarchically in branching diagrams referred to as dendrograms or cladograms, depending on the methodology used to construct them. For the most part, especially among ichthyologists, phylogenies follow methods proposed by the German biologist Willi Hennig (English translation, 1966)—the method of phylogenetic systematics. This approach has been further developed by Niles Eldredge and Joel Cracraft (1980), Gareth Nelson and Norman Platnick (1981), Edward Wiley (1981), and many others.
Basic principles of the cladistic method are that (1) relationships among groups are based primarily on branching points in evolution and not on degrees of divergence; (2) recognized groups should be derived from a single ancestral group (the principle of monophyly); and (3) taxa should be recognized on the basis of possessing
