Birder's Guide

OCT 2014

Birder's Guide is the American Birding Association's newest publication. Each issue focuses on a key subject, providing tips from experienced birders on a wide variety of topics like Travel, Listing & Taxonomy, Gear, and Conservation & Community.

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Page 23 of 57

22 Birder's Guide to Listing and Taxonomy | October 2014 Species & Subspecies : A Brief History into one. How narrow a hybrid zone must be to constitute a barrier to speciation is subjective and varies from species to spe- cies (Cracraft 1997), and there are plenty of taxa with apparently stable hybrid zones that are currently not considered separate species, such as Myrtle and Audubon's warblers (Brelsford and Irwin 2009) and "California" and "Woodhouse's" scrub-jays (Delaney et al. 2008). The Rise of Historical Species Concepts The BSC focuses on what is happening today: Are two populations readily inter- breeding? But some scientists have argued for a more historical approach to defn- ing species—one which emphasizes the evolution of populations through time. Perhaps the most frequently cited histori- cal approach is the Phylogenetic Species Concept (PSC). Under this approach, species are delimited according to their po- sition within a phylogeny. A phylogeny is a diagram showing the branching pattern of ancestry and descent, created by scien- tists to study evolution. These diagrams are created by assembling a dataset of traits which indicate similarity between species; these traits could represent parts of the anatomy, plumage, or genes. Computer algorithms are used to analyze the data and group species together on the basis of shared traits. For a species to be rec- ognized under the PSC, all members of a population should be more closely related to one another than to other populations. Species which fulfll this criterion are con- sidered to show monophyly. They should This isn't to say that the BSC is perfect. The concept's emphasis on interbreeding (or lack thereof) has meant that the theory is hard to test in populations which exist in allopatry. For instance, many birds show regional varia- tion across different island chains with no evidence of intermingling of populations. If two taxa never come into contact but are closely relat- ed, how does one determine whether they can interbreed? Ornithologists often must use vocalizations and differences in plumage to attempt to make this determination, but voice is often plastic within songbirds, and using plumage differences requires a cer- tain degree of arbitrariness in designating what level of distinctiveness is necessary. In addition to diffculty with allopatric species, the BSC also has trouble dealing with hybridization. Many closely related species have hybrid contact zones, includ- ing Carolina and Black-capped chicka- dees (Bronson et al. 2005, 2003), Lazuli and Indigo buntings (Baker and Boylan 1999; Carling and Brumfeld 2008), and Townsend's and Hermit warblers (Rohwer and Wood 1998; Rohwer et al. 2001). Generally speaking, if a hybrid zone re- mains comparatively narrow and doesn't widen over time, then the hybrids pro- duced from such interspecies coupling are not likely to reproduce and probably will not contribute to the two species merging between two species from ever happening; thus, they prevent the formation of a zy- gote. They can take the form of differences in reproductive timing, display, vocaliza- tion, or reproductive anatomy. Postzygotic barriers occur after fertilization and result in sterile hybrids or hybrids that are either less adapted to the environment than their parents or less able to produce offspring. These barriers exist in the wild, but may not be present in captivity. The BSC rev- olutionized taxonomy because it trans- formed what had been a glorifed fling system into an adjustable framework that allowed for testable hypotheses. This presumed "Grant's" Storm-Petrel, photographed off North Carolina's Outer Banks in May, is one of the many cryptic spe- cies that seem to make up Band-rumped Storm-Petrel. Photo © Mike Danzenbaker Italian Sparrow (right) is the result of (long- ago) hybridization between Spanish Sparrow (left) and House Sparrow (center). Even though Italian Sparrow has a hybrid origin, Spanish and Italian sparrows have been found to only breed with their own kind in areas of sympatry in southern Italy. Photos © (left to right) Jan Wegener, Jacob Spendelow, Mark Piazzi.

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