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 22 of 57

21 October 2014 | Birder's Guide to Listing and Taxonomy Traverse City, Michigan Morgan Churchill Linnaeus' views on species derive in part from earlier Greek philosophy, especially the works of Aristotle. Linnaeus believed that spe- cies and genera were discrete categories that remained stable over time and refected real en- tities. Thus, the taxonomy created by Linnaeus refected categories of similarity, and not any form of implied descent from a common an- cestor. These ideas were carried over into the Typological Species Concept (TSC). Under this species concept, a species is distinguished on the basis of discrete features of anatomy that distinguish it from other species. Early use of this concept led to over-recogni- tion of different species, mostly due to poor sam- ple sizes available to researchers and poor com- munication among scientists. The publishing of Charles Darwin's On the Origin of Species in 1859 presented further problems: If species gradually evolved into other species, then we should ex- pect a wide range of intermediate forms. Indeed, in that book Darwin highlighted the ramifca- tions of evolution for our understanding of spe- cies when he wrote, "It will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not es- sentially differ from the term variety, which is given to less distinct and more fuctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake." These inher- ently arbitrary lines that we use to separate spe- cies from mere races, varieties, and subspecies have created a problem that scientists continue to grapple with. Integrating Evolution and Speciation Although Darwin's theory of natural selection was published in 1859, the TSC would remain the only species concept used for much of the next half century. This did not change until the modern evolutionary synthesis, which integrat- ed genetics and natural selection. After that, an appreciation of the importance of reproductive isolation in speciation processes was developed, most notably in landmark works by Theodosius Dobzhansky (1937) and Ernst Mayr (1942). The work of the above authors ultimately led to the creation of the Biological Species Concept (BSC), one of today's most well-known species concepts, and the concept explicitly and currently used by the AOU. The BSC defnes a species as "groups of actually or potentially in- terbreeding natural populations which are re- productively isolated from other such groups" (Mayr 1940). The BSC provided a clear, (sup- posedly) testable criterion for delimiting species. Unlike the earlier TSC, the BSC was not solely focused on categorization but also on the process whereby species become distinct entities, with an emphasis on formation of reproductive barriers. These barriers could take two forms. Prezygotic barriers prevent fertilization of eggs or mating Indigo and Lazuli buntings regularly hybridize where they overlap, yet they are not widely considered the same species. Why not? Read on to fnd out! Photo © Scott Carpenter

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