Evolution in the Rock Dove: Skeletal Morphology

-Domestic pigeons were derived from Rock Doves (Columba livia) by artificial selection perhaps 5,000 ybp. Feral pigeon populations developed after domestics escaped captivity; this began in Europe soon after initial domestications occurred and has continued intermittently in other regions. Ferals developed from domestic stocks in North America no earlier than 400 ybp and are genealogically closer to domestics than to European ferals or wild Rock Doves. Nevertheless, North American ferals are significantly closer in skeletal size and shape to European ferals and Rock Doves than to domestics. Natural selection evidently has been reconstituting reasonable facsimiles of wild size and shape phenotypes in feral pigeons of Europe and North America. Received 17 April 1991, accepted 13 January 1992. Man, therefore, may be said to have been trying an experiment on a gigantic scale; and it is an experiment which nature during the long lapse of time has incessantly tried [Darwin 1868]. Of the many kinds of animals examined for the study of variation under domestication by Charles Darwin, only for pigeons (Columba livia) did he describe fully the chief domestic strains, along with "their history, the amount and nature of their differences, and the probable steps by which they have been formed" (Darwin 1868: 1 [vol. 1]). He did artificial selection and studied inheritance of plumage colors, color patterns, and body size and shape in domestic pigeons; the results of these studies were important to his work on natural selection (Darwin 1859, 1868). Darwin's findings supported the idea that the range of colors, patterns, sizes, and shapes shown by domestic strains had antecedents in the variation of wild Rock Doves. He also thought that feral pigeons were an understandable consequence of domestic birds escaping captivity. In the late 1850s, however, Darwin was heavily involved in writing his "big book" (Stauffer 1975), so origins of ferals from domestics were barely mentioned. Details of such origins, involving character variation molded presumably by natural selection, are nevertheless of interest to thinking concerning population differentiation; some details inferred from skeletal morphology are reported here. Rock Doves were domesticated in the period 10,000 to 5,000 ybp, earlier than has been previously suggested (e.g. Sossinka 1982). Domestications evidently occurred many times throughout the Mediterranean Basin, Near East, and southwestern Asia; this is known to be true in more recent time (Darwin 1868; N. E. Baldaccini, pers. comm.). Later, pigeons escaping captivity either rejoined wild colonies or became feral, and are now found in most of the world (Long 1981). European, North African, and Asiatic ferals may have histories of several thousand years. North American ferals have a significantly shorter history, stemming from British dovecote pigeons (the earliest of which were brought to Britain by the Romans; Levi 1974) introduced by Scottish and English immigrants to American Atlantic seaboard localities in 1600-1610 (Schorger 1952). North American ferals, therefore, are not directly lineally related to ferals of the Old World (Fig. 1). Additionally, founder gene frequencies seemingly departed significantly from those of European domestics, judging from allozymes of North American and European ferals (Johnston et al. 1989). Thus, the evolutionary derivation of ferals is more complex than it might have been. The complexity is most useful-it is, for example, important that feral pigeons were derived from domestics more than once, because the development of ferals in North America can be viewed as an independent replicate of the natural experiment in ferality tried in Europe and Asia. Without the replicate, this study would almost certainly not have been undertaken, nor would it in any event have a satisfactory conclusion. Getting to that conclusion employs assessment of skeletal similarities and differences among wild, domestic, and the two feral lines of Rock Doves, and approximating how the similarities and differences could have occurred.