was initially self-selected for tolerance to humans, rather than actively selected by humans for tameness or for desired services such as companionship, hunting or guard duties, or food. According to the authors’ reconstruction, cat domestication probably began near some of the earliest agricultural settlements of the Neolithic, in the Fertile Crescent region of the Near East, as wildcats became accustomed to feeding on rodents and refuse near human towns. If so, their evolution to companion animals, and their ecological isolation from wildcats, was initially a response to natural selection more so than to conscious artificial selection.
Apart from appraising the phylogenetic histories of domestic organisms, the field of molecular genetics is also uncovering the genes responsible for key phenotypes that have emerged from artificial selection. In Chapter 6, Feng Tian, Natalie Stevens, and Edward Buckler IV provide cases in point involving domestic corn (maize), the ancestors of which are wild teosinte grasses native to Mexico. The evolutionary transformation from teosinte to maize ranks among the most impressive of all feats of artificial selection. For example, teosinte lacks a cob-like inflorescence and instead produces only 6–12 kernels in two rows protected by a hard covering, whereas each cob of modern maize consists of approximately 20 rows with numerous exposed kernels; and teosinte has long lateral branches terminated by male tassels, whereas modern maize has short lateral branches tipped by female ears. The authors review current knowledge about the genetic loci responsible for these and other such morphological transitions. Several genes with major effect can be specified, and many others are implicated, including a newly discovered region on chromosome 10 that spans more than 1,000,000 base pairs and retains the molecular footprints of strong artificial selection during the domestication process.
In Chapter 7, Fred Allendorf and Jeffrey Hard describe another form of human-induced selection that they term unnatural selection. When breeders artificially select domestic animals for food or companionship, they purposefully try to propagate traits that people deem desirable. However, hunting and fishing (especially for trophies) routinely violate such ground rules by culling rather than propagating the animals that humans prize most. In other words, unnatural selection via hunting, unlike artificial selection by people (or natural selection by nature), often eventuates biotic outcomes that run counter to what humans (or nature) otherwise would strive to achieve. For example, the evolutionary responses to the continued selective removal of larger or healthier animals from a population of deer or fish could include, in principle, earlier sexual maturation and smaller adult body sizes. The authors review arguments and empirical evidence for unnatural selection imposed by human harvests of wild animal populations, and they discuss the management problems generated by such selective mortality. Darwin mostly overlooked this important topic,