Host Race Formation and Sympatric Speciation in Rhagoletis Fruit Flies (Diptera: Tephritidae)

In 1866, some 200 years after the introduction of apples to North America, a local newspaper reported that larvae of an unknown fly were infesting apples (Malus pumila) in the Hudson River valley of New York (Illingworth, 1912). Shortly after the maggots were found in apples, Benjamin Walsh (1867) described the fly as Trypeta pomonella (later recognized as Rhagoletis pomonella (Snow, 1894)). Walsh's description was based on specimens reared from apples grown in eastern United States, as well as from hawthorn fruit (Crataegus spp.) gathered in Illinois where, he reported, the fly did not attack apples. Walsh suggested that the flies infesting apples represented a newly established local host race biologically distinct from the flies associated with native hawthorn. He had proposed earlier (Walsh, 1864) that in phytophagous insects a host shift by individuals bearing a heritable change in host preference could give rise to new host races in the absence of geographic isolation. He reasoned that there would be sufficient "phytophagic isolation" (host specificity) to allow the adaptive evolution of new species. Not only was Walsh the first biologist to propose that insects could speciate sympatrically via a host shift, but he also was quite clear