Fruit Fly Rhagoletis pomonella

The apple maggot fly (Rhagoletis pomonella) of North America is an example of sympatric speciation that may be happening now.The fly originally laid its eggs only in the fruit of native hawthorn trees. Adults find mates on the host fruit, females lay eggs in the fruit, and the larvae (maggots) feed inside it.European settlers brought apple trees to North America. In the mid-nineteenth century, some flies began laying eggs in apples. Two groups, called host races, now exist in the same area: a hawthorn race and an apple race.The two races are becoming reproductively isolated in several ways:1. Ecological isolation: flies usually mate on the type of fruit they grew up in, so apple flies mostly meet apple flies, and hawthorn flies mostly meet hawthorn flies.2. Temporal isolation: apples ripen several weeks earlier than hawthorn fruit. The life cycle of the apple race has become timed to emerge and breed earlier, so the breeding seasons of the two races overlap only a little.3. Different selection pressures: the two fruits differ in ripening time, chemistry and the parasites that attack the larvae, so different alleles are favoured in each race. The races already differ in the frequencies of several alleles.Gene flow between the races is now low, though not zero. The two races are not yet separate species, because they can still interbreed. If gene flow continues to fall, they may become fully separate species, without any geographical barrier ever dividing them.