An example of sympatric speciation: Phytophagous insects.

Rhagoletis pomonella is a tephritid fly and a pest of apples (opposite). It lays its eggs in apples and the maggot then ruins the fruit. This was not always so: only in 1864 were these species first found on apples. Since then it has expanded through the orchards of North America and exploited cherries, pears and roses. These moves to new food-plants are called host shifts. In the host shift of R.pomonella, speciation may be happening before our eyes.

The R. pomonella on the different hosts are currently different genetic races. Females prefer to lay their eggs in the kind of fruit they grew up in: females isolated as they emerge from apples will later choose to lay eggs in apples, given a choice in the laboratory. Likewise, adult males tend to wait on the host species that they grew up in; mating takes place on the fruit before the females oviposit. Thus there is assortative mating: male flies from apples mate with females from apples, males from hawthorn with females from hawthorn.

Gel electrophoresis shows that the two races have evolved extensive differences in their enzymes. They also differ genetically in their development time: maggots in apples develop in about 40 days, whereas hawthorn maggots develop in 55 - 60 days. This difference also acts to increase the reproductive isolation between the races, because the adults of the two races are not active at the same time.

Apples and hawthorns differ and selection will therefore probably favor different characters in each race; this may be the reason for their divergence. If it is, selection may also favor prezygotic isolation, and speciation. We can be practically certain that the initial host shift, and formation of a new race, has happened in sympatry; however, it is not a full example of sympatric speciation because the races have not fully speciated.