About 200 years ago, an ancestral population of flies laid their eggs on tree fruits called hawthorns. Once domestic apple trees were introduced, some flies in the population chose to lay their eggs on apples, instead. The maggots that developed in the hawthorn fruit grew into adult flies who preferred to mate with other hawthornreared flies; maggots from apples developed into flies who preferred to mate with other apple-grown flies. This is an example of ction of offspring between two different species. prevents the production of offspring between two different spection.