Disruptive selection

Review types of selection (Lecture 4)

This could cause speciation if there is a link between the trait undergoing disruptive selection and assortative mating

Sympatric speciation in Rhagoletis pomonella (apple maggot fly) ?

Native host is Hawthorne tree

• Larvae develop on fruit.
• Adults mate, deposit eggs on host plant.

1860s discovered on apples.

• Different fruiting time
• Temporal Isolation

1960s discovered on cherries

• Different fruiting time
• Temporal isolation

Overall, matings between hawthorn and apple flies has been reduced to only ~6% and the two are now considered incipient species.

Details

Hawthorns were the original host plant of Rhagoletis pomonella until about 150 years ago, when the fly was observed on cultivated apple trees (introduced to the Americas). The apple maggot fly is now widespread in the northeastern US and causes millions of dollars of damage each year to apple crops. Rhagoletis courts, mates, and lays eggs on its preferred host plant. Individuals that changed their preferred food source to apple trees (= a host shift), consequently tend to mate with other individuals who prefer apples. Since the host shift, flies that prefer hawthorns and apples have diverged genetically. Feder et al (1988, 1990) found statistically significant differences in allele frequencies at six loci, differences that allow the two populations to be identified. Changes in the timing of mating have also occurred: mating on apples occurs ~ 3 weeks earlier. Overall, matings between hawthorn and apple flies has been reduced to only ~6% and the two are now considered incipient species.