Exercise 6.2

Selection for Longer Pollen Tubes in Irises Presumably Due to Pollinator Pressure

(This exercise is based on Alexandersson, R., and S. D. Johnson. 2002. Pollinator-mediated selection on flower-tube length in a hawkmoth-pollinated Gladiolus (Iridaceae). Proc. R. Soc. Lond. 269: 631–636.)

(Note: The reference above links directly to the article on the journal’s website. To access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)


Gladiolus longicollis is an iris that has spurs, narrow flower tubes that contain nectar to reward hawkmoth pollinators. Spurs are found in many plant species and are assumed to have evolved due to selection pressure generated by the pollinators. Darwin hypothesized that pollen removal and deposition would be favored when flower tubes were longer than the pollinators’ tongues.

Ronny Alexandersson and Steven Johnson examined the selective pressures acting on the irises by looking at the correlations between the spur length and fitness-related traits in a natural population near Mount Gilboa in South Africa.


Use the information in Table 1 to answer questions 1 and 2.


Hawkmoth species

Mean tongue length

± s.d. (mm)

Individuals carrying Gladiolus pollen (%)


 Agrius convolvuli

 102.9 ± 6.7



 Basiotha schenki

 39.5 ± 2.9



 Hippotion celerio

 36.8 ± 1.7a



 Nephele sp.

 37.0 ± 3.0



 Theretra cajus

 27.0 ± 2.6



 Theretra capensis




an = 5

Table 1 Species of hawkmoths caught in the area, length of tongue, and proportion of individuals carrying pollen from Gladiolus longicollis.


Question 1. Which species was most likely to be pollinating the iris?


Question 2. Relative to the other species, does this species have a long, a short, or an intermediate-length tongue?

Use the information in Figure 1 to answer questions 3 through 5.


Figure 1 Distribution of flower-tube length in the study population of G. longicollis.


Question 3. What type of distribution of flower-tube lengths do these plants have?


Question 4. What is the mean flower-tube length in this population?


Question 5. Is the mean flower-tube length of the flowers longer or shorter than the mean tongue length of the most prevalent pollinator?

Use the information in Figure 2 to answer question 6.


Figure 2 Percentage of plants that set fruit as a function of their flower-tube length.


Question 6. What is the relationship between flower-tube length and setting fruit?

Use the information in Figure 3 to answer questions 7 through 10.


Figure 3 The mean number of seeds produced as a function of their flower-tube length.


Question 7. What is the relationship between flower-tube length and the number of seeds produced?


Question 8. Assuming that seed production and fruit set are closely associated with fitness, what do the data suggest about selective pressures on flower-tube lengths?


Question 9. If the variation in flower-tube length in this population were due solely to environmental factors, what would be the expected change in flower-tube length over several generations?


Question 10. What possible experiments can be done to examine whether Agrius convolvuli is the selective agent acting on the length of flower tubes in G. longicollis?