Postzygotic isolating mechanisms result in selection against hybridization

478

Genetic differences that accumulate between two diverging lineages may reduce the survival and reproductive rates of hybrid offspring in any of several ways:

Natural selection does not directly favor the evolution of postzygotic isolating mechanisms. But if hybrids are less fit, individuals that breed only within their own species will leave more surviving offspring than will individuals that interbreed with another species. Therefore individuals that can avoid interbreeding with members of other species will have a selective advantage, and any trait that contributes to such avoidance will be favored.

Donald Levin of the University of Texas has studied reinforcement of prezygotic isolating mechanisms in flowers of the genus Phlox. Levin noticed that most individuals of P. drummondii in most of the range of the species in Texas have pink flowers. However, where P. drummondii is sympatric with its close relative, the pink-flowered P. cuspidata, most P. drummondii have red flowers. No other Phlox species has red flowers. Levin performed an experiment whose results showed that reinforcement may explain why red flowers are favored where the two species are sympatric (Investigating Life: Flower Color Reinforces a Reproductive Barrier in Phlox).

investigating life

Flower Color Reinforces a Reproductive Barrier in Phlox

479

experiment

Original Paper: Levin, D. A. 1985. Reproductive character displacement in Phlox. Evolution 39: 1275–1281.

Most Phlox drummondii flowers are pink, but in regions where they are sympatric with P. cuspidata—which is always pink—most P. drummondii individuals are red. Most pollinators preferentially visit flowers of one color or the other. In this experiment, Donald Levin explored whether flower color reinforces a prezygotic reproductive barrier, lessening the chances of interspecific hybridization.

image

work with the data

Donald Levin of the University of Texas proposed that Phlox drummondii has red flowers only in locations where it is sympatric with pink-flowered P. cuspidata because having red flowers decreases interspecific hybridization. To test this hypothesis, Levin introduced equal numbers of red- and pink-flowered P. drummondii individuals into an area with many pink-flowered P. cuspidata. At the end of the flowering season, he assessed the genetic composition of the seeds produced by P. drummondii. The results are shown in the table below.

Morph (flower color) Number of seeds (progeny)
P. drummondii Hybrid Total
Red 181 (87%) 27 (13%) 208
Pink 86 (62%) 53 (38%) 139

QUESTIONS

Question 1

Check the 95% confidence intervals for the proportion of hybrid seeds in red- and pink-flowered P. drummondii in the graph in the Results of the experiment. There are many websites available for calculating confidence intervals; a good one is the Vassar College statistical computation site, VassarStats.net. You can go to this site and select “Proportions” from the left-hand menu, then select “The Confidence Interval of a Proportion.” What are the numerical values of the 95% confidence intervals?

For red flowers, the proportion of seeds from hybrid matings is 0.1298 (27/208). The 95% confidence limits of this proportion are approximately 0.09–0.18.
For pink flowers, the proportion of seeds from hybrid matings is 0.3813 (53/139). The 95% confidence limits of this proportion are approximately 0.30–0.47.

Question 2

Notice that the proportions of hybrids among the seeds of red- versus pink-flowered samples are significantly different, because the 95% confidence intervals do not overlap. To quantify the significance of this difference, use the website suggested in Question 1, but select “Significance of the Difference between Two Independent Proportions” from the “Proportions” menu. What null hypothesis are you testing in this case? (See Appendix B if you need help.) What is the P-value of getting results at least as different as these two samples if your null hypothesis is true?

The null hypothesis is that there is no significant difference in the proportion of hybrid seeds produced from red versus pink flowers. This hypothesis is rejected; the probability that the null hypothesis is true is P < 0.0002.

Question 3

How would you extend or improve the experimental design of this study? What kinds of additional test sites or conditions would you want to examine? How might replicate or control sites make the study more convincing?

Scientific studies often raise new questions that can be addressed by follow-up experiments. Repeating this study in several different sites of sympatry and allopatry would address the unlikely possibility that red flowers are favored at the study site because of local conditions that have nothing to do with the presence of P. cuspidata (such as the local abundance of pollinators that prefer red flowers). A possible control experiment would involve removing all P. cuspidata from an area of sympatry and testing to see if the absence of P. cuspidata eliminates the selective advantage of red-flowered P. drummondii.

A similar work with the data exercise may be assigned in LaunchPad.

Likely cases of reinforcement are often detected by comparing sympatric and allopatric populations of potentially hybridizing species, as in the case of Phlox. If reinforcement is occurring, then sympatric populations of closely related species are expected to evolve more effective prezygotic reproductive barriers than do allopatric populations of the same species. As Figure 22.11 shows, the breeding seasons of sympatric populations of different leopard frog species overlap much less than do those of allopatric populations. Similarly, the frequencies of the frog mating calls illustrated in Figure 22.12 are more divergent in sympatric populations than in allopatric populations. In both cases, there appears to have been natural selection against hybridization in areas of sympatry.