|Figure 17.12 Flower Color and Reproductive Isolation
Donald Levin at the University of Texas proposed that Phlox drummondii has red flowers only in locations where it is sympatric with pink-flowered Phlox cuspidata because having red flowers decreases interspecific hybridization. To test this hypothesis, he 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. Of the seeds produced by pink-flowered P. drummondii, 38 percent were hybrids with P. cuspidata. By contrast, only 13 percent of the seeds produced by red-flowered P. drummondii were genetic hybrids. These findings demonstrated that the red color acts as a prezygotic reproductive barrier against interspecific hybridization.
Sexual isolating mechanisms such as the one described above are typically considered to be the most important genetic barriers leading to speciation. With regard to Phlox plants, the presence of a color signal increases the likelihood of receiving intraspecific pollen. Given that the Phlox hybrids that occur in sympatry produce almost no viable seeds, it is clear that reproductive advantages exist for individual Phlox plants that donate and receive primarily intraspecific pollen.
Levin, D. A. 1985. Reproductive character displacement in Phlox. Evolution 39: 1275–1281.
University of Texas: Phlox drummondii
University of Texas: Phlox cuspidate
University of Texas: Donald Levin’s Home Page
UC-Berkeley: Understanding Evolution: Reproductive isolation
Evolution, 3rd ed.: Speciation - What is the role of reinforcement in speciation?
Science Daily: Getting A Whiff Of Speciation By Reinforcement
Kay, K. M., and D. W. Schemske. 2008. Natural Selection Reinforces Speciation in a Radiation of Neotropical Rainforest Plants. Evolution 62: 2628–2642.
Kimball’s Biology Pages: Speciation
Johnson, N. 2008. Hybrid Incompatibility and Speciation. Nature Education 1(1)