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1 October 2008 Natural Selection Reinforces Speciation in a Radiation of Neotropical Rainforest Plants
Kathleen M. Kay, Douglas W. Schemske
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The importance of reinforcement, that is, natural selection that strengthens reproductive isolation between incipient species, remains controversial. We used two approaches to test for reinforcement in a species radiation of Neotropical gingers in the genus Costus. First, we conducted an intensive study of Costus pulverulentus and Costus scaber, two recently diverged species that co-occur and share hummingbird pollinators. The hummingbird pollinators transfer pollen between these Costus species, but hybrids are rarely found in nature. By performing pollinations between populations of C. pulverulentus and C. scaber from three sites across the species' geographic ranges, we find that pollen–pistil incompatibilities acting prior to fertilization have evolved only between locally sympatric populations, whereas geographically distant populations within the region of sympatry and allopatric populations remain fully interfertile. Second, we conducted a comparative study of isolating mechanisms across the genus. We find lower seed set due to pollen–pistil incompatibility between species pairs that co-occur and experience pollen transfer in nature compared to species pairs that are otherwise isolated, regardless of genetic distance. Taken together, these studies indicate that crossing barriers prevent potentially maladaptive hybridization and effectively reinforce the speciation process. Our results add to mounting evidence for reinforcement from animal studies and show that plant speciation may also involve complex mate recognition systems. Reinforcement may be particularly important in rapidly diverging lineages where ecological factors play a primary role in reproductive isolation, as may often be the case in tropical communities.

Kathleen M. Kay and Douglas W. Schemske "Natural Selection Reinforces Speciation in a Radiation of Neotropical Rainforest Plants," Evolution 62(10), 2628-2642, (1 October 2008).
Received: 26 March 2008; Accepted: 23 June 2008; Published: 1 October 2008

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crossing barriers
differential fusion
pollen–pistil incompatibility
Reproductive character displacement
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