RAPID SPECIATION AND THE EVOLUTION OF POLLINATION
Evolution of Hummingbird pollination in Costus
Neotropical forests are well known for plant diversity. Though botanical researchers have suggested various reasons for this plant diversification, the speciation process and the evolution of pollination still remains as one of the less understood phenomena in plant evolutionary biology. Hence many research works have been conducted in this field using Neotropical Costus Subgenus Costus Erythrina Heliconia and similar other plants.
The diversity of plant species that can be found within the Costus subgenus Costus (Costaceae) will definitely prompt a botanist to research the speciation processes involved in the diversification of plant species within the genus. As we know, the critical factor that plays in the speciation process is the reproductive isolation. Main reason behind the reproductive isolation is pollination ecology or more specifically, postpollination barriers. These pollination barriers act as the barriers or limiting factors that limit the gene flow (fertilization) between two closely related species (sympatric). In this research paper, an attempt has been made to understand plant diversification process by taking two sympatric species, Costus pulverulentus and Costus scaber as examples.
Both of these Costus species have a remarkable floral biology that is suited for hummingbird-pollination but their pollination process differs mainly due to the difference in their floral characters. Though flowers of both the species are odorless, diurnal and with high content of nectar, other characteristics are different. Thus in the first study, we are able to confirm that the flower characteristics play a critical role in reproductive isolation process of genus Costus. Major floral parameters that contribute to the process of reproductive isolation are parental style length differences and floral morphology.
It is widely believed that postpollination barriers show evidence of reinforcement or natural selection. Even though, the hummingbirds transfer pollen between Costus pulverulentus and Costus scaber, hybridization rarely occur. Pollenpistil incompatibilities are considered to be the major reason for this non-hybridization process. So it can be stated beyond doubt that postpollination barriers prevent incompatible hybridization and thus reinforce the speciation process within the genus Costus.
Evolution of Hummingbird pollination in Heliconia
Study of rapid speciation and the evolution of hummingbird pollination in Neotropical Heliconia are equally interesting for a botany enthusiast. Heliconia, as in the case of Costus, is also a humming bird-pollinated plant and found in all sorts of habitats at all elevations. Heliconia species that are considered in this research paper are Heliconia beckneri and H. tortuosa, two species that cooccur in most of the habitats. Even though the sympatric species, Heliconia beckneri and H. tortuosa are similar in the form and structure of their flowers, hybridization does not occur in their natural habitats. It is found that this happens mainly because of the difference in pollen deposition sites. Though the shape of the Heliconia flowers matched with the shape of the birds bills, what differed was some f the flower characteristics, especially the curvature of the flowers. The birds are forced to feed from the flowers at different angles due to the difference in flower curvature and this naturally facilitates the pollen deposition process by making it fall at different parts of the flower such as anthers, stigma, lower lobe and upper lobe of the flower. Pollen deposits at different sites act as a pollination barrier and thus the risk of hybridization is reduced and that in turn facilitates rapid speciation process.
Evolution of Hummingbird Pollination in Costus and Heliconia A Comparative Analysis
Researchers have many well-supported theories on the rapid speciation and the evolution of Hummingbird pollination that supported the plant diversification found in the Neotropical forests. But, in fact, understanding sympatric speciation process is a difficult task for even a hard-core botany enthusiast. Since Costus and Heliconia are species rich and many sympatric species within the genus are available in both of these plant geneses, it is reasonable to suggest these plant geneses for further research studies related to the rapid speciation and the evolution of Hummingbird pollination processes.
The best way to understand the sympatric speciation process is to study the evolutionary biology of any two sympatric species within a genus. Costus is one of the best examples for studying sympatric speciation process as Costus is the most species rich and shows most of the plant diversity within the genus. Another criterion used for selection is floral morphology. Costus has a remarkable floral biology where a spiraling stem supports a terminal inflorescence that produces a large showy flower with full of nectar during flowering season. Costus also has diverse pollination syndromes, mainly bee-pollination syndrome and humming bird pollination syndrome. But only the humming bird syndrome is discussed in this research paper for study purposes. Two sympatric species of Costus, Costus pulverulentus and Costus scaber are studied in this research paper for understanding the speciation process associated with the plant diversity.
Heliconia may, as well, be included in study of rapid speciation and the evolution of hummingbird pollination. Because both of the geneses, Costus and Heliconia, are found in same natural habitats that is predominantly tropical, and their flower morphology and pollination processes are also similar. Heliconia flowers are large and full of nectar as Costus flowers and in both of the geneses, this floral character attracts hummingbirds, which in turn facilitates hummingbird pollination. The presence of postpollination barriers such as certain flower characteristics play a similar role in both of the geneses to limit cross-fertilization or hybridization and thus maladaptive fertilization processes are barred. This is how, nature reinforce the speciation process by creating a reproductive isolation between the species within the genus.
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