The evolution of reproductive systems in plants, and particularly, the emergence of flowers, has had reciprocal effects on the diversification and proliferation of all other life on this planet.
Today, angiosperms are widespread, diverse, and hugely important to the global ecosystem. Their success can be largely attributed to the mutalistic relationships that flowers have evolved with their animal pollinators, who facilitate sexual reproduction, and thus ensure genetic diversity in these plants.
Although the origin of flowering plants are still poorly understood, both phylogenetic and fossil evidence suggest that water lilies are among the most ancient groups of angiosperms.
Surprisingly little is understood about the origins of flowering plants. Without the ability to rewind history, scientists are forced to use creative methods for resolving questions like,
How did the first flowers evolve? What pollinated these flowers? What motivated these pollinators?
One strategy hinges on the assumption that ancient angiosperm species retain qualities of the earliest flowering plants. By studying these flowers and their modern pollination strategies, we might make inferences about the first adaptations for pollination in angiosperms.
Predicting what the first angiosperm pollination systems looked like involves answering a number of questions.
1. Were the first flowers pollinated by wind/water or by animals?
Pairing clues from ancient floral morphology with our knowledge of modern wind and animal pollinated species helps us to answer this question. Based on this information, scientists have determined that the first flowers were almost certainly animal pollinated.
2. What kind of potential pollinators existed before the first angiosperms evolved?
We know from studying the fossil record that insects evolved before the first flowering plants. Many of the insect orders that, today, are major pollinators of modern flowers already existed before angiosperms first arrived. These include beetles (Coleoptera), flies (Diptera), wasps (Hymenoptera), moths and butterflies (Lepidoptera), and to some extent, thrips (Thysanoptera). The feeding structures on these animals tell us that the earliest insects fed on carrion, detritus, and plant tissue. (Damaged fossilized plant tissue supports this theory). This strongly suggests that many of these insects were already adapted to search for food at non-flowering plants.
3. Prior to the evolution of flowers, did any plant species involve animals in their pollination strategies?
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| Image: New York Botanical Garden |
4. How did the first flowers entice insects to visit them?
We are still answering this question! As stated, insects were adapted to feed on plant tissue before angiosperms first evolved. This means that insects were already visiting plants in search of food. But how did flowering species persuade these insects to visit their novel reproductive structures, while also preventing herbivory?
One hypothesis is that the plants used what they already had. Before flowers evolved, plants already possessed the chemical equipment to defend against herbivory and disease. It is postulated that primitive floral odors were simply derivatives of these defensive volatile chemicals. In fact, we already know that these two functional classes of compounds share a similar biosynthetic pathway.
It is possible that some insects may have developed a tolerance for certain plant chemicals. Instead of acting as a deterrent, the plant volatiles could have evolved to signal such things as a food source or mating site for those insects. Furthermore, some evidence suggests that odor was the first floral characteristic to have evolved as a pollinator attracting in angiosperms. Flower color and morphology may have become increasingly important as these plant-animal mutualisms proliferated and specialized.
The arching theme of this study and of pollination biology, in general, is asking,
What modes of communication do plants use to signal to pollinators?
Flowering plants invest immense resources into their floral structures, in order to ensure that their genes are distributed. It is consequently important that plants successfully attract pollinators to their flowers.
Floral morphology, color, and odor are the main cues used by flowering plants to advertise their rewards. Variations in these traits and in the combinations in which they are used by flowers is dependent, primarily, on which pollinators these plants are selected to attract. Understanding innate pollinator preferences to these modes of floral communication helps us to predict the evolving dynamics of flowers and their pollinators.
