A world in full bloom

The way flowering plants came about and the manner in which they partnered with insects in a trade that allowed both to grow and diversify is yet another example of how a beautiful and complex environment is designed without a designer, accident upon accident, during evolution. The first flowering plants, or angiosperms, are believed to have evolved during the Early Cretaceous Period, approximately 125-135 million years ago, based on fossil evidence such as pollen and leaves. While the fossil record is one kind of a clock, looking at the DNA sequence of extant species of plants gives information about the tree of evolution. Assuming a specific uniform tempo in DNA changes during evolution, one can ask how far back the earliest ancestors of flowering plants could have come about. These studies suggest that their origins could date back much earlier, possibly between 149 million years and 256 million years ago, or even as far back as 350 million years ago, indicating a significant gap in the fossil record.

Thus, when we try to determine when flowering plants appeared in the history of life, we get two possible answers. Fossil evidence shows a rapid diversification of flowering plants during the Cretaceous, coinciding with the evolution of pollinating insects, which played a key role in their ecological success. On the other hand, molecular studies suggest a cryptic evolutionary phase before their appearance in the fossil record, possibly beginning in the Jurassic or even earlier. The Jurassic Period spanned from approximately 200 million to 145 million years ago. This period is known for the dominance of dinosaurs, the break-up of the supercontinent Pangaea, and the diversification of reptiles, early mammals, and conifers. The Jurassic preceded the Cretaceous Period, which lasted approximately 145-66 million years ago, making it the final period of the Mesozoic Era. It was marked by the rise of flowering plants, continued dinosaur dominance, and ended with the mass extinction event that wiped out most dinosaurs, which shaped today's world in tumultuous ways.

The quip that "God has an inordinate fondness for beetles" is attributed to evolutionary geneticist J.B.S. Haldane, who moved to India from London. It was evidently made in response to theologians asking what could be inferred about the creator from studying nature. Haldane highlighted the astonishing diversity of beetles: there are over 400,000 known species, making them one of the most species-rich groups of organisms on Earth, accounting for about 40% of all insect species and 25% of all known animal species. Much of the diversification of beetles came from a partnership with early flowering plants.

Beetles became the primary pollinators of early flowering plants due to their ecological roles and the co-evolutionary relationships that developed between them and basal angiosperms. Beetles were already abundant, feeding on plant parts like pollen and ovules in gymnosperms (non-flowering plants) before the emergence of flowering plants. This pre-adaptation made them suitable candidates for pollination when flowering plants appeared. Early flowering plants, such as magnolias and water lilies, evolved traits to attract beetles. These included bowl-shaped flowers, strong fruity or fermented scents, exposed reproductive structures, and moderate nectar production. These floral characteristics catered to beetles' sensory preferences and feeding behaviours. Beetles benefited from feeding on pollen, nectar, and floral tissues, while plants gained efficient pollen transfer as beetles moved between flowers. This mutualism enhanced reproductive success for both groups. During the early evolution of flowering plants, other specialised pollinators like bees and butterflies had not yet evolved. This allowed beetles to dominate as pollinators without significant competition. Fossils show that beetles carried pollen from non-flowering plants and early flowering plants as far back as 99 million years ago. This suggests their role in bridging the transition from non-flowering to flowering pollination systems. Beetles' feeding habits, abundance, and adaptability allowed them to become the dominant pollinators of early flowering plants, shaping the evolution of floral traits through millions of years of interaction and, in turn, allowing the diversification of beetles themselves.

Beetles are messy pollinators, and while they had their dominance, the evolutionary race in flowering was speeded by the evolution of more efficient flying insects. Flying insects and flowering plants co-evolved, forming one of evolutionary history's most significant mutualistic relationships. This co-evolution began during the Cretaceous Period, over 100 million years ago, and has profoundly shaped the diversity of both groups. Flowering plants evolved traits such as colourful petals, enticing scents, nectar rewards, and specialised structures to attract specific pollinators. These traits are known as pollination syndromes. Flying insects, such as bees, butterflies, moths, and flies, developed adaptations like long proboscises for accessing nectar and enhanced sensory systems to detect floral cues like colour and scent. The radiation of flowering plants coincided with the diversification of significant insect groups like bees (Hymenoptera), butterflies and moths (Lepidoptera), and flies (Diptera). Plants benefited from precise pollen transfer by insects, reducing the need for excessive pollen production compared to wind pollination or even the messy ones by beetles. In return, flying insects gained reliable food sources such as nectar and pollen, which supported their survival and reproduction.

Some plant-pollinator pairs exhibit extreme specialisation. For example, orchids like Angraecum sesquipedale evolved long nectar spurs matched by the proboscis length of their moth pollinators, as Charles Darwin famously predicted. Darwin's orchid, native to Madagascar, has a nectar spur approximately 30-35 cm long, which led him to conclude that only a moth with a proboscis of similar length could access its nectar and facilitate pollination. In 1862, Darwin studied its floral structure after receiving the orchid from horticulturist James Bateman. He published his observations in On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, and on the Good Effects of Intercrossing (bit.ly/orchid-fertilisation). He hypothesised that the flower was pollinated by a hawkmoth with an extraordinarily long tongue. At the time, this prediction was met with scepticism, as no such moth had been documented. Decades after Darwin's death, in 1903, the hawkmoth Xanthopan morganii praedicta was discovered in Madagascar. Its proboscis matched the length required to reach the nectar at the base of the A. sesquipedale spur. Direct observations of X.morganii praedicta pollinating the orchid were not made until much later, between 1992 and 1997, using a video camera. Darwin's insight into this relationship highlighted the concept of co-evolution. The orchid's long nectar spur likely evolved to ensure that only moths with long proboscises could access its nectar, thereby increasing pollination efficiency. Conversely, moths with longer proboscises would have an advantage in accessing nectar from flowers like A. sesquipedale, driving their evolution toward longer tongues. This "arms race" between flower and pollinator exemplifies how natural selection can shape interdependent traits in different species. Darwin's prediction about A. sesquipedale and its pollinator remains one of the most celebrated examples of evolutionary foresight and evidence for natural selection.

One of the prominent researchers studying how the shapes, visual and textural properties evolved to attract insects is Beverley Glover of the University of Cambridge. Glover studies how the surface structure of petals influences their optical properties and attractiveness to pollinators. For example, petal cells' microscopic and nanoscopic textures can create effects like glossiness or iridescence, which enhance visual signals to pollinators. In the flower of Hibiscus trionum, she has explored how petal cell shapes and arrangements contribute to structural colouration, which produces iridescent effects that may help guide pollinators to the flower's reproductive structures. Nectar spurs are tubular extensions of flowers that store nectar and play a critical role in plant-pollinator interactions by favouring specific pollinators with matching morphology (for example, long-tongued insects such as the hawkmoth). Glover's research investigates the genetic and developmental mechanisms underlying nectar spur formation, including cell elongation processes.

Structural colouration is the result of physical interactions between light and the structures on petal surfaces, rather than pigments.

Many flowering plants allow multiple insects to pollinate them. Here, both insects and plants are generalists. Other plants and insects have a more specialised relationship, such as Darwin's orchid and its pollinating hawkmoth. Sometimes, this specialisation can pose challenges when plants are bred in non-native environments. Some flowers require human pollination, either due to the absence of natural pollinators, environmental conditions, or agricultural practices. This process involves manually transferring pollen from the male part of a flower (stamen) to the female part (stigma). Vanilla plants are native to Mexico, where specific species of Melipona bees historically pollinated them. Hand pollination became necessary when vanilla cultivation expanded to other regions without these bees. Fine motor skills are required to transfer pollen in these fragile flowers. Cacao flowers are small and complex, and their natural pollinators (tiny midges) are often inefficient in cultivated settings. Farmers sometimes hand-pollinate cacao flowers to ensure sufficient yields. So, your vanilla ice cream with chocolate shares a common pollinator: humans. In parts of China, heavy pesticide use has caused a decline in natural pollinators such as bees. Farmers now rely on human pollinators to brush pollen onto flowers manually to grow apples and pears. Date palms are dioecious (male and female flowers occur on separate plants), and hand pollination is commonly practised to ensure high fruit yields. While human pollination can secure crop yields, it is labour intensive and costly, making it a last resort in most cases.

The evolution of flowering plants has profoundly impacted other species and ecosystems, driving biodiversity and reshaping life on Earth in several key ways. Flowering plants introduced diverse ecological niches, allowing other plants and animals to thrive. Their ability to form complex forest structures provided habitats for countless species, significantly increasing biodiversity per unit area compared to ecosystems before the rise of flowering plants. After the extinction of dinosaurs, flowering plants dominated terrestrial ecosystems, driving the Flowering-Plant Terrestrial Revolution. This period saw the rise of modern tropical rainforests and other biodiverse habitats, which supported new groups of animals such as mammals, birds, and insects. The efficient photosynthesis and energy capture of flowering plants enhanced ecosystem productivity. This additional energy flowed through food webs, supporting herbivores, predators, and decomposers, thereby enriching entire ecosystems. The seeds and fruits of flowering plants became the foundation for human agriculture. Their domestication over the past 10 millennia has been critical for the development of human civilisation. Flowering plants revolutionised terrestrial life by fostering biodiversity, and enabling co-evolutionary relationships. We owe the diversity of much of what we see in our flora and in the food we eat to the partnership between insects and the flowers that attract them.

Also Read
Glover, B.J. Understanding Flowers and Flowering: An integrated approach (2007). bit.ly/glover-flowers