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SCIENCE New discovery unravels Darwin's 'abominable mystery' of butterfly and moth evolution

Published 12 Feb 2018 11:23AM

Words by Joellie Hale | Staff Writer

Which came first, the butterfly or the flower it feeds upon?

The answer to the famous naturalist Charles Darwin’s ‘abominable mystery’ has finally been uncovered.

It has long been thought in the scientific community that flowering plants and the insects that feed upon them evolved at roughly the same time: in the Cretaceous era, 130 million years ago. This widely accepted theory is known as the ‘angiosperm co-evolution theory’. Angiosperm is the technical term for ‘flowering plant’.

A team of scientists, led by Boston College’s Professor Paul Strother, discovered unexpected remains in pond scum from over 200 million years ago. Trapped in sediment deposited during a cataclysmic event, the fossils looked suspiciously like insect wings. Insect remains are rarely preserved.

The delicate details in the wings, intricate and essential for identification, are usually destroyed prior to preservation. The conditions provided by this cataclysmic event – rapid burial to prevent scavenging or decomposition, fine sediment, numerous specimens for true analysis – granted the researchers rare insight into the murky history of insect evolution.

They spent 5 years painstakingly deciphering clues locked in the scum. Their research, presented last month, demonstrated a surprising finding.

Moths and butterflies actually evolved fifty million years before flowering plants.

In the absence of the flowers fed upon by the moths of today, the primitive insects developed the sucking proboscis to draw nutrition and water from water droplets on the immature seeds of conifers and injured leaves. They transferred their feeding preference to the nectar provided by emerging flowers, 50 – 70 million years later.

Before the relatively recent evolution of flowers, primitive plant species dominated the land. Ferns, gymnosperms (conifers and cycads) and mosses decorated the surface of the Earth. These ancient plant species reproduce without the assistance of a pollinator, using wind or water to carry their spores. Some can also reproduce asexually: part of the plant breaks off and regenerates, growing into another plant.

Angiosperms are reliant on pollinators to reproduce. Flowers house the reproductive system, and act as a lure to attract pollinators. The striking colours and sweet nectar attract insects, which have been feeding on other flowers. Pollen is collected as they brush against it to harvest the nectar, then deposited on another flower. This combines the different sexual parts, found on other sections of the flower, or on flowers of the opposite gender.

The angiosperm co-evolution theory is demonstrated in the massive morphological variation seen in flowering plants and insects. While plant groups such as ferns and conifers are generally consistent in design, angiosperms exhibit extensive morphological variety to attract specific types of insects.

If sufficient at attracting specific insects, provided that insect persists, the plant will have reduced competition with other plants for pollinators. The same is true for insects, which have developed morphologies to harvest pollen from specific flowers, at maximum efficiency. Being able to quickly extract the nectar results in the consumption of more nectar from different flowers, increasing the reproductive success of the plant. Specificity will also reduce the transferral of unusable, foreign pollen.

This specificity can be seen across the globe. On the biodiversity hotspot island of Madagascar, seemingly endless varieties of orchid can be seen, each designed to attract a different hawkmoth. When sent a sample of the Star-of-Bethlehem orchid by a colleague, Darwin was intrigued by its unusual morphology.

The nectar was accessible only through a narrow, foot-long tube. He asked in a letter to a friend: “Good heavens, what insect can suck it?”. He predicted that there must exist an insect with a foot-long proboscis. And, some years later, a huge hawk moth with a foot-long proboscis was discovered. Darwin’s theory, supported by the lesser-known Alfred Russel Wallace, was proved to be correct.

And, perhaps a little closer to home: can you imagine a delicately-winged butterfly, crumpling itself awkwardly into the tubular entrance offered by a foxglove? Foxgloves have evolved to attract honeybees. Butterflies seek out plants to offer more welcome morphology, typically more open flowers such as Buddleia.

There are many astounding examples of this mutualistic relationship progressing beyond the coincidental transferral of pollen. The Yucca moth belongs to the same sub-order as the prehistoric specimens uncovered by Strother and his colleagues. Having mated, the female deliberately deposits pollen into a cup-type structure inside the flower. She then lays her eggs inside the ovaries – precisely where the seeds will develop. The timings are perfect.

The caterpillars hatch in time to feed on the developing seeds. They do not devour all of the seeds, ensuring the future of this relationship. Some species of Yucca plant have even lost the ability to produce nectar, and are now entirely dependant on this specific moth for reproduction.
Then there’s the firecracker plant. It features flowers which hang in such an awkward way that there is only one species of butterfly with the aviation skills necessary to access the nectar. The orange barred sulfur butterfly performs breath-taking aerobatic displays, exceptional even for a butterfly. The displays seen across the butterfly family are also used in courtship; a skilled flyer will be favoured as a mate, as it is more likely to produce offspring able to easily feed.

And of course, those clever bees. Encased in electrostatic charge to attract pollen to the awaiting hairs, they fully enter the flower to harvest nectar. Specialised hairs on the hind legs form baskets, allowing for the transportation of significant amounts of pollen. They careen back to the hive, where the pollen is converted into honeycombs. This production and storage of food allow for the complex social dynamics that have given rise to bees which do not have to work.

That said, just imagine spending your days flying through the air, visiting flowers. Flowers emanating your very favourite smells and colours, literally designed to hook you in. You may be able to think of less desirable careers.


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