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Kew research is unearthing answers to Darwin's 'abominable mystery' of the origin of flowering plants

Kew's research is revealing answers to how and why flowering plants evolve and spread so rapidly.

Portrait photograph of Charles Darwin (Image: RBG Kew)

Some 20 years after publishing On the Origin of Species, Charles Darwin wrote a letter to his friend Joseph Hooker (then director of Kew) musing on the fossil record of flowering plants.

‘The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery,’ wrote Darwin. And 130 years later, botanists are still searching for answers.

Flowering plants (angiosperms) are the most diverse group of plants on Earth. They occupy almost every habitat and have a dazzling array of adaptations. Although we may not realise it, we’re highly dependent on them for food, medicine, shelter and clothing. It’s estimated that there are more than a quarter of a million species of flowering plants, and it’s how and why such an astonishing diversity of species evolved that captivated Darwin.

Flowering plants appearance 100 million years ago

Fossil evidence for the apparently rapid evolution and spread of early flowering plants was just beginning to emerge when Darwin raised his question. Flowering plants are unknown in rocks of the Jurassic age (about 150 million years ago). But Darwin was aware of work published by Swiss botanist Oswald Heer that flowering plants of remarkable diversity and seemingly modern character had appeared by the middle Cretaceous (about 100 million years ago), and that by the late Cretaceous (about 70 million years ago, when the upper chalk of the south of England was being formed) they had come to dominate floras from Europe to the Arctic.

What concerned Darwin, and prompted that much-quoted remark, was the apparent rap-idity of the evolutionary changes involved. To Darwin, this conflicted with his concept of slow, gradual evolution by the process of natural selection.

Evolutionary explosion

The dramatic rise of flowering plants is the most striking example of an ‘evolutionary explosion’ in the entire geological history of plant life on Earth. The major plant groups that we recognise today were quickly established in the middle and late Cretaceous period, although much evolution was still to occur.

The extraordinary diversity of flowering plants that we see today results from a delicate balance between the evolution of new species and the extinction of others, as shaped by changing environments and conditions over tens of millions of years.

This has captured the imagination of botanists and many others interested in how biological evolution has unfolded through time. There are many questions associated with the abrupt appearance of flowering plants in the fossil record and their rapid evolution. What were the earliest flowering plants actually like? From what group of plants did they arise? Are there any living representatives of these ancestors among today’s plants? What particular feature allowed them to evolve and rise to ecological dominance? 

A great deal of evidence has been gathered since Darwin’s time. The fossil record of those earliest flowering plants has been the subject of intense research, and, although trying to trace plant evolution using fossils is a complex business due to the rarity of fossilised flowers, we now have a much fuller picture than was available to Darwin.

Kew research

Current work on the DNA of living flowering plants – on which the Jodrell Laboratory staff at Kew have made significant advances – has also contributed much to our understanding of how most living flowering plants are related and how they may have evolved.

 It appears that if there was one factor that allowed them to evolve so rapidly, it is one that had already been grasped by Darwin, and is a subject on which he did critical research. This is a process called ‘biotic pollination’ – the use of insects and other animals to transfer pollen from the male parts of one flower to the female of another. The adaptations associated with this – coloured petals, nectar, scent and the enormously varied structure of flower parts – are the distinctive features of most flowering plants. 

Insect and wind pollination

Although we have no direct evidence of colour or scent in the fossil record, most of the earliest fossil flowers show structures associated with insect pollination. Wind pollination, used by conifers, is enormously wasteful and haphazard, compared with the efficiency of the biotic pollination of flowering plants. It’s no surprise that the fossil record of bees, butterflies and moths, which today are important pollinators, undergo a parallel evolutionary burst in line with flowering plants.

While wind-pollinated plants often take their time over reproduction, some herbaceous flowering plants (especially many familiar weeds) can run through their entire life cycle, from seed dispersal to the next generation of seed production, in a few weeks. This quick, reliable and efficient way of reproducing was a critical factor in allowing flowering plants to rapidly exploit genetic mutations that improved their competitiveness and contributed to their success.

Our knowledge of early flowering plants took a great leap forward in the 1980s when a young Danish scientist, Else Marie Friis, first studied some tiny fossil flower buds that had been preserved as charcoal in a late Cretaceous clay in Sweden. Flowers preserved in this way show amazing detail of structure and give a much fuller picture of the early diversity attained by flowering plants some 80 million years ago.

Despite the great evolutionary advantage that biotic pollination seems to have given flowering plants, a remarkable feature of their evolution is the evidence that several separate lines chose to revert to the wind pollination of their cone-bearing ancestors (the gymnosperms).

This is the case with several of our native trees – birch, alder, hazel and beech, but most strikingly in the grasses. It is interesting that the latter are one of the most species-rich families of flowering plants, and formed the basis for agriculture, enabling development within the human race.

To return to Darwin, some aspects of the rapid rise of flowering plants in the Cretaceous are still mysterious, but perhaps less abominably so now than they were in the late 19th century. Using molecular techniques, we can now explore how different flowering plants are related and we can link this understanding with the search for their earliest fossils. This offers the best prospect of advancing our knowledge of the origins of flowering plants. But, undoubtedly, the fossil record will still reveal some surprises.

Authors:

Professor William Chaloner FRS, Department
of Earth Sciences, Royal Holloway, University of London

Professor Sir Peter Crane FRS, Department of the Geophysical Sciences, University of Chicago

Kew magazine 

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