Friday, 12 August 2016

Environment, Elms and Evolution

Dead or dying English elm sapling.
Go into the English countryside right now and you could see a major ecological change occurring. I saw this example on a walk earlier today with my grandson. This change started in about 1976 or 1977 and is continuing still.

What you will see in most hedgerows at least in Southern England will be the sad sight of English elm saplings succumbing to Dutch elm disease and quickly turning their leaves brown and dying. Around them you will very likely see what look like perfectly healthy smaller elm sapling.

When Dutch elm disease first struck in the mid 1970 it altered the English landscape as massive elms that had stood for a century or more along the side of our country lanes and whole woods of elms quickly turned brown and died in August. It looked in places like Autumn had come a full two months early. Many feared that the elm was doomed and frantic efforts were made to conserve a few trees by annual 'injections' of fungicide to kill off the culprits (there are three related ascomycete microfungi which cause the disease).

But the English elm didn't die out completely. Instead it became a hedgerow plant as young saplings sprang up from the roots of the old dead trees. Spreading by root suckers rather than seed is a characteristic of the English Elm. But to understand why they didn't also die off it's necessary to understand the way Dutch elm disease is spread. It doesn't spread by wind-blown spores like many other fungi but depends entirely on being spread by the elm bark beetle. This beetle lays its eggs underneath the bark of the elm by depositing them via an ovipositor into the layer immediately beneath the bark, transferring the fungal spores beneath the bark as it does so.

Heathly-looking elm saplings just a few feet away from the dead one.
However, the elm bark beetle only chooses bark that is above a certain thickness and as it begins to develop the characteristic cracked appearance, so the younger saplings, with smooth bark, don't get infected.

A dynamic balance has now been established where young elms live for a few years until they become large enough then they die. Manewhile, they will have produced enough rootsuckers for a new generation of small elms to survive for a few more years. The beetles still find enough 'mature' trees to maintain their population and the fungi still have somewhere to produce the next generation of spores.

Under the intense selection pressure of the disease, the English elm has changed in form and habitat. It is no longer the magnificent long-lived giant anyone born the the English countryside much before 1970 will remember but a modest, short-lived hedgerow plant, probably barely recognisable to anyone who doesn't recognise it by its leaf. Natural processes, acting without direction or intelligence and following the chance introduction of non-native fungi, maybe in a single load of imported elm wood, has transformed both the English elm and the English countryside.

The ecological explanation for this observable phenomenon which was recorded in great detail and witness by millions, is quite clear and complete. There is no magic or mystery involved. What, I wonder, would be a creationist explanation in terms of intelligent (sic) design? Why did some putative intelligent (sic) designer suddenly decide to alter the growth habit and life cycle of the English elm in about 1975 and why did it need to use fungi and beetles to make the change happen? Why did it quite suddenly want the magnificent elms it had put in the English countryside for hundreds of thousands of years since the last Ice Age to suddenly disappear. Was it just maybe bored one day and decided to spoil beautiful views for the sake of it? Did the Devil find work for God's the intelligent (sic) designer's idle hands?

Perhaps just as interestingly from an evolutionary biologist's point of view, why did it leave the English elms with the full genetic potential to grow into giants still? And why did it chose a method which looked for all the world like one not requiring intelligence or design?


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