Thursday, 26 February 2015

Hybridity’s contribution to Evolution

Part 3
No, not an evolutionary tree, merely the ginkgo maiden hair emblem of a Japanese district

I have argued in Part 1 and Part 2 that the most frequent occurrence of hybridisation is when closely related animals or plants breed together and that their line of issue repeatedly backcrosses within the group of one of their parents. The effect of the original cross is hugely diluted but never-the-less new genetic material will have  transferred from species to species. Such genetic information gives natural selection a greater pool of genes to work with.

I have said little about incredibly rare successful hybridisations between organisms genetically distant. Such events of varying ‘distance’ have been recorded and continue to occur. They neatly explain the mysterious appearance of completely new animals or plants in the fossil record. Such animals or plants are not so different to be completely bizarre and are consistent with reasonable deduction of the identity of their parents - albeit this is made difficult by the fact that the parents will often be extinct.

The consequence of hybridity is that the ‘tree’ of evolution is not merely a series of simple branches. Instead, from it’s very beginnings the branches and twigs are entwined and grafted together. Even the roots, not normally depicted, are finely matted.

I had intended today to use the three ‘fossil trees’, Ginkgo biloba, Taxodium distichum and  Wollemia nobilis to both further my argument and to celebrate the wonders of evolution. I now find there is so much of interest about ginkgo and its cycad ancestry that I need to write further posts after this one!

About ginkgo
A fine Ginkgo biloba in Kew Garden

Ginkgo, The Maidenhair tree is repeatedly recorded in the fossil record over a period of 270 million years. Fossils have even been found here in North Yorkshire in Scalby. I wonder if I can claim to be native? Over this time ginkgo  has remained remarkably constant although it did evolve into several now extinct species and once had worldwide distribution. Ginkgo biloba is the only species surviving. 
It must be very difficult to classify the extinct ginkgo species when, for example, the leaf shape of the modern plant has numerous variations on a single tree. There is very little difference between the present day form and the oldest of fossils. One might ask why over such time it has not made small Darwinian changes into something quite different.

Known from the fossil record, ginkgo was thought by western science to be extinct until discovered by a dutchman in Japan in the palindromic year 1691.
It had been preserved planted around ancient Chinese temples. It no doubt had traditional herbal uses as well as sacred traditions. Resistant to fire there is an old legend that a dense planting of ginkgo once protected an ancient temple from being burnt down. 
It would now seem that in China some present day ginkgo forest has survived directly in nature.
Propagation material had been taken from China to Japan about a thousand years ago where it also became sacred.

Ginkgo has survived and now continues to thrive in disturbed, rocky, wet, yet well drained soil. It is known to grow in soils as acid as pH 5 but seems fairly pH tolerant. I grew it in York at a neutral pH 7. In fact it is very easy to grow and is remarkably tolerant to pollution and susceptible to hardly any pest and disease. It makes an excellent street tree where it can survive at very low light intensities.
Beautiful yellow Autumn colour on my tree

It is reported to live up to 3000 years old and achieve heights of more than forty meters. Its longevity is aided in that it freely vegetatively propagates from basal ligno-tubers or broken attached branches that root into the ground. Ginkgoes are usually monoecious (single sex, male or female) although dioecious trees have also been recorded. Males are favoured  for public planting because females carry large seeds that smell rancid!  A possible factor in it’s decline and near extinction might be that dinosaurs were significant vectors of its stinking seed.

My young tree initially developed a tall unbranched trunk rather reminiscent of young ginkgoes competing in forest
Revered in Japan, several ginkgoes were within a kilometre of the epicentre of the Hiroshima explosion. Although charred they fully recovered and perhaps relevant to the theme of this post and ginkgo’s genetic stability over 270 million years, no mutations were reported.

So where did ginkgo come from? Although thousands of its fossils are known, there is no fossil record of its immediate parent. Botanists could not agree how to classify something that had both conifer and cycad characteristics and invented a division in plant classification to especially contain it. 
Genome studies have shown that ginkgo contains cycad genes. Gingko in common with cycads, but in common with no other tree, achieves fertilisation by means of  mobile flagellate spermatozoa.  
(Botanical note. Fertilisation is a distinct process to pollination and the pollen tube which precedes it. Motile ginkgo sperm do not have to swim very far)

So could ginkgo be a hybrid? Even today this is a heretical notion and would appear to have never even been considered. Perhaps it has been suggested and dismissed with a sneer. 
If it were a hybrid what might be the two parents? Conifers are the only tree that grew alongside the early cycads as far as I know. Perhaps ‘the other parent’ wasn’t a tree, cycads themselves are woody and very tree-like. I felt I must read about cycads!

Cycads
I thought I had better check out cycads and to find out if this most ancient of plants actually hybridises.
It is very promiscuous indeed according to modern breeders. Cycad propagation text books actually advise how to avoid  hybridisation. 
Most evolutionists do not seem to consider hybrids a possibility! Interspecifically ginkgo hybridises very freely. Between distinct cycad genera a significant numbers of successful crosses have also been recorded. This is a long way from creating a ginkgo but a distant hybridisation event or a series of crosses seems no less believable than standard evolutionary theory. 
If cycads were shown to be one of the parents of a past hybridisation would it challenge neo-darwinian theory? I think not. In my next hybrid post I will write about an iconic conifer that has been shown to be an ancient hybrid. Of it having any significance to evolutionary theory there is not a whisper!

A rare slate blue cycad in Funchal botanic garden

Cycads are fascinating plants that share ginkgo's monoecious life cycle, longevity and resistance to fire. 

I was fascinated to discover the story of the cycad Encephalartos woodii. Now extinct in the wild, only one specimen was ever recovered. It is a male that produces fertile pollen. 
Plants have the advantage over most animals that they can reproduce vegetatively. E. woodii propagates very freely from suckers and grows into a six meter high vigorous 'tree'. It is now to be found worldwide in five hundred botanic gardens and many more are in private collections.

Where did it come from? We are in the realm of speculation. Had the females become extinct? A very real possibility. Was it a hybrid (between two cycad species) and a female form had never existed?

Is it possible to recreate a female encephalartos and obtain viable seed? There are various possibilities that are being tried. 
A recognised fact about cycads is that they sometimes change sex if subjected to stress. It needs a very strong will to almost kill your prize plant to actually try! 
Another approach, close to my story, is to use fertile pollen to hybridise with a close encephalartos species and to repeatedly back cross over several generations using E. woodii pollen to recreate a female version of the original plant! Note the time between germination and production of seed is a long one. It might be a long wait.

Micro propagation can be used to vegetatively propagate thousands of 'spare' plants for the breeders to play with.

I have no information what-so-ever that ginkgo did not evolve in a straight line from a single now extinct cycad. What grieves me is that no one seems to have looked at the possibility of hybridisation and without evidence just assumes the usual story.
cycad in Funchal botanic garden

Never give up on an apparently dead cycad. It can be vegetatively propagated from leaf, root or stem! I haven’t a clue about the history of this cycad in Steven’s new garden.

Neanderthals and humans 
If you condense the four billion year age of the earth to that of a day it was a single second before midnight when these two very close relatives got together. Perhaps a very good time for a fling. Being genetically close and sharing the same number of chromosomes many unions were destined for procreation.

I lay in my bed and got thinking about recent revelations that we share with neanderthals 2 to 4% of our genetic make up. I got to wondering that when, as we surely will, further analyse fossil neanderthal DNA, that a similar ratio of human DNA will grace their own genome.

I thought that 4% is quite a high number and surely represents frequent liaisons. After all, when hybrid genes are subsumed into a populations over hundreds of generations of back-cross-dilutions they will virtually vanish. Reminiscent of dubious homeopathic molecules that disappear with their repeated logarithmic dilutions!

But was I now dreaming?  Surely my hybrid posts are about how natural selection selects from a widened pool of genes made available by hybridisation. If neanderthal genes held advantages, then of course they would remain. Indeed how much better the chances of several linked genes being passed on than those of a single mutation as typified by neo-darwinian theory. 
Recall that several genes from hybridisations are often associated together and change is not always just an isolated mutation.
I wondered how many beneficial Darwinian random mutations never saw the light of day?

I then had a nightmare. What if the 4% figure was achieved after statistical binning? I awoke with a jolt and realised that as they had been looking for neanderthal genes they would not on this occasion have dumped them!

And news of potential hybrid ancestry millions of years before modern humans and neanderthals emerged

From the December 20/27  2014 issue of the New Scientist
According to the New Scientist, the science community are eagerly awaiting results from findings in the Rising Star cave in South Africa from where several skeletons have been taken for analysis. They would appear to be between 2 and 3 million years old, the time when Homo was splitting from Australopithecus.
I loosely quote the New Scientist. “instead of a neat family tree it looks like a number of ape like creatures existing a few million years back interbred to produce a messy collection of species, from which we eventually emerged”


4 comments:

  1. I have never seen a few big Ginkgo, but none as large as the one on your picture from Kew Garden. It is a magnificent tree.

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    1. It is, but nothing like specimens in China!

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  2. Hi Roger, like your last post which created copious comments, this one is excellent reading but not something that I can really comment on. You can answer a question that has intrigued me though, 1691 is not a palindromic number unless you flip it over on itself, have I fallen into a trap or did you put it in to see if we were paying attention :).

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    1. I put it in to be a clever-clogs Rick! :}- And of course got it wrong!! Is there a word for flipping a word over and reversing it?

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