Monday 12 January 2015

Hybridity in Evolution

How hybridity is often overlooked in evolutionary theory
Part 2

There are tens of thousands of new species that are known to have have arisen naturally from crosses between similar species and infinitely more at the plant and animal breeders’ hand.
I know of no verified cases (as opposed to assumed cases) where simple natural selection of the conventional kind has created a new species and yet conventional wisdom always states that in the absence of evidence of hybridity, that for every new species discovered, accumulation of slow small Darwinian changes in a straight line was how it evolved.

You will imagine by now that I claim hybridity leads to dramatic changes in plant and animal form. Sometimes it does and hence my comments in my previous post about abrupt new appearances into the fossil record. More usually changes are relatively small and indeed new hybrids are mainly between similar species and are of intermediate form. Such change is normally diluted when a new hybrid and its off spring repeatedly backcross with one of the parent species. An exact parallel with normal plant breeding.

The genes in this rose are of worldwide origin and have been introduced from many hybridised species 

Many naturally occurring species of rose  have contributed to modern garden forms and their breeding has continued from ancient times. In most cases the contributing species would be quite incapable of directly fertilising one another. How can it be that genes from such plants can be shared?  I will answer this vaguely but to say that frequent backcrossing and recrossing creates fertile hybrid plants where sharing of information between formally distinct forms can be made.

Hybrid lilies are frequently more fertile than their parents

Can such ‘sideways transfer’ of genetic  code be equally subtle in nature? Imagine species A can successfully, albeit rarely, hybridise with species B. And species B is able to hybridise with species C, but species A will not cross with species C. If an AB hybrid lives in a zone dominated by species B, after repeated backcrossing it is effectively subsumed into species B. Never-the-the less some genetic information has moved from species A to species B. When the modified species B hybridises with species C, genes from species A will enter the genome of species C!
This is not something from my fevered imagination, it really happens. And one might say “and so on ad infinitum”. When I wrote recently about mistletoe my title was ‘So good nature invented it five times’. I am beginning to wonder, tongue in cheek, whether nature invented it once and it was passed on!

My recent post on holly discussed how holly evolved a shared ability with other genera in ancient laurel forest  to cast off water by developing waxy and elongated leaves and pointy leaf drip tips. Convergent evolution might very well correctly explain such relatively simple modifications. But really?

A short horsey digression

At the end of the nineteenth century, a museum assembled a demonstration of purported successive fossils that demonstrated the slow evolution of the horse over tens of thousands of generations of accumulation of small changes. This sequence appears in almost every school text book that teaches evolution.

There is no doubt of the accumulation of small changes over the millenia, just doubt that the changes were in a single species, were in a continuous straight line and were not punctuated by a series of small advances as closely related species bred and created new genomes. Hybridity theory explains the evolution of the horse in a much more better way.
Most geneticists are ashamed of the claimed sequence of horse evolution and there is no certainty that all the supposed ancestors are actually linked and at least one is in the wrong order of time.
No longer on public display, this historic ‘teachers demonstration’ languishes, in disgrace in a museum dungeon!

I am NOT suggesting that the evolution of the horse is not reflected in the generalised picture painted by the fossil record. I am suggesting the progression is not in a straight line but in a series of small hybrid leaps and might be described as rather reticulate. No doubt numerous close hybridisations occurred ‘along the way’ in a similar manner when in our own ancestry, neanderthals and denisovan genes entered our genome.

Some stories of plant hybrids

Fatsia crossed with Hedera gave the interspecific hybrid Fatshedera x lizei (and subsequently a specimen ‘sported’’ to give Fatshedera x lizei aureomaculata)

If you read wikipedia you will find that  the evolution wheat is a result of a series of mutants. Clearly written by a politically correct hybrid denier!

Closer to the truth (but not exact) is the following hypothesis. Neolithic farmers grew starchy grasses. Wild einkorn Triticum boeoticum and goat grass Aegilops speltoides hybridised together to produce emmer Triticum dicoccoides. Emmer subsequently hybridised with another goat grass Aegilops squarrosa to produce durum. Several promiscuous relationships with the original parents were to produce wheat. There is of course no reason to deny that there might also also have been beneficial mutations ‘along the way’.
Conventional  opinion is that these crosses were all natural hybrids and man’s only contribution was to sow them together and opportunistically select out improved forms. Certainly true, but I wonder when ancient man, who was much closer to nature than ourselves, began to transfer pollen?

Forty years I rooted a cutting and (to my current regret) subsequently planted it.

The Leyland cypress, despite my generally snide comments makes a very fine tree. It is a bigeneric hybrid between Xanthocyparis nootkatensis and Cupressus macrocarpa. It is recorded to have naturally occurred in gardens on at least three occasions.
It produces fertile seeds albeit rarely. Not unusual for a hybrid and one wonders in the absence of man’s propagation whether it would survive in nature. Read RHS botanist James Armitage who discusses its fertility.

You might wonder where each of the fifteen known cultivars of Leyland cypress come from. One of three mechanisms will be involved - vegetatively propagated mutation (often called a ‘sport’), selected fertile seedlings or re-creation of the original cross by a plant breeder.

I can find in my garden several examples of plants that are interspecific and intergeneric hybrids. Apparently crosses as distant as between different families have been recorded although I cannot find in the literature suitable plant examples. 
There may not be any reason to think that distant crosses are more significant in evolution than an accumulation of repeated smaller hybrid ‘advances’. Distant crosses in nature are extremely rare with odds against their occurence sometimes many millions to one and odds against any such a hybrid being fertile and having selective advantages make the likelihood of survival extraordinarily small  - but not vanishingly small. On the scale of evolutionary time such events would seem likely!

I was interested to try and find references to very distant plant crosses. The strangest I found was this ‘proof of principle research’ where oenothera pollen was transferred to oryza (rice). I am afraid I did not fully understand to what extent the cross was successful and if you are interested you can read it yourself!

On further reading I discerned that this transfer of the genetically distant pollen was followed 48hours later with normal self pollination with another rice plant. The resultant seedlings showed marked differences to normal inbred rice. There was no evidence of actual transfer of distinct oenothera genes although such gene transfer is not unknown. Several generations of inbreeding were subsequently studied. In several lines there had been restructured patterns in the rice genome and significant epigenic  change. The researchers postulated that such novel hybridisation would occur in nature.

Oenothera is regarded as something of a genetic maverick that does not obey simple Mendelian rules

I have written before about that ‘alien invader’ Rhododendron x super ponticumin my post ‘Musings from York’. Peter Williams also eloquently explained in his guest post ‘A passion for rhododendrons’ how interbreeding between doubtfully hardy Iberian Rhododendron ponticum and three other introduced species created thuggish and very hardy Rhododendron x super ponticum, now described as a hybrid swarm.

Peter tells me that the chances of a plant invading the countryside is a mathematical thing and these opportunities are vastly increased when there are large numbers of seeding plants growing together. Historically the new hardy ponticums were widely planted on gentlemen’s shooting estates. Seed was very cheap and quickly scattered by unskilled labour was very fecund. There was every opportunity for the plant to become an invader.
Peter tells me that in Norway where the climate and soil is infinitely more suitable for rhododendrons, without our mass planting there is no R. x super ponticum in their countryside at all!

The term ‘hybrid swarm’ puzzled me as it suggests plurality. R. x super ponticum is regarded a single species but is highly variable and readily self and cross pollinates with itself and no doubt with garden forms. Within a large clump in the wild there may be separate seed sown clones but all are regarded as the same hybrid species.

In view of the evolution of Rhododendron x super ponticum here in the UK perhaps we should restore its good name by dubbing it native?

The interspecific hybrid Primula kewensis appeared at Kew in 1900 growing amongst Primula verticillata and Primula floribunda. Genetic analysis confirms that it is a hybrid.

  plant world seeds                              

P. kewensis is a yellow primula covered with lovely mealy white farina.
For many years it was sterile - a very common feature in new hybrids - and was propagated vegetatively by division. Several years later it produced a fertile flower and there was an increase in plant size. It had doubled its chromosomes in a vegetative mutation to create a tetraploid shoot. Described as a ‘failure of mitosis’  this has occurred on the sterile form on three separate recorded occasions. Polyploidy enables efficient pairing of genes in meiosis and is often exploited by plant breeders.

P.kewensis now prolifically produces fertile seed. The young plants show no variation other than the degree of farination. That’s very curious and indeed is a very interesting genetic story, but not for today!
As a no dig gardener I was fascinated to see that some of the early research was carried out by Miss Digby.

Senecio squalidus is native to Mount Vesuvius and was an early guest in Oxford Botanic garden. It escaped over the wall, or more accurately into the wall and then over! It is known as the Oxford ragwort.
The railway line is thought to be the agent of its spread. There is a story, probably fiction, that a botanist traveling by train observed a seed float into his carriage. At the next station it floated away onto burnt vegetation on the railway banking.
At several locations S.squalidus is known to have hybridised with the common weed groundsel Senecio vulgaris. The Scots and Welsh versions are now known as the Cambrian ragwort and are very similar. 
More recently the Oxford ragwort travelled to York, my home town, and again procreated with groundsel! This time to give a very distinct plant from its Celtic relations. Unfortunately the York Parks Department sprayed the York ragwort away where it was flourishing under Lendal bridge. Was this vandalism or a service to humanity? Fortunately the botanists have a large supply of viable seed for future investigation.

An interesting point is illustrated in the fact that the Cambrian crosses are very similar and that our York version is completely distinct. Geneticists  agree that depending on which of two hybridising species is the male or female parent, their can be a profound difference in the character of any offspring. Peter tells me that the direction of the York cross is the odd one out. Unfortunately my fount of all wisdom is now on vacation!

Animal stories

Duck billed platypus

When you have eliminated the impossible, whatever remains, however improbable must be the truth. (Conan Doyle)

If ever there was a candidate for being a hybrid it is the duck billed platypus. What a mixture of mammal and bird characteristics! It was shown in 2004 and then confirmed in 2008 when its genome was published, that at a genetic level it shares sex chromosomes of both mammals and birds.
Now I have no idea whether it really is a hybrid and must conclude I am missing something when none of the scientists involved in deciphering its genome seem to have considered that the said platypus might be of hybrid origin. Are they so indoctrinated in current theory that they give it no consideration? Do they fear they will be subject to ridicule? Are there no research grants because the idea of hybridity is so grotesque? It is apparently extraordinarily difficult to demonstrate hybridity from the map of the genome - you can see what is there but is very difficult to elucidate where the genes have come from. In the case of Duck Bill, no one seems to have tried.
It is even quite difficult to show that even a modern plant of known hybrid origin is actually a hybrid from it’s genome alone. From millions of years ago it must be virtually impossible.
Look at Eugene McCarthy’s list of bird characteristics sported by a duck billed platypus! 

The more original a discovery, the more obvious it seems afterwards. (Arthur Koestler)

Bird family tree

Another matter concerns me. An excellent huge co-operative study claims to have accurately elucidated the descent of birds from dinosaurs. There are a few surprises in the developmental Darwinian ‘tree’ but never the less it seems to fit fairly closely with expectations. It is recognised now that the dinosaur was the ancestor of birds and the bird that resembles it’s ancestor most closely is the common chicken. The hybrid history of the hen would appear to be very simple (although it probably isn’t) and it results from a hybrid cross between the grey and red jungle fowl. Both parents look very similar and such ‘close’ hybridisation in relatively recent times (several thousand years) does not challenge its certain evolution from dinosaurs (note my provocative plural). Regarding the chicken’s ancestry scratch around in friznecker’s blog and find some very juicy worms.

Is this Kafkaescue comedy or am I just cynical?

Are they throttling the black swans?

Apparently the analysis of the bird data was analysed by a process called ‘statistical binning’ This is way-way above my comprehension and I am sure that it is statistically valid although I am reminded of the phrase ‘rubbish in, rubbish out’.
Now I know statistical binning does not literally mean ‘ignoring inconvenient data’ but I really do wonder. Especially so when they are happy with the result when the previous standard method of analysis shows a much more jumbled inheritance. A jumbled result is what you would expect if ‘inconvenient’ genes had been introduced by hybridisation.

Or just strangling the chicken?

I asked Peter whether I should write these provocative words. He advised that I should not let the facts get in the way of a good story! I am concerned that the idea of the exclusivity of ‘straight line inheritance’ is held by some evolutionists with such conviction that their eyes are closed to any contradictions.

Daphne ‘Eternal Fragrance’ is an exciting new interspecific hybrid that ticks all the boxes for a fine garden plant. Mine showed it’s hybrid vigour by flowering repeatedly from the day I bought it in April right through until Christmas. (Note the ‘drip tip’)

I promised a third part to my hybridity story. Unfortunately their is so much to say about the fossil trees and other items and I am afraid there will be even more!

Link to Part Hybridity Part 1


  1. Roger, I am baffled. My poor little brain cannot absorb all the wisdom you are throwing at it! Where do you manage to get all this info from? BTW: are you a contributor to Wikipedia or any such knowledge bases?

    1. Flattery will get you everywhere Mark! I think my limited knowledge but keen interest enables me to ask questions without losing any reputation - I have not got one. (And I think you will agree Mark one of the pleasures of blogging is that you can vent an opinion)
      Perhaps on the horticulture side I might sometimes speak with a modicum of authority - you will have every right to disagree when I write about brussel sprouts next month!

  2. I await your treatise on the Brussels Sprout with eager anticipation! Presumably you will reveal how it came into existence...

    1. I like the irony Mark!
      Since you ask I think development of cabbages, sprouts, caulis etc are quite interesting and as far as I know are all examples of man selecting for characteristics from the original wild form of Brassica oleracea. Breeders have hybridised some of the brassicas for example with radish

  3. Roger you may have wondered why I don't always leave a comment just after you have published your post but in fact it is because I prefer to think about your words for a few days before I comment. In this case I am giving it much thought and will return.

  4. I have to say I was a bit lost in your post and like Mark have the same feeling of "absorbing your wisdom". The post is very interesting though.

    1. Wisdom, what wisdom? It’s probably not very wise to sound off like I have from what some will regard as very superficial knowledge.
      I know readers will have very varying existing knowledge of evolution and things genetic and some of my argument may be very hard work. I fall back on the fact that most of my readers are interesting in plants and where they have come from.
      I know from your own blog Donna, that you sometimes have challenging things to say.

  5. On the subject if hybridisation - if a F1 hybrids is pollinated by the same F1 hybrid will the seeds come true? Martyn has saved seed from a Crown Prince squash. We have only grown Crown Pince and there are no other squash growing anywhere near ours

    1. Before an F1 cross is made to create seed for commercial sale, the two parents are usually produced by the seedsman over several generations of inbreeding by self pollination. The result is that each parent is a pure breeding line and becomes genetically very uniform. This stock of the two parents is maintained by the seedsman for each new generation of F! crosses for future sales.
      Because the two parents are themselves uniform there is uniform pairing of genes when the F1 cross is made. F1 hybrids are remarkably uniform. If the F1 hybrid cross pollinates with itself the next generation is a much more jumbled. In scientific terms such an F2 generation is rubbish! Well it certainly is with tomatoes and most other flowers and veg!
      Seedsmen love F1 hybrids, you keep having to go back for your seed!

      In some cases such as the pelargonium, F2 seedlings are very worthwhile growing.
      I have no specific knowledge with squashes and have not cheated by looking it up on the net!
      I expect you will be blogging (or knowing you Sue, you will have already started) about your squash. I will follow with interest how your squashes do next year.

      En passant, in the same family as squashes, for all female cucumbers, the only way they can be recreated is by repeating an F1 cross!

      When plant breeders( as opposed to seedsmen) carry out cross pollinations, eg between species, they are not crossing uniform lines and any F1 hybrids produced will be a glorious mixture although they will still potentially display ‘hybrid vigour’

    2. Martyn's put something on his blog as it is his project really. By the way is an oxlip a natural hybrid?

  6. I found very few references to its hybrid origin except for Darwin who states that it is a hybrid between Primula veris and Primula vulgaris!
    What I do know is it hybridises very freely especially with the primrose and many colonies of oxlip are in danger because of such hybridizations.

    1. For good or bad, hybridity is everywhere and has been so through evolutionary time. Thats my point really.(sorry to 'go on')
      Peter tells me there are so many places where zones of bluebells just merge.

    2. Sorry Sue, thats not clear - where Spanish and our native bluebells are planted close together they hybridise and merge
      (too much vino tonight)

  7. I always thought that hybrid lilies were sterile, I have left some of mine to seed but they don’t seet seeds. I will try to hand pollinating this summer and see if I can do a better job than the insects (probably not!). Perhaps crossing the hybrids will be more successful than expecting them to pollinate each other within the same hybrids? Thanks for yet an interesting post from you, I will digest it, can’t claim to have understood all of it in one go!

  8. I must confess I have not personally hybridised lilies. As a complete digression I did spend a happy hour in my greenhouse yesterday potting up little bundles of bulbs of Lilium formosanum pricei, an exquisite dwarf lily I sowed last year from my own seed.
    Yes I know my post takes some very careful reading and many folk have better things to do!. I do hope that everything does make sense when you read it again.

  9. Hi Roger, finally finished reading your blog for the third time and I also had a look at the pages regarding the rice/ Oenothera cross although WHY escapes me. I think I will sit firmly on the fence and say that both arguments are valid and probably run in parallel, although I can see the point about hybridization I can also see that, what I suppose we know as conventional evolution, is far more likely when there are outside influences such as changes in climate which could well account for the holly leaf. I can't think that this type of occurrence could spark hybridization, but who knows? I loved the section about wheat and your examples although incidentally I have always understood the true Oxlip to be Primula elatior and its many sub species, the P.veris and P.vulgaris cross being the False Oxlip although I am open to those with greater botanical knowledge and DNA results. Thank you for an interesting piece and if anyone else asks the answer is the number 42.

    1. Hi Rick, I have been waiting with some intrepidation for your comments. They are as thoughtful and questioning as ever. I nearly said ‘wise’ but I did not want to be too complimentary!
      You might ask ‘why’ to the oenothera /rice cross but had it shown a very distant hybridization it would have been very exciting. I know you are a primula expert and I did find something about a primula that sets apomictic seed (Without fertilization) but did need the stimulus of a primula pollen to set. A bit similar to the oenothera pollen effecting the rice but short of fertilization.
      My bit about drip tips was a bit of a shot in the dark, but it does seem to me that parallel evolution is often invoked when gene transfer from related species might be the real explanation.
      My research on Sue’s question on the oxslip was a bit hasty and the only reference to the vulgaris / veris cross was the great man himself. I did not read on to see whether he regarded it as the false oxslip.
      I did plant some oxslips next to primroses several years ago and they have all now merged together. Primulas can be very promiscuous. By former neighbour had the Barnhaven primroses and just about every combination of primrose characters were in her seedlings.
      Actually in the Christmas addition of the New Scientist in a piece about the genetic code the author said the actual answer was 37!

  10. Thanks Roger but words like "wise" and "expert" are far too much of an exaggeration. I did forget to confirm that the False Oxlip is a natural hybrid and you are absolutely correct in saying that many primulas are extremely promiscuous but they also have the added benefit of producing copious amounts of seed, my other favourite genus Meconopsis behaves in exactly the same way and if anything its nomenclature is even more mysterious.

  11. This blog is excellent because it demonstrates how horticulture involves lots of different scientific disciplines as well as aesthetics.
    However, I don't understand what you mean by evolutionists saying that evolution is linear. We get our genes from our ancestors. Theoretically and logically I must be able to trace my ancestors back in a line to the first life on earth. Is that unreasonable?
    You are creating problems that are not there. There is no conspiracy. You know about these things because scientists have written about them in books. There is some of evidence of evolutionary hybridisation for example: "Evolution"(1993) Skelton, "Cultivated wheat is derived from the hybridization of two species einkorn wheat and the emmer which are sympatric for a small part of their range. [Their natural ranges overlap]. The hybrid subsequently became polyploid and hundreds of varieties have been developed"
    My work on the hybridization of Quercus robur and Quercus petraea to produce the hybrid Quercus x rosacea on the Ercall Hill in Shropshire was done as as part of an undergraduate evolution course.
    As you have said, and was discovered by geneticists, sometimes sterile hybrid plants become fertile because of the duplication of genomes. This is accepted - no conspiracy. It is in basic undergraduate text books.
    Understanding what a species is and how they diverged and converge is called cladistics. It is based, as all good science, on evidence rather than speculation. Lots and lots of measurements -I know cus I've been there - of fossils and living organisms together with DNA analysis puts them into 'groups'; sometimes in the wrong group because we don't have enough data. This is why botanical names seem to change every year. Cladistics can work out which groups had common ancestors and how long ago they diverged. Regardless, our knowledge about evolution is built up painstakingly from evidence.
    Bacterial antibiotic resistance is brought about by exchange of plasmids – rings of DNA which can naturally transfer between different species and even between bacteria and fungi. There is nothing ground breaking about the movement of DNA between species. Viruses are doing it all the time. The Ebola virus moved from fruit bats to humans and probably bought some bat DNA with it. Plasmids and viruses are used by scientists to move DNA around to produce genetically modified organisms.
    DNA is very promiscuous and will go where it can survive and replicate. As Dawkins says it is very, very selfish.

    1. Thank you Anthony for your three long comments on my two hybridity posts and on my latest post on soils!
      Good to meet up here in the blogosphere as I often read your own fine blog. I suspect that we actually have much in common, although on soils in particular, our philosophies are very different.
      Forgive me if I suggest there is a conspiracy. I do not think this, although I do believe that many scientists are reluctant to accept hybridity’s huge contribution to the process of evolution as I suspect you do yourself. Of course scientists accept that hybridity takes place and has always done so and one of my purposes in my series is to bring such examples of hybridity to readers attention.
      I do suspect many researchers who understand hybridity just keep their heads down and don’t want to rock the boat. It seems to me that the teaching of evolution in schools is a Micky Mouse version of the real thing. Perhaps this is inevitable.
      I don’t want to challenge you on your wish to trace your ancestry back to the beginning of evolutionary time when life, perhaps on just one single occasion. started. It’s just that if hybridity is a major player that line is not completely straight but rather wobbly!
      I don’t know if you have studied Eugene McCarthy’s blog but he covers most of the issues you raise about fertility and permanence of new hybrids and what he has to say about stabilization of hybrids is both significant and a fascinating read.
      PS I have read most of Richard Dawkin’s books on evolution and he is one of my all time heroes, although since I have read McCarthy…..


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