Sunday, 7 December 2014

Hybridity’s significance in evolution....high-lighting hybrid plants.

Part 1. 

I am above my pay-grade today! Writing about one of my passions, evolution. I am questioning Darwin’s received wisdom that life’s evolution comes down in a straight line in a continuity of small steps. Not that I suggest anything that greater minds than my own do not espouse. My source of inspiration is that great American geneticist Eugene McCarthy and if my post today interests you, you will constantly need to refer to his website to clarify the numerous ‘loose ends’ that I fear I will fail to tie.
Don’t get me wrong, the fact that life has evolved slowly from primitive beginnings is a fundamental part of my personal belief system. 
The bone of contention is that although we gardeners personally experience the results of hybridisation on an everyday basis, conventional and might I say outdated ‘popular’ macro-genetic theory dismisses it as an insignificant aberration.

Let me please emphasis I do not deny the slow and gradual pace of macro-evolution, nor the accumulation of small genetic changes. What I do now believe is that when genetic changes are shared from one organism to another and when a huge number of genes are transferred and recombined and expressed in novel combinations, the contributions of hybridity to evolution is a very powerful force indeed.

Some adherents of hybridity theory  suggest that Darwin himself who carried out hybridisation of domestic birds was well aware of it’s significance but perhaps thought the world not to be ready for it yet.

This nerine is an interspecific hybrid derived from several different species. It’s hardiness was introduced from Nerine bowdenii into otherwise tender nerine. It’s large flower spike is no doubt an example of ‘hybrid vigour’


Strawberry (Fragaria, itself an interspecific hybrid) hybridised with Marsh cinquefoil (Comarum) to create intergeneric hybrid Fragaria ‘Pink Panda’
My own Paulian conversion is an unexpected consequence of writing my blog! You can witness my enlightenment in my post ‘Musings from York’ when ‘Fool-On-A-Hill’ - perhaps it is a pseudonym? - wrote illuminating things in my comments section. Please go there, his words are more eloquent than mine. Where fools rush in angels fear to tread!

There are many popular intergeneric hybrids between Heuchera and Tiarella. Breeders create new varieties of Heucherella by repeating  the original cross or selecting nice seedlings from Heucherella.
Harry and I always argued. He would sometimes storm home and I would think it would the last I would ever see him. I would bang the table! He accepted evolution in principle but insisted there was a fundamental flaw as shown by the fossil record. Animals and plants appear without any clear immediate and obvious ancestor and remain relatively unchanged for millions of years up to their eventual extinction or to the present day. I used to insist that ‘missing links’ were constantly being filled and would argue that misinformed deniers did not recognise that when one gap in the record was filled, two others were created.
After extensive reading I am now convinced Harry was right and that the fossil record does show a punctuated progression. How we miss him and I wish I could just pop round and tell him!

Some general observations about evolution

Before I get my teeth into the hybridity thing there are a few things about evolution that I think gardeners might misunderstand. And perhaps I do too! Very often the press records examples of rapid evolution such as the change in the colours of moth wings or very recently the height that bats nest in their caves under the threat of a competitive alien bat species. I wrote myself how natural selection of plants growing near polluting former Welsh lead mines had over very few generations developed the ability to survive and thrive despite toxic heavy metals. 
None of these changes require new mutations but only the diversity which already exist in the ordinary variability of the population.

Such rapid change in contrast to stability of complete organisms over perhaps millions of years. 

Nearer to my theme is when hospital bacteria develop resistance to antibiotics. It is widely and accurately reported that the spread of this resistance is a more acute danger because different species of bacteria can exchange new genetic data. Resistance developed as a consequence of for example, overuse of an antibiotic  in agriculture, is passed on to other bacteria by a process of conjugation. Not quite hybridity but the result is in some ways similar. When genetic information is passed from species to species it is a very powerful force of evolution indeed.

Some general observations of hybridity

“I’m not a mongrel but this wretched streptocarpus has multiple parents”
“Poppy, I have serious doubts about your origin but the text book agrees with you”

When I sit here in our conservatory today I see numerous plants such as my calamondin orange, fuchsia, and pelargonium that are known acknowledged hybrids between two or multiple species. I also see orchids and christmas cacti and hippeastrelia which are hybrids between distinct genera. In gardening terms hybrids are very common and indeed almost all of our modern garden plants are the result of many years of selective breeding where genetic information has been mined from a huge range of distinct albeit (usually)related plants.

The Christmas cactus has parents from several genera. In my post about this plant I mentioned it is ‘invading’ the wild in South America. Should we worry or just admire evolution?

Inter specific hybrid calomondin’s several citrus sources are said to be ‘lost in antiquity’

It is sometimes suggested hybrids are less fertile than ‘pure breeding species’
I had no trouble germinating my calomondin pips.
Hybridisation within an established species is an everyday occurrence to  plant breeders and seedsmen. Gardeners routinely sow F1 hybrids where two closely related parents have been crossed by the seedsman ‘on demand’. Even the small differences between such close relatives produce hybrid vigour. How much more vigour and variation there will be when whole chromosomes containing thousands of new genes combine when species cross.

Compact Winter-flowering Corydalis elata has been crossed with taller June-flowering Corydalis flexuosa to produce several named hybrids such as Corydalis ‘Spinners’.
My hybrid is intermediate in size between it’s parents — a common characteristic of hybrids — and flowers at the same time as elata. 
As a bonus many of my hybrid plants continue flowering for the rest of the Summer.This picture is in early December! Surely an example of hybrid vigour?

Wolves, coyotes and dogs

All are different species of the genus Canis, share the same chromosome number (78) and interspecifically hybridize relatively freely. Not in everyday terms, but on an evolutionary time scale not uncommon.
I had not intended to say very much about animals today but have been drawn to two recent news items. One was about coyote-wolf hybrids causing concern in north east America.The other - no lesser source than the Daily Mail(!) - reported real research that challenges the conventional view that dogs are descended from grey wolves.

On reading further about coyote/wolf hybridisation I found that it is not a new phenomenon and genome studies and actual recordings show it has taken place hundreds and perhaps thousands of times over, say, the last hundred years. The coyote genome seemingly contains genes from grey, red and eastern wolves. It is a magnificent coyote/wolf mixture.

As is normal with hybridisation events, a new hybrid usually merges into the species of one of it’s parents by repeated backcrossing. All over America there are slightly differing ‘strains’ of coyote that further intermingle as they migrate. Hybridisation has given natural selection opportunity to work with an increased library of genes to create the superb cocktail that proudly fulfils a ‘top predator role’ in the USA.

A caveat here. In nature rare hybridisation normally take place where the ranges of plant or animal geographical distribution overlap. In most cases repeated backcrossing within the population of one of the parents reduce ‘new genes’ to barely perceptible levels. A population’s integrity is often maintained for thousands of years. In most places coyotes may carry very few ‘wolf genes’ and many of my American friends might argue that in their locality there are none!

Some coyotes, dubbed ‘coy-wolves’ contain more wolf genes than ‘their own’. Talk about ‘a wolf in sheep’s clothing’!

The new data on domestic dogs  is that they have not evolved from the grey wolf as previously thought, but from a slightly earlier and now extinct wolf-like common-ancestor. Dogs as a distinct animal have been around much longer than previously thought. The picture is clouded, in that, since dog’s domestication, dogs and grey wolves have sometimes bred with each other.

In evolutionary terms, thousands of years is relatively recent. On that small timescale canid hybridisation has had significant impacts on their own evolution. 
Why does conventional theory assume that hybridisation has not been happening for all of millions of years?

I must confess to a small difficulty in understanding what hybridity really is! The division of organisms into families, genera, species (and subspecies, varieties, cultivars and all) is a man made artificial construct. What really is the difference between sharing new genes by intra species mating to inter-species, inter-genus and inter-family crossing? Yes I know that usually within a species there is simple pairing of genes via the process of meiosis and that between more distant parents the crossing of information to form a new genome is sometimes at first a more messy process and that most crosses fail or are infertile (but not all). To me it seems that from the closest of liaisons to the more distant successful couplings, in all cases genetic information is shared.

This is the first of three posts on this subject - the next in January

18 comments:

  1. Well, Roger, you have certainly given us food for thought! I read a book the other day about "Commensual" animals (ones that cohabit with humans), which made me realise that the concept of keeping pets is perhaps a bit artificial. Some animal species made a conscious decision to live with humans simply because it was in their interest to do so. I know this is not the same subject as the one you are discussing, but equally absorbing, I think! :)

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    1. Very absorbing Mark. It seems very likely that dogs and perhaps - even more so cats - sought us out.
      If we move away from pets, what about cockroaches and in the garden fruit tree red spider mite which only eats moss and algae in nature but as a result of the selective action of our spraying since about 1930 'eats' the leaves of our apples and pears!

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  2. Thoroughly enjoyed this post (and the older one Musings from York) Roger.
    It's not a topic I know an awful lot about but I'd like to say that your posts have been very informative and have given me plenty of food for thought.

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    1. Part two is already written and gestating on my computer Angie. Thanks for the nice words.
      Its a relief that we have kicked off with some nice comments. I await the brick-bats. I expect Sue will have something incisive to say!

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  3. As always you have written a well-balanced post with lots of information and thoroughly researched. I really enjoyed this post, and I also read the Musings from York which I must have missed, with the comments, really interesting. By the way, I have read all the books of Jean M Auel about the Earth’s Children, I have all 6 of them here in my bookshelf and enjoyed getting history and science presented to me in such a digestible way. Eugene McCarthy’s book sounds interesting, I clicked on the link to his blog and started reading – and got absorbed! I haven’t got time to read it all now but it is an interesting topic I know just a little about (too little). I was also under the impression that hybrids were sterile, at least in the world of mammals, nice to have that cleared up! Looking forward to your next chapter :-)

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  4. I have also read all six of 'Earth's children' and McCarthy's book twice!
    It is former lack of knowledge about the potential fertility of hybrids that has held back thinking. There are so many well documented examples of animal hybridisation now that perhaps hybridity is being re-examined. My next post in this series looks at several histories of hybridity in plants. I don't mention lilies but they have interest in as much lily hybrids are often more fertile than their parents

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  5. @Roger

    That is cruel of you to make us wait so long for your next instalment... really enjoyed Part 1

    @Helene

    I likewise have Jean Auel's Earth's Children series (in my Kindle). I found it fascinating that Jean had been able to so neatly distil and focus on a number of fundamental traits which drive animals into the near opposite directions of speciation and diversification.

    Although I don't doubt it works wonders for sales, she picks up the near omnipotent importance of sex drive. Without the need for, and pleasure of, sex, there would be no breeding. She also managed, although in a limited way, to portrait the ability of sex drive to span from one animal to another, although she stopped short of revealing the reality that the power of the drive extends across wildly divers animal forms (provided they are both of a mind to enjoy the moment) and right into the family between children and parents. This unrestrained drive for sex is the engine of genetic diversity and is almost certainly the cause of 'Darwins Dilemma' - the Cambrian explosion.

    But set against this unbridled lust for copulation, animals have an opposing driving force - the attraction of like for like and a repulsion against anything different. While 'sex drive' will create genetic diversity and hybrids, repulsion against 'difference' is what drives enhanced fertility and eventually 'speciation' - ie the development of a refined, isolated genome, optimised for breeding and survival in a given environment. But that very isolation is what nails the coffin lid on that species when 'the environment' changes and the species is left with inadequate diversity to allow adaptation. Again Jean weaves this revulsion wonderfully into the story line.

    There is also the amazing fact that a child of one species that has been brought up by another species, thinks of itself and is sexually attracted to its adopted parents - again Jean wove this drive characteristic into her novels.

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    1. Thank you
      For those of you who have not read Earth’s Children the first book covers the story of the finding and and adoption of a little girl no more than a baby by Neanderthals who bring her up as one of their own. Although she grows up as a Homo sapiens beauty she regards herself, as do the neanderthals, as ugly. She does carry a child to the leader of the clan. In Book 2 she finds her own race and Mark Willis will be interested to find that she personally domesticated both the wolf and the horse!
      Her early life is a fictional example of imprinting. This phenomenon is when a newly born animal bonds on the first animal (or even thing!) that it encounters. Harry rescued a fertile wild duck egg and put it in with his broody hens. The duck that hatched thought itself to be a hen. The duck even corralled its sisters into the shed at night! In this case the duck eventually paired with a wild duck and returned to the wild. For several months they would do a ‘fly pass’ of its old home.

      Apparently our neighbour’s father believed hybridity to be a significant aspect of evolution. He managed a huge farm in Africa. He was trained as a vet and with his biologist’s eye he closely observed jungle life for several decades and based his view on the evidence he had seen.

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  6. Beautiful photos and fascinating post.

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    1. Evolution is a really interesting subject that is taught badly and shallowly - if at all - in schools. Glad you enjoyed my effort.

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  7. I remember Daisy Duck! Her hen mum was absolutely horrified the first time Daisy jumped in the pond! The other hens just treated her like a 'special needs' chicken!
    And yes, my Dad always said that he saw stuff in the remoter parts of Southern Africa that would make your hair curl! Put is this way, zoonosis never surprised him!

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    1. I had forgotten your propensity to give animals names. I can just imagine Daisy's foster mother's reaction at her diving in the water.
      Interesting how your duck still took to water and eventually mated with its own kind. There is much discussion these days about the balance of instinct and learned behaviours in animals.
      (You have also since mentioned to me that Daisy stayed in the area for several years)

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    2. I think learned behaviour is extremely interesting. I rescued a day old sparrow last year. I think he'd been turfed out the nest for being a bit 'wrong'! With only me to teach him, he now, at a year old, makes the same sparrow noises that I do! Distinctly different from normal outdoor birds!

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    3. He is the happiest little bird I know. I know he attempts Poppy's 'hello' squawk when he stays at your house!

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  8. Roger between yourself and ‘Fool-On-A-Hill’ you have made my meagre brain hurt. I am currently cursing the work of geneticists for revising what seems to be most of the nomenclature with which I am familiar and now you want to have an in depth discussion about the very basis of life. Evolution appears to be a subject which is all too often skirted round due to fear of our PC society and being burnt at the stake. My own simplistic view is that I accept Darwin's theories are correct as we have proof of this in that changes can occur in relation to circumstances in a very short space of time so it would follow this is also true over a longer period, however if one is to accept that life itself appeared because of a unique set of conditions, in other words a fluke, then it would appear to be quite feasible that hybridisation is another, maybe more potent, force of evolution. Interesting stuff.

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    1. I agree that in the same way that natural selection can be observed over very few generations as with my opening examples, hybridity too is a very common occurrence that all us gardeners see in our plants.
      There is no reason to assume that what happens now has not happened over evolutionary time.
      I don't think we can compare all the mechanisms of the process, the more I learn the more I think that nature is opportunistic and does not abide by rigid rules.

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  9. Darwin says, "Is it improbable [that] variations useful in some way to each being in the great and complex battle of life should occur in the course of many successive generations? If such do occur can we doubt that individuals having any advantage however slight, over others, would have the best chance of surviving and of reproducing."
    Darwin was well aware of hybridization through his correspondence with many of the most famous head gardeners of the time such as Paxton, pigeon fanciers and farm animal breeders. It was his observations of these hybrids that led him, among other things, to write, 'Origin of Species'
    The fossil record is very sketchy as Darwin says, " The noble science of Geology loses glory from the extreme imperfection of the record. The crust of the earth with its embedded remains must not be looked at as a well filled museum, but as a poor collection made at hazard and at rare intervals."
    Even with a poor fossil record I think there is mounting evidence for punctuated gradualism and punctuated equilibrium for some species. Darwin admitted that the rate of evolution was variable although he still seems to have insisted on phyletic gradualism.
    I think that you are making a problem that does not exist in modern evolution theory. Most biologists would agree that there are differing rates of evolution. However, remember that evolution depends upon whether you can survive and reproduce and hybrid breakdown, hybrid sterility and hybrid inviability all contribute to loss of fitness. Fitness in evolution theory is defined as the relative ability of an individual organism or genotype to survive and leave offspring that themselves can survive and leave offspring. So even in F1 hybrid vigour when these individuals reproduce their offspring will revert to the characteristics of the original parents. Just because we can maintain a hybrid artificially through gardening does not mean that it will survive in evolutionary time. Nature has made lots of experiments but because of lack of fitness they have not survived to modern times.
    Incidentally, the evidence that Darwin's theory of natural selection is mounting so a reasonable person would accept that it is the best description of how organisms change over time however I would not accept that it has been 'proven' and it most certainly was not correct in all areas. Punctuated evolution being one example.

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    1. I have answered some of the matters you raised, Anthony, on your very much appreciated comments on my other hybridity post
      I don’t think the fact of evolution as put forward by Darwin is in doubt, just the nuts and bolts of the process. Darwin himself would be fascinated by all the knowledge we have now of matters genetic and it is amazing how many of his conjectures remain sound .
      I strongly disagree that the sudden appearance of new stable life in the fossil record has been adequately explained by the various fairly desperate theories of different rates of evolution and the punctuated nature of the process!
      I also disagree with your hints of the ephemeral nature of changes brought about by hybridity. I know I am repeating myself, but Eugene McCarthy examines these issues on his chapters on stabilization on his blog where his complete book is reproduced

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