Sunday, 17 January 2016

Why is hybridisation still not generally considered to be a force in evolution?


For many hybridity is peripheral and others fervently reject it.


As a lifelong horticulturist I have always been intensely interested in evolution. To me it is a fact that that life as we know it has evolved from primitive beginnings.

I know as a gardener that many of our garden plants are hybrids. I read that most farm animals are too. Whenever I search the net for a story about the origins of any plant or animal I want to write about I invariably find hybridity is involved. Why don’t we learn more about it in our school evolutionary text books? Today I want to explore why.

Other than my biological background and intense interest I have no special knowledge of genetics. I struggle to understand much of what I read. When I discovered Gene McCarthy’s writings it was revealing that someone with his authority, intellect and lifetime dedication to genetics should believe hybridisation was a prime driver of evolution. Such notions had been knocked out of me at school. I shamelessly return to his writings whenever I have doubts. Reading McCarthy has brought home to me not only that hybridity is equally relevant to both plants and animals but that there is a mountain of evidence that hybridisation has always been common in nature and that known and frequently recorded genetic mechanisms exist that enable the results of hybridisation to join the evolutionary line. Or should I say lines?

Many geneticists take it as a given that hybridisation is significant in evolution. Perhaps it is so obvious it is not even worthy of their attention and spending time in foolish argument? 
In contrast, within narrow genetic specialisations others might not bother to address the big picture? Perhaps they don’t want to bang their heads against a brick wall? Perhaps backwoodsman cannot accept new thinking and change their mind.
Despite all the evidence the standard story of evolution is that hybridity somehow has very little or zero significance. They even use statistical techniques to remove genetic evidence to construct their pure family trees!

Most evolutionary texts emphasise that evolution has been in a completely straight line. No hesitation, no deviation just like an arrow. This implicitly rejects the notion of hybridisation.
On the other hand the concept of hybridisation does not  deny that all life has evolved from that first single spark. (Nor does it confirm it as a single event).
Hybridity does suggest that the path of evolution is somewhat striated, that new mutations can be shared, that chromosomes can be restructured and new genomes created.

The belief that the arrow of evolution is precisely a straight line denies that an evolutionary line can share with others exciting new options. An organism passes new configurations on to its own progeny but not others. Sexual sharing is limited within its own species.
One might reasonably argue that normal sexual exchange is a form of hybridity. Why is such sharing acceptable but is taboo when breeding occurs across so called ‘species barriers’?

Cathi’s soay sheep remain true to type as they interbreed freely. If they got together with a different breed of sheep they would  readily create a hybrid, albeit a genetically close one

Although evolution has no preordained direction, nature has proved endlessly innovative in its procession. Evolutionary mechanisms unimaginable to Darwin are now understood in everyday currency. Genes are now known to have undreamed of mobility from species to species. I cannot believe that hybridity across so called species ‘barriers’ has not been a powerful force in evolution’s speed and direction.
My subject today is to wonder why many people disagree.

Why is hybridisation swept under the carpet?
Culture
I attended a lecture recently where mention was made of a project on the distribution of trees and the success of significant varieties in a dissimilar range of European climates - thought to be of interest in terms of conservation. The speaker showed us a picture of the magnificently handsome best performing tree and in a lowered voice stated that it was an interspecific hybrid. There was a perceivable frisson.

Listening to Jim jal-khalili’s excellent programme where he interviews great scientists, his guest for the day was a man whose knowledge of the botany of cereals had transformed whisky production. He recalled his first job interview as a young man armed with his botanical degree(s). He was asked what he knew about wheat! His answer was “nothing” , his course did not include hybrids, they were not considered real plants!

Our language is pervaded with words that suggest ‘pure breeding’ is a good thing. Mongrels are regarded as inferior. In truth everyone knows that many mongrel dogs outperform expensive pure breds most of the time.

The popular perception of a mongrel

Rocking the boat
Half the world does not accept the fact of evolution. It’s deniers know no limits in their propaganda that demeans the concept. Not only have many such people never tried really to understand evolution, much of the believing public only sketchily comprehend it.
Since Darwin’s time much of the detail of his so called ‘theory’ has been expanded and generally confirm his brilliant deductions. He would have been delighted to know about mechanisms that have now been discovered that he could never have dreamed of. 
Charles Darwin himself was a great hybridist and wrote extensively about hybrids. He did not permit himself to speculate about them in ‘The Origin of Species’

New discoveries can be wrongly portrayed as original error. Proponents of evolution are hoist on their own petard in being dogmatic about certain aspects of the process and over simplifying explanations. To admit that hybrids are a fundamental part of the process gives deniers ammunition.

Fear of public ridicule
Darwin withheld from the public his insights most of his life. As a devout man he worried how it would impact on public belief. Perhaps he also feared ridicule. The suggestion that man was descended from the ancestor of a monkey was a startling revelation. (Scientists these days are a little more specific when they name our ancestor as a chimpanzee).

The huge range of modern pigs bare little resemblance to their wild ancestors

My inspiration, Eugene McCarthy has postulated that in addition to chimps that the pig  family also features in our early human ancestry. Only a hypothesis, not  a statement of fact. His website includes a very long list of human characteristics that we share with pigs and no other primate. I am inclined to believe McCarthy’s suggestion.

It is not my purpose today to persuade you of the right or wrongs of this hypothesis. My point is that takes a lot of courage for an eminent geneticist to share his ideas when they might destroy his reputation.
I was recently distressed to read a virulent and ignorant attack on Eugene McCarthy. Written on a blog that google statistics show to have thousands of followers, it demeans itself by suggesting that only an idiot would perpetrate such nonsense. It scornfully distorts McCarthy’s  achievements and creates a caricature of of his theories. It displays total ignorance of science when it distorts his opinions before it triumphantly knocks them down. Such a level of argument would disgrace kids in a schoolyard. When I showed it to Cathi she observed that those who shout loudest have usually lost the argument.
One of this man’s scathing ‘put downs’ revolved around the fact that pigs and monkeys are distant on the mammalian family tree. I was delighted to recently read that new genetic studies have placed the two lines very much closer together.
A female academic was more subtle in her insults. She writes that McCarthy was somehow given his PhD and that ‘he writes about birds’. Some understatement about  a world authority on bird hybridisation.

Diverse animals can be friends 

Public distaste

Sex rears its ugly head

Paleaolithic cave art. This picture has an enormous similarity to a roman depiction I have seen from Pompeii 

It’s easy to accept hybridity in plants where innocent albeit potentially promiscuous pollen is carried to the stigma by wind or animal transfer. Not so animals that indulge in copulation. Sex across species lines and animal behaviour that might involve incest for many is repugnant.

Bisquit’s father is his grandfather! 

Worse when one wishes to study reported cases of historic hybridisation there is a long history of records that include both the genuine and the manipulations of charlatans. We have a history of Victorian circus and a morbid interest in monsters.

Belief that hybridity is an evolutionary dead end and that hybrids are sterile

Hybrid crosses like the 'cockapoo' might become popular. I presume they are repeatedly created each time by the breeder producing a litter by hybridising a spaniel with a poodle. 
If cockapoos were to self fertilise over several generations and a breeder were to select in each new generation, they would eventually give a fertile pure breeding line.

When a horse and a donkey breed together a mule is created. It is a frequent occurrence. Much is made of the ‘fact’  that all mules are sterile and this is often quoted as evidence of the sterility of all hybrids. 
What might be true with mules is patently untrue as any gardener will testify in the cases of tens of thousands of hybrid plants. The same with animals, especially where crosses are not genetically very distant and in particular where different species and genera share the same chromosome number. 
More distant crosses are much less fertile and sometimes are not fertile at all. I have written in my previous posts about plant hybrids such as Primula kewensis originally sexually infertile which after many generations of vegetative propagation produced fertile seed. Many ‘more primitive’ animals also reproduce vegetatively and share this opportunity. (En passant I mention that this morning’s New Scientist carries an article on so called ‘virgin birth’ over multiple generations in sharks).

Some successful crosses might be extraordinarily rare. In terms of evolutionary time  ‘rare’ might be reinterpreted as ‘certain’.

If this palaeolithic cave art is thirty thousand years old and if we take a billion years of evolutionary time and scale it down to a 24 hour clock, then this painting was done at about three seconds to midnight. 

Take mules as an example of a rare case of an infertile hybrid producing offspring.  A geneticist estimated the chances of a mule procreating as one in five million. A dubious figure up or down! The figure of five million mules born must in earth’s history have been exceeded several times over and indeed as the mathematics would suggest there are reported examples of a very small number successfully breeding.

Mules are very fine animals and display hybrid vigour

Another objection often quoted in the case of hybrid plants is that although they may be produced by a successful cross and also be fertile, further seed production gives highly variable offspring. This is absolutely true! 
Such variation provides new opportunities for nature. And the plant breeder too. This is the stuff of evolution where selection from new innovations wends life on its way.
I suppose all those posh pure breeding dogs started as mongrels.

Teaching evolution
When the scientific community does not agree whether hybridity is a small blip in evolution's alleged beautiful straight line or is a prime driver by the process of sharing and the fabrication of new genomes it is not surprising that the public presentation of evolution is shallow.
Pity the poor teacher. For most subjects these days syllabuses are restrictive. How much more in a world where many do not accept evolution. Worse where their is ignorant resistance to teaching evolution at all.
It must be a delight to teach evolution to a group of eager young scientists. It must be hell on earth in a school where pupils have come to the class armed with preconceived  propaganda.
There is little scope for the teacher to speculate that the story of the descent of the horse although correct in outline is in detail seriously flawed. Nor to discuss the notion that many species appear in the fossil record with a relative suddenness and remain very little changed over millions of years.
It will be years before hybridity receives the attention it deserves.


Bisquit’s mother was unable to suckle and rejected him and he was raised by Cathi. Brenda was stepmother to feed him in the daytime. Now returned to his flock, whenever Cathi approaches he is first to come running

It’s amazing how variable a lamb’s diet can be when not in a field

This is the first of a new series about hybridisation. Below is a resumé of my previous posts

Hybridity 5. Traces the discovery of the Wollemi pine and relates the genetic mystery of why all wollemis whether raised from seed or vegetatively propagated are genetically identical. Also references to horizontal gene transfer and stabilization of new genomes

Hybridity 4. Metasequoia is not only a fossil tree that has maintained its genetic integrity over millions of years it is also thought to be the hybrid parent of the giant Sequoia sempervirens. Growing taxodium and metasequoia. Russian genetics and hybridisation, the evil Lysenko and the sad story of Vavilov. Taxodium fossils. References to ancient plants that hybridise freely

Hybridity 3.  Gene transfer across hybrid zones. The apparent ‘saltation’ of new organisms in the fossil record. The amazing ginkgo. Speculation that cycads might be an ancestor of Ginkgo. Tentative talk of hybridity in human ancestry

Hybridity 2. Numerous cases of garden hybrids including Leyland Cypress (hybrid vigour anyone?) The question is asked why with so many known and verified hybrids is it always assumed that any new species has developed by Darwinian ‘straight line’ selective mechanisms. The curious duck billed platypus that has bird and mammal characteristics, both in its physical structure and its genetic mechanisms. Similarity of a chicken to a dinosaur. Sarcastic comment about statistical binning

 Today’s New Scientist reports that the domestic chicken has been known to give parthenogenic birth

Hybridity 1. Numerous cases of garden hybrids. My own enlightenment to hybridity Examples of rapid ‘natural selection’ over very few generations. Gene transfer between microbes. Dogs, coyotes and wolves. Deficiencies of plant classification.

addendum January 2016
More negative publicity for the idea of hybridisation
Today’s morning newspapers, in their latter page headlines, splatter the notion that we inherited our ‘bad genes’ from the neanderthals. My paper states from ‘limited liaisons’ (we must not scare the children) that nicotine addiction and depression come from Neanderthal genes. Such information comes from research in Nashville, Tennessee where I understand they talk about nothing else other than evolution.

My hackles rose at the suggestions of human superiority and the implication that other neanderthal genes will not have made positive contributions in our genesis. The way that the press report the research we are left with the notion that hybridisation although generally insignificant is also a bad thing.

8 comments:

  1. Our family have had poodles since my teens. I worry that the modern trend to hybridise from them will lead to the demise of the pet poodle as they will all be kept for breeding purposes. What sort of life will thr breeding machines have then?

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    1. I don't know enough about the politics of the doggie world. I might imagine that if a popular form emerges from a cross it would become stabilised in a pure breeding line but then we would have the problems of inbreeding, not to mention any stupid fashion characteristics that might be selected for. Give me a mongrel any day, although I do like....
      I don't really see why such a lovely dog as the poodle should not have a long continuing future

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    2. It's strange how many people have a totally wrong impression of poodles. Ours have always had barks that fool people into thinking they are supersized - no yapping for them. Also because they only have a 'lamb' cut rather than pom-poms etc they don;t recognised them as poodles. To be honest they are also too intelligent to let us get away with anything!
      When we bought Tivvy tracking down a miniature poodle breeder not miles and miles away was tricky.

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  2. Roger, your posts are always so thought-provoking! I am going to have to consider the origins of the duck-billed Platypus a lot more carefully now...

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    1. The genetics of the duck billed platypus are beyond me Mark.
      I think it would be a career destroying move to be 'daft enough' to apply modern genetic knowledge to the problem and you would never get a grant

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  3. An other excellent post - keep at it Roger.
    Fortunately, whatever our opinions of hybrids, it has very little effect on hybridization, that just goes on.

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    1. Very profound Alain. I have been pondering your words of wisdom for a couple of days now….

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  4. Very thought-provoking post. Very well thought out too. Alain is right. Does not matter what the buying public thinks if that is what we get in the end.

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