How hybridity is often overlooked in evolutionary theory
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.
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!
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.
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