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we gardeners personally experience the results of hybridisation on an everyday basis, conventional and might I say outdated ‘popular’ macro-genetic theory dismisses it as an insignificant aberration.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.
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