We are hybrids

We are just good friends

When I told Brenda that I was yet again writing about hybridisation she wondered if I was wasting my time and that I was knocking at an open door. Surely it is accepted that hybridity is a significant source and distributer of genetic variation and a driver of evolution?

I was talking to a well informed botanist recently. He said that some botanists thought perhaps a third of all natural plant species were known hybrids. That is amazing. To demonstrate past hybridisation is extremely difficult and as you go back over millions of years genetic evidence becomes vanishingly small. Not to mention geneticists have more rewarding things to do. Perhaps all plants are derived from ancient hybrids? We just don’t have the means to know.

The world’s tallest tree the coast redwood was an ancient hybrid between metasequoia and a sequioadendron

Modest examples of the parent species

A  natural hybrid between two primula species happened at Kew more than a century ago. Initially sterile after several years a flower was able to self pollinate and set seed. It is a common plant now. 

Distinct new plant hybrids have a much easier time than animals to stabilise a new genome. Even if they are infertile, vegetative propagation will often permit their spread and survival; plenty of time and potentially huge numbers  of candidates for fertility to arise.

One successful hybridisation event can produce numerous seeds and plenty of brothers and sisters. Furthermore unlike animals, plants often self pollinate and a new hybrid requires no mate.

Most gardeners know the huge contribution over more than a thousand years of plant breeders crossing distinct species of rose to eventually create those grown today. Less  well known is that Rosa canina is a very complex ancient natural hybrid. Its pollen and female germ cells contribute genetic code unevenly in seed formation and it does not subscribe to Mendelian rules 

Rosa canina is one of the wild ancestors of modern roses. Here it grows wild on the village plot

Animals have it different - but hybridise too

On the BBC ‘earth’ website  Rebecca Ackermann of Cape Town University states that 7-10% of primates have been known to hybridise in nature. That means they have mated and brought forth viable (and often fertile) offspring. Considering their geographical distribution and how few primates overlap this is very revealing  how easy hybridisation can be.

You might well ask what happens to such rare animal hybrids. Their line only continues if they mate with one of their parent species and after a few generations their identity is lost. Never-the-less novel genetic information has moved across so called species barriers. It’s called introgression.

What is really interesting is when the results of any new hybrid crosses becomes geographically isolated and inbreed within a small community leading to new species.

The Appalachian butterfly is a known hybrid and its area of distribution is bordered by the habitat of its original parents

The  power of hybridisation can come from the fact that new genomes are created and genetic code comes together in exciting new combinations.

More mundane  - but perhaps of greater significance  - is the introgression of code of advantageous small mutations - the very stuff of natural selection. There are seven billion of we humans. If a new selective advantageous variation arises on a single occasion (such as the ability to digest lactose or tolerate alcohol)) it has the potential over countless generations to be shared within the complete human race by sexual reproduction. If a new and different mutation occurs elsewhere in the population it is likewise brought into the fold.

This to my mind is a long way from evolution being in straight line when the changes would have taken place in the line of a single organism and the chances of pieces of beneficial new code being shared is billions of times less. (Another way to look at this is that although our ancestry is a straight line all the way back to the first dawn of life this does not hold for the genes)

Sexual reproduction within a species is the way that nature brings together new information - and we even talk about hybridisation between different communities of any said species.

Sexual reproduction across so called species barriers is essentially the same except the potential for novel combination from a much wider gene pool is hugely enhanced.

Evolution has avoided a complete cross species breeding ‘free for all’ by fostering barriers between dissimilar groups.The more genetically similar two species the more likely these evolved prohibitions to successful cross breeding do not apply. The other side of the coin is that crosses between distant cousins are very rare. Never-the-less when distant crosses happen as they surely do, their significance in evolutionary terms is so much the greater.

Species barriers

Enlightened biologists these days question the whole concept of family, genus and species. We retain the facade because it is so useful although cross-species barriers between distinct organisms often resembles a colander.

Wikipedia lists sixty distinct species of epimedium, many newly discovered in China. Word from the plant breeders is that they all seem to be mutually fertile

The so called genetic tree of life is still a useful indicator of of life’s direction. It is not however a straight line and better than the analogy of a tree it would be better to depict it as an infinitely stranded vine constantly dividing and re-binding together.

I still resent black arts such as statistical binning which clean up evidence of horizontal gene transfer every time the tree of life is amended.

Stability of species

Despite genetic code being able to cross species barriers, life’s ability to maintain distinct species is truly amazing and insects found in amber look the same as those today after many million years. Hybridisation can be rare and crosses are ever diluted by back breeding.

Little changed over two million years

The fossil record shows new plant and animal species seemingly appearing  from nowhere and maintaining themselves very little changed over millions of years. Significantly almost identical fossils continue from first appearance either up to extinction or up to the same organism still living today. I once read an imaginative simulation of the nearly two million years that Homo erectus existed. I could not get over the fact that in all that time change was barely perceptible from fossil evidence. 

As I argue today it is only from extraordinary hybridisation events that such stasis is broken and new species arise, including our own. 

Human evolution

The fossil record of 'recent' events

The above text adds to my previous posts that argue for hybridity’s general significance in evolution. Today I now concentrate on the contribution of hybridisation to the human line. I draw on a recent article on the BBC ‘life’ website which adds veracity to my words as an amateur observer. Even more commanding are the links on that website to state of the art genetic investigation.

Neanderthal skull

We shared the planet with neanderthals as recently as 40,000 years ago. ‘Modern neanderthal’ fossils are encountered from back 300,000 years and their emerging line much longer. Blinks of time in evolutionary terms. Even our earliest purported homo ancestors barely clock up ten million years.

There is real genetic evidence that we repeatedly crossed with neanderthals. We contain in our cells up to 4% neanderthal genes. Not only neanderthals but other ‘ancient ‘races such as denisnovans. (Not so ancient as the timescale of our canvas. I love the notion that they were named after Denis - a hermit who had previously lived in the discovery cave!) Some Asian populations have 6% denisnovan genes in their genome

Although none of us go beyond 4% neanderthal they are not all the same genes! It would seem that if all surviving neanderthal genes were brought together we would be 20% neanderthal.

There was a fascinating find in Romania of a 40,000 year old human fossil whose skull showed clear evidence of neanderthal introgression in the jawbone. Known as the Oase individual he had perhaps 9% neanderthal genes and it would seem he was only between four and six generations beyond the actual human x neanderthal hybridisation event. Apparently none of those neanderthal genes are thought to be have passed on into our identity today. 

Going back a very long way

Homo naledi

1500 remarkable skeletons were found in the Rising Star Cave in South Africa 2013 and have been examined worldwide by hundreds of specialists. In 2015 it was named as a new homo species. Not yet accurately dated the skeletons are perhaps a million years old. It is an extreme mixture of archaic and modern human features. Variously described as mosaics or curious mixtures of ancient Austropithicene and modern homo, the word hybridisation seems to be studiously avoided.

It would seem that obscure homo species occurred all over Africa and fossils are now being found all over Asia too. For example you might have read of the dwarf human species Homo floriensis now confirmed not to be a nutritional aberration!

They often describe them as relic populations or evolutionary dead ends. This is true but I cannot  believe they left no contribution.

Fortunately more and more scientists are recognising hybridisation as a very significant part of our own evolutionary history. Many don’t choose to shout it too loudly even when they agree.

I am only too keen to tell the world I evolved from dinosaurs

Final thoughts

If the fate of a new fertile hybrid is to be subsumed into the parent population or survive as a new entity (probably after a few generations of breeding within one of the parent species) then in the latter case there is no time for natural selection into a new niche in the classical way. The new hybrid either lives or dies in finding a new suitable environment. Hybrids are frequently commoner in hostile new challenging conditions.

Humans have evolved with outgoing generalist characteristics that have enabled us to rule the world. We have gone out into the world and exploited myriads of new niches. We are the perfect example of the power of hybridisation.

In old thinking hybrids were aberrant genetic challenges to the natural order. In truth they have always been a principal route to the future

Epilogue

I had just completed this post when Cathi celebrated my three quarter century birthday by giving me the weighty updated version of Richard Dawkins’ wonderful book ‘The Ancestors Tale’. Dawkins is my longtime hero but I have not recently read him. Although always at the forefront of genetic research and the most brilliant exponent of its translation to layman like me I feared he might not embrace modern thinking about hybridisation.

I need not have worried. There would seem to be a subtle sea change in the genetic establishment’s attitude to hybridity. It’s suddenly respectable and although in my opinion the enormity of it’s significance is not yet fully reflected it moves nearer to the insights of that  brave pioneer geneticist Gene McCarthy.

Links

To really learn about evolution this book is required reading

The BBC 'earth' article about human evolution

Eugene McCarthy's website