2009 is the 200^th anniversary of the birth of Charles Darwin. So how are his ideas holding up?

Two hundred years is a long time for any scientific idea to survive in one piece. It is the business of science to test ideas to destruction.

Nobel prizes are won by iconoclasts. Reputations- and funding- can be secured by those who offer new ways of seeing the world. Darwin's ideas have been subjected to every test that raw ambition and the desire for truth can design.

Out of this onslaught, doubts have arisen- on the role of natural selection, for example- but it is only recently that a key Darwinian idea has been in danger of collapse. Molecular scientists and the increasing power of gene sequencing techniques are to blame. 'The Tree of Life' is in danger of being uprooted.

The idea of the Tree of Life was fundamental to Darwin’s thinking and he regularly referred to it in 'The Origin of the Species'.

It was also one of the key ideas which allowed him to win wide acceptance of his ideas of evolution because it appealed to a common sense view of the living world. It seemed obvious to Victorians that there were 'families' of plants and animals. Zebras, after all, were similar to horses, less similar to bears and very different to snakes.

A classification system for animals from Karl Linneas in 1753 had gained wide acceptance among natural historians. Darwin's ideas provided a simple and compelling account of how differences and similarities in the natural world could arise and how families of animals were interrelated.

Karl Linnaeus

Linear Evolution

A central idea in the Tree of Life theory was that evolution occurred in a linear way. One kind of animal gives rise to another kind of animal under the pressure of natural selection. Every animal had only one immediate ancestor but since any particular animal or group of animals could give rise to more than one type of new animal, the tree of life had many branches.

For two hundred years evolutionary biologists have studied the bones and organs of related species to understand the similarities and differences that evolution has wrought. The evidence produced by anatomists is one the main strands of evidence in support of Darwin’s ideas. It seemed possible to follow linear evolution in any branch of the evolutionary Tree of Life by studying anatomical relatedness.

Molecular biologists expected to find similar evidence when they studied plants and animals at the molecular level. If the 'Tree of Life' idea was correct, species shown to be closely related by anatomical studies should also be closely related at the cellular and molecular level.

At first the evidence was all positive. The molecules studied in the 1970’s were RNA- chemicals similar to DNA but smaller and easier to analyze with the techniques and equipment of the day.

These early studies of RNA seemed to support the Tree of Life theory. The more closely species were related on anatomical grounds, the more similar their RNA was.

Then, as new, more powerful techniques of molecular analysis became available and the studies shifted to DNA itself, strange anomalies were found.

In the most primitive life forms such as bacteria it was impossible to demonstrate any kind of linear evolution. The genetic codes of related and unrelated species showed routine mixing together of genetic material- either as a result of virus activity or cross species breeding.

In a major study of primitive organisms, 80% of genes showed evidence of being transferred from other species rather arising through linear evolution.

Instead of confirming a 'Tree of Life', molecular studies suggested a complex and confusing 'Web of Life"- at the bacterial level at least.

For a long while, evolutionary biologists resisted the importance of these findings. They stuck to the idea that the tree of life was the important thing and the cross contamination of genetic material caused by viruses was trivial, just noise in a bigger picture. Besides, they argued it could only apply to the unimportant microbes not to the animals and plants we are most familiar with.

Now that view is under serious threat. Recently, as more animal species are having their complete genomes sequenced it is obvious that cross species genetic exchanges are common and have had a big impact on evolution.

Not only do viruses transmit pieces of genetic code from one species to another, it seems that hybridisation- the interbreeding of unrelated species has a long and important history.

Theories of non-linear evolution

Is the transfer of Genetic Material Significant for Higher Animals?

Some studies suggest that something like 40% of the genetic material in a human being is the result of viral genetic transfer. If this is verified, it will make the 'Tree of Life' look completely redundant.

It would, however, help to explain sudden and great jumps in evolution that have puzzled scientists for a long while. A new piece of the right DNA entering the right species at the right time- by pure accident- could propel change in a way that the minute changes propelled by random mutation cannot.

Gene transfer in humans

Hybrids are created when different species interbreed. A mule is a cross between a horse and a donkey, a liger is a cross between a lion and a tiger.

On land, cross-species breeding isn’t so easy. Size differences, habitat difference and behavioural differences all introduce obstacles. A lion would rather eat a gazelle than breed with it. An amorous robin has little chance with a squirrel. In the oceans however, most species release eggs and sperm in huge numbers and they float together in a great reproductive soup.

Michael Syvanen of the University of California recently compared 2000 genes that are common to humans, frogs, sea squirts, sea urchins, fruit flies and nematodes. He was trying to use genetic data to construct an evolutionary tree showing the relationships between the six animals. He failed- in a spectacular way- with sea squirts. Half the genes of a sea squirt come from a chordate (an animal with a primitive backbone) and half come from sea urchins. This is quite a mash up and difficult to explain unless a hybrid of strange parentage came into being many millions of years ago.

Human Hybrids

There is evidence that early modern humans hybridised with our extinct relatives, such as Homo erectus and the Neanderthals. Did drunken nights have unforeseen consequences?

http://dx.doi.org/10.1098/rstb.2008.0021

http://blogs.discovermagazine.com/80beats/2008/08/08/neanderthal-dna-shows-they-rarely-interbred-with-us-very-different-humans/

These new ideas have even made some scientists say that Darwin’s idea of linear evolution and the 'Tree of Life' have had their day.

Just as Newton’s simple, elegant ideas of the physical universe were superseded by Einstein’s complex but real world theories, so Darwin’s ideas of linear evolution may be superseded by new theories of evolution which will reflect the true complexity of life.