Neutral evolution
Charles Darwin was not yet 30 years old, when he got the basic idea of the theory of evolution.
He knew that it will not be easily accepted.
He spent 20 years methodically compiling evidence for his theory.
One of the strong objections he had to overcome, was that gradual evolutionary process,
could not produce complex structures.
Consider the human eye.
It has many parts, a retina, a lens, muscles, jelly, and so on.
All these parts must interact for vision to occur.
In fact all the parts need to have certain specific properties.
If Darwin was right, then the complex eye developed from simple precursors.
Even Darwin found this hard to believe.
He found a way that complexity could have evolved.
In each generation individuals varied in their trades.
Some variation increased their survival rate, and allowed them to have more offspring.
Over generations these advantageous variations would become more common, or 'selected'.
A new variations emerged and spread, they would gradually produce more complex structures.
Conventional wisdom holds that complex structures evolve from simpler ones, step by step,
through a gradual evolutionary process.
Recently scientist have proposed that complexity can arise from other means.
For example, it can be a side effect even without natural selection to promote it.
They suggest that random mutations can fuel the emergence of complexity,
in a process known as constructive neutral evolution.
The human eye could have evolved from a simple light catching tissue,
that animals such as flatworms have.
Step by step, natural selection would drive this process to increased complexity.
Today biologists can probe the eye, and other organs in detail, at the molecular level.
They find immensely complex proteins joining together to make intricate structures.
Such structures can evolve from simpler ones, favoured by natural selection.
Some scientist have suggested that complexity could arise from other routes.
Some say that life has a built in tendency to become more complex, overtime.
Some say that random mutations arise, and complexity emerges as a side effect,
even without natural selection to help it along.
Scientists suggest that we not only look at the sheer number of parts, making up living things,
but all the types of parts.
Our bodies are made up of ten trillion cells.
If they all were the same type, we would be a featureless heap of protoplasm.
We have different type of cells, like muscle cells, red blood cells, skin cells and so on.
Even a single organ can have multiple type of cells.
The retina for example has 60 different kinds of neurons.
Each has a distinct task.
By this measure, we can say that humans are more complex,
than an animal called a sponge, which has only 6 cell types.
Whenever organisms reproduce one or more of their genes may mutate.
Sometimes these mutations give rise to more types of parts.
Once an organism has more parts, these units have the opportunity to become different.
The organism's complexity increases.
As complexity arises it may help an organism to survive better or have more offspring.
If this happens it will be favoured by natural selection, and spread through the population.
Mammals smell by binding odour molecules, to receptors on nerve endings in the nose.
These receptors genes have repeatedly duplicated over millions of years.
The new copies mutated, allowing mammals to smell a wider range of aromas.
Animals like mice and dogs, have more than thousand of these receptors genes.
Sometime complexity may have a negative impact.
In this case natural selection will ensure that the trait does not spread through the population.
Some scientists propose that complexity increases even in the absence of natural selection.
They call this the zero force evolutionary law.
Scientist wanted to test this theory with fruit flies.
For more than a century scientists have bred these flies, to use in experiments.
They lead a pampered life, as they are provided with a constant supply of food,
and a steady warm climate.
There wild relatives, have to contend with starvation, predators, cold and heat.
Natural selection is strong among wild flies.
It eliminates mutations unable to cope with the many challenges.
In the sheltered environment of the lab natural selection is more feeble.
The zero force evolutionary law makes a clear prediction.
Over the last century the lab flies should have been less subject to the elimination of disadvantages mutations.
They thus should become more complex than the wild ones.
Scientists found that lab flies, were indeed more complex than wild ones.
Some flies had irregular legs, some acquired complicated patterns of colours on their wings.
Some had different shapes of antennae.
Freed from natural selection, the flies revel in complexity.
Some scientist started to think about the fact, that some mutations,
have no effect on the organism at all.
They refer to them as neutral mutations.
They proposed that the mutations could give rise to complex structures,
without going through a series of intermediate selections.
They called this process 'constructive neutral evolution.
They found some examples in the cells of fungi.
In fungi, cells have to move atoms, from one place to another to stay alive.
One of the ways they do so, is with molecular pumps called vacuolar ATPase.
A spinning ring of proteins, shuttle atoms from one side of the membrane to another.
The ring is a complex structure of six protein molecules, with 3 protein types.
In animals the ATPase complexes also have spinning rings of six proteins.
But there is a crucial difference.
They have only two unique protein types.
The fungi is considered more complex, in this context, because it has three protein types.
By comparing the difference in the genes of two proteins, unique to animals and fungi,
scientist reconstructed the ancestral genes from which they both evolved.
Fungi and animals had a common ancestor about a billion years ago.
They used the DNA sequence of the reconstructed gene to create a corresponding protein.
In effect they resurrected a 800 million old protein.
They injected this protein into yeast.
They also shut down the genes, that make proteins for the ATPase rings, in the yeast.
Normally this should be fatal, because the yeast could no longer make the critical rings.
However, they found that the yeast could survive by making use, of the injected ancestral gene.
The yeast was able to still make the rings.
Scientist formulated a hypothesis using these findings, how fungi ATPase became more complex.
Fungi started out with rings made from only two proteins, the same ones found in animals like us.
The proteins were versatile.
They were able to bind to themselves or to their partners.
The genes generated two variants.
The new proteins kept doing what the old ones had done.
They assembled into rings for pumps.
Over millions of generations of fungi, they began to mutate.
Eventually the ring could form successfully only if all the three proteins were present,
and only if they arrange themselves in one pattern.
This experiment precisely pointed to the kind of evolutionary episode,
predicted by the zero force evolutionary law.
Over time life produced more ringed proteins or parts.
The parts began to diverge from one and another.
The fungi ended up with a more complex structure, then their ancestors had.
But it did not happen by natural selection.
The fungal ring degenerated its way into complexity.
Scientists have found another example of neutral evolution.
This happens in the way many species edit their genes.
When cells need to make the given protein, they transcribe the gene into RNA.
RNA is the single stranded counter part of DNA.
It then uses special enzymes to replace some RNA building blocks, called nucleotides.
RNA editing is essential to many species, including us.
The unedited RNA produce proteins that do not work.
There is something unusual about this.
Why don't we have genes with the correct original sequence, making RNA editing unnecessary?
Scientist proposed a theory to explain this.
An enzyme mutates, so that it can latch on to RNA, and change certain nucleotides.
The enzyme does not harm the cell or help it.
Doing no harm, it persists.
Later a harmful mutation occurs in a gene.
Fortunately the cell has the RNA binding enzyme, which can compensate for this mutation,
by editing the RNA.
It shields the cell from the harm of the mutation.
This allows the mutation to pass from one generation to the next, and spread throughout the population.
This process was not driven by natural selection.
The extra layer of complexity evolved on its own neutrally.
Once it becomes widespread, there is no way to get rid of it.
Scientist cautioned that though neutral evolution could exist,
it does not negate in the evolutionary process.
They emphasise the important role that natural selection plays.
Some examples are the biochemistry that builds a feather,
and the photosynthetic factories that exist in leaves of trees.
Neutral evolution could be just one more way, in which evolution happens.