Sunday, 29 December 2013

Understanding Evolution

Jean-Baptiste Lamarck
In this blog I'm going to look at what were once competing theories for explaining the fact of observable evolution, Lamarckian and Darwinian evolution as proposed by Jean-Baptiste Lamarck and Charles Darwin with Alfred Russel Wallace respectively. Although these are often presented as two opposite and irreconcilable theories, with the former being utterly defeated by the latter, as with so much in science, things are not always as they seem. Jean-Baptiste Lamarck, if not being entirely rehabilitated, is at least being shown to be not entirely wrong.

You only need spend a few minutes reading creationist materials to realise that these two theories are often, and often deliberately, confused.

By the late 18th-century, especially following the work of Carl Linnaeus it was becoming obvious to serious biologists that living organisms could be arranged in a hierarchy of families, families into orders, orders into classes, classes into kingdoms, etc. We were also beginning to realise that Earth had not been created suddenly exactly as we saw it but that it was the result of change over time. One event which had given impetus to this growing philosophical movement had been the Portuguese earthquake which had destroyed Lisbon in 1755 (for more on this see An Earthquake in Theology).

The question was what process, operating over time, had given rise to the observed diversity of life on Earth?

Charles Darwin, 1874
Jean-Baptiste Lamarck's hypothesis was that by reacting to challenges in their environment organisms acquired characteristics which were then inherited by their offspring. Giraffes needed to stretch their necks to reach the leaves in tall trees so their necks got slightly longer. This was inherited by their offspring and, repeated over time, gave rise to giraffes with long necks.

One of the pieces of 'evidence' Lamarck presented was that sons tended to follow their fathers in trades such as blacksmiths. He argued that fathers developed the muscles and coordination needed to beat hot metal on an anvil. These were then inherited by their sons who naturally became blacksmiths.

This might seem laughably naive now but, in the absence of any knowledge of DNA or genetics, and a primitive view of embryology, it seemed superficially to be a perfectly good hypothesis. What did they know about sociology? The tendency of sons to follow their fathers into trades was not because of biological inheritance but because it was accepted that children born into certain social classes would remain in those classes at least partly because it was assumed that the social order was a God-given thing. It was natural that boys would be trained by their fathers and take up the trade they followed because the opportunity was there and there was little opportunity to do anything else. Children adopted by blacksmith families were more likely to be blacksmiths than to follow their natural father's trade, as were children produced by a blacksmith's wife's surreptitious dalliances with the local baker.

What was being inherited was essentially an expression of culture. Hold that thought!

Another problem for Lamarckian evolution was that, even if a giraffe's neck could be stretched by reaching for the acacia tree leaves, and even if this stretched neck could be inherited by the next generation, what equivalent mechanism was there for the acacia tree getting taller? Did the tree strive to lift its branches out of reach?

Darwin's and Wallace's hypothesis was that the environment naturally favoured those with small variations which made them better able to survive and reproduce in that environment. In this way an organism tends towards fitness for survival in its environment. All that is needed is inherited characteristics, variation in those characteristics and a selective environment and evolution over time will be inevitable.

Both Lamarckian and Darwinian evolution initially suffered from the same problem - there was no known mechanism for passing these characteristics on to the next generation. Both depended on a hypothetical entity which passed from parent to offspring and so carried the changed information into the next generation. Gregor Mendel then showed that certain characteristics seem to be inherited according to rules which strongly imply a discrete entity or entities which are inherited in discrete ratios.

We now know these are genes arranged on chromosomes and composed of DNA and carried in pairs in all the sexually reproducing species. We now know how these contain information, how this is translated and how it is inherited and so the triumph of Darwinian evolution over Lamarckian inheritance seemed complete.

But. We have recently discovered the epigenome.

The epigenome is an integral part of the genome and controls which parts of the DNA are active and which aren't; which genes are switched on and which are switched off. We have long known that all normal cells in an individual have the same DNA derived from the DNA which went into the first cell at fertilization. Yet different cells carry out different functions depending on the organ in which they find themselves. Muscle cells are different to liver cells which are different to neurones and so on. This is achieved by different genes being deactivated as necessary. One way in which genes can be deactivated is by a methyl group being attached to the cytosine molecule in DNA. Methylated cytosine continues to behave much like regular cytosine in that it pairs with guanine but areas of DNA which are heavily methylated tend not to be transcribed so the gene in that piece of DNA is switched off. This mechanism is still not fully understood but it is known that methylated DNA can be inherited through the germline by the next generation.

So here we have a mechanism for characteristics acquired after birth being inherited by the next generation. In other words, epigenetics can be Lamarckian in its inheritance.

But there is another way in which Lamarck was undoubtedly, though unwittingly, and ironically, right.

Ironically, because with his blacksmith example he was actually describing cultural inheritance which we now know, at least in sentient species like Man, is similar to genetic inheritance but the units of inheritance are not chemical genes in the form of DNA but ideas in the form of memes.

It seems highly likely that we are born with no memes at all and acquire all of them after birth by classical Lamarckian inheritance.

And, since our memetic inheritance is Lamarckian we can change it under conscious control. We can choose which to pass on and we can choose which to accept. In short, unlike our genes where, with the possible exception of genetic engineering, we have no choice in what we got from our parents and what we give to our children and are slaves to our 'selfish' genes, we are the masters of our cultural inheritance and are neither bound by it nor compelled to pass it on.

If there is something wrong in our cultures we can change it, as we are doing as we increasingly reject religious bigotry and superstition in favour of humanism. We are not destined to follow the dictates of earlier, less enlightened generations.

We are free to look at religions and say, if that's what your religion tells you then your religion is wrong.

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1 comment :

  1. Hello, I was just curious if you had heard about the debate between Bill Nye the science guy and Ken Ham, the founder of the Creation Museum in Kentucky, about evolution. It's set to take place on Feb 4th. I don't know if it is going to be broadcast or not, but I hope so. Though it looks like it could pull in quite a lot of cash for the museum, which could be why they are doing this debate as they seem to have refused invitations in the past. Here is a link to an short article on it:

    Anyway, keep up the great posts! I enjoy reading this blog. =)


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