Lamarckian Evolution

Once upon a time there was a man who liked to mutilate rats.  As you might expect, he was German.  Not that he was some proto-Nazi, mind you.  He was just a good, solid, merciless, German scientist, trying to make a point.

August Weismann was an evolutionary biologist, back in the 19th century, when the actual mechanisms of inheritance and evolution, genes, were unknown.  And if you don’t know how a mechanism works, you can hardly say what it can’t do.

Specifically, Weismann, a good, orthodox Darwinian, didn’t have a good way to debunk the neo-Lamarckians who were cropping up.  Lamarckians claimed that experiences in life could be passed down to offspring.  The favorite example is the idea that generations of giraffes stretching up to reach leaves soon resulted in animals with very long necks.

The operative word there is “soon.”  If the Lamarckians were right, then evolution didn’t have to be a matter of eons but generations.  In effect, each generation would impact the mutations of its descendants.

Now Weismann hated this idea.  But without genetics, how could he prove what dummkopfs the neo-Lamarckians were?

Enter the rats.

Weismann took 68 mice and cut off their tails.  When they had offspring, he cut off their tails.  And when they had offspring…

For five generations.  A total of 901 offspring.  And none of them were born tailless.  In fact, none of them even had shorter tails.

By most standards, this is a pretty ridiculous experiment.  And I will always suspect that was precisely Weismann’s intention.  From then on, whenever the Lamarckians spoke, someone would be sure to ask, “Yeah, but what about those rats?”  And everyone would giggle.

Actually, he had a much better, longer-term experiment he knew about, but in his Victorian age, it just couldn’t be used: had Jewish males lost their foreskins?  Some Lamarckians actually claimed that there were Jewish males born without foreskins, but obtaining empirical evidence was simply unthinkable.

So he was left with his rats.

And through many generations of scientists, through the discovery of genes and the deciphering of DNA, they have cited the Weismann Barrier.  Anything that happens to an individual that doesn’t affect the DNA in the germline cells dies with that individual.  We can only pass along mutations in our DNA to our offspring.

And yet…
In the winter of 1944/1945, the Netherlands experienced a completely manmade famine.  The Allies had taken southern Holland, but stalled.  After the north tried to help the Allies, the Germans imposed an embargo on food imports.  Thousands of people died.  Those who survived showed all the medical effects of starvation.

Life was especially hard on women who were pregnant.  As expected, they gave birth to low-weight babies who were to suffer many chronic conditions throughout their lives.  What was not expected was that their children, too, were smaller than average.  Without any detectable genetic change, the experiences of their grandmothers had affected them.

What the heck is going on here?

Think about this: We all started out as an undifferentiated fertilized egg, a zygote.  We like to think of it as a simple container for a new organism’s DNA.  But it is actually more than that.

How does that single package of DNA, as it divides and re-divides, know how to destine one cell line to become a hand and another to become a liver?  In other words, if all those divisions were simply creating perfect copies of our original DNA, no specialization would be possible.  We would simply be a blob of undifferentiated cells.

Or think about this: If identical twins start out as identical DNA copies, and if they both go through identical processes of differentiation, how come “identical” twins aren’t?  They may look the same to us, and they may come up as perfect matches in a DNA test, but they are different people with different natural abilities and different susceptibilities to diseases and such.

The key lies in the fact that genes can be turned on and off.  That genes have to express themselves to be useful.  The control mechanisms that regulate gene expression can alter the organism without tampering with the basic genome in any way.  And those mechanisms are, themselves, subject to external modifications.

The non-DNA changes occur because the germ cell doesn’t simply get a naked DNA molecule.  Other stuff comes along.

For example:  A naked DNA molecule would be 1.8 meters long and pretty useless.  Instead, DNA is wrapped around protein spools called histones.    And it is the physical manner in which DNA is spun around these protein spools that determines whether or with what intensity specific genes are expressed.  Both maternal and paternal histones get passed to the zygote.

Point here is that maternal and paternal events, like starvation, can affect the molecular tools we pass onto our children.  Just how that happens, whether it is heritable, and if that inheritance can be trans-generational,  lies in the field of epigenetics: The study of changes in gene expression that are not caused by changes in the DNA.

The effects on the Holland children were trans-generational, but not permanent.  Succeeding generations have gotten bigger.  It appears to be a transitory response to a transitory problem.

Evolutionary biologists will tell you that natural selection occurs when there is some major change in the environment, like an Ice Age.  The rest of the time mutations are not favored and fade away after a few generations.  Kind of a neat system to keep organisms stable until real, long-term change is needed.

But what about short term changes in the environment?  How do we respond to them?  Imagine that the Dutch famine had been due to some catastrophic environmental changes that only lasted a few generations.  It would be very useful under those circumstances for people to get smaller for a few generations and then revert to a larger size when things stabilized again.

Could it be that epigenetics is really a natural mechanism we have to survive short term disasters?

How remarkably Lamarckian.

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