A recent study published in PLoS Biology reveals that so-called "identical" (monozygotic) twins are not completely genetically identical. Visible differences in phenotype (such as height, freckles, etc.) have hinted to differences, so have difference in disease susceptibility, but researchers couldn't eliminate purely environmental factors as the cause. This recent work definitively shows that there are genetic differences, specifically with the number of copies of genes. Because the twins start from the same cell, they must have started genetically identical. So something in the environment during development must explain the deviation in the genes. A person's genes change over time. That's supposed to be a radical idea, but perhaps because I am not deeply steeped in the dogma of this research field for me this is pretty obvious. It's quite easy to hypothesize several plausible mechanisms through which this happens and that's what I'll do here.

The genetic code programs for, among many other things, the machinery that copies the genome (polymerase enzymes, etc) and the machinery that turns genes into RNA and into individual proteins and enzymes (the ribosomes, etc.). But environmental factors such as the presence of nutrients and sufficient energy will affect the exact structure and number of this genetic machinery. So, for example, if a pregnant mother's diet is high in starch then we would expect that starch related nutrients would be more plentiful. It's also easy to imagine that the genetic copying machinery is structured in such a way that the more starch-related compounds there are around the more copies of the gene that encodes for the enzymes to break down starch that it would insert while copying the genome.

There is a clear Darwinian evolutionary advantage for having one's enzyme production capacity be contingent in this way. Furthermore, as a corollary of this hypothesis we would expect different parts of the body to be affected differently. This means that the genetic material will be (slightly) different for different parts of the body. Is this true? As it turns out, yes. There are variations in genetic material that can occur in, for example, 75% of a person's blood cells. Blood cells are made is many different parts of a person's body and so the genes in the marrow in one's arms is different than the genes in the marrow in one's legs. This is rather easy to explain if the change in the genome happened during the fetal development process because a change in one cell early on could propagate to all the cells that split from it.

With blood cells it's even easy to imagine that life events (changes in nutrients) would affect the bone marrow's stem cell's machinery so that it created slightly different blood cells over time. I'm not talking about mistakes. I mean feedback mechanisms so that if there is a deficiency or excess in some chemical structure the cellular machinery works differently and these differences result in the insertion or deletion of the appropriate genes that encode for the processing of those chemical structures. I would expect that within one's intestines,, for example, there is significant variation in the genetic details of the cells and thus in what they metabolize and that such variation is key to proper functioning.

How does this result in Lamarckian evolution? That's just the name attached to the idea that environmental effects that alter one's phenotype can be passed to offspring. Usually this is something like giraffes stretching their necks through life so children are born with longer necks. Clearly that doesn't happen. For anything like that to happen a person's zygotes would have to be genetically altered through life and I'm not claiming that such a thing is happening. So even though population genetics isn't Lamarckian the genetic competitions within the microcosm of your own body are. Cells that reprogram themselves to make better use of local resources will be more efficient at replicating and your body will have more of them. Also, the offspring will have the code for the machinery that the parent cell had at the splitting time.

Diversity is key in all complex systems and so we'd expect that a person's (or twins') genes, though all borne from a single strand, will reveal nuanced and important variations from that point forward.