Few may pause on February 12 to note the 200 year anniversary of the birth of Charles Darwin and 150 years since the publication of “On the Origin of Species” (click here to download). To some extent, this may be expected since much of the controversy (creator vs. autonomous biochemical processes) seems to have abated – NOT. Politically, the issues are still red hot – in Kansas – and elsewhere across the globe.
But what about the science ? Do we accept the basic tenets of evolution by way of genetic variation and natural selection ? For goodness sake, I mean, we’ve just about (or soon will have) sequenced every living organsim-on-the-planet’s genome. Surely there is no doubt about the validity of the so-called neo-Darwinian synthesis of basic/population genetics and the theory of evolution by natural selection. Is there ? Perhaps you can’t blame folks for trying to poke holes (as covered extensively by Sandwalk), especially on the big 200th anniversary.
One place where I am hearing some buzz on the teetering of neo-Darwinism and the Modern Synthesis lately is in the area of epigenetics. Consider the paper by Arai et al., entitled, “Transgenerational rescue of a genetic defect in long-term potentiation and memory formation by juvenile enrichment” [doi: 10.1523/jneurosci.5057-08.2009]. In this paper, the researchers measured a trait known as long-term potentiation (LTP), wherein a synapse fires in a longer and stronger fashion. This type of potentiation is thought to be a basic mechanism that neural networks use in learning and memory formation. In their paper, the team found that certain synapses in the hippocampus were potentiated when animals were exposed to an “enriched” environment (normally mice are caged in empty bins lined with woodchips, but an enriched environment is one filled with tunnels, hidden passages, toys, ropes to climb & other stuff to discover and learn about). The team shows that, in response to an enriched environment, the mice acquire the LTP trait.
The next thing the team found was that the offspring of female (but not male) mice that had acquired the LTP trait – did also show the LTP trait – even when they, themselves, did not experience the enriched environment. Thus, the so-called acquired trait (LTP) was inherited by the offspring. Hmmmm – sounds a bit Lamarckian to me, or, as the authors of this research article suggest, “Lamarckian-like”. Is this a case that violates core tenets of the modern synthesis ? Does it besmirch Darwin on his 200th birthday ?
No. Here’s why in a nutshell. The LTP trait is not passaged via the female germ line. That is, the physiological and genetic (gene expression) changes that lead to LTP in the mothers are not encoded in the genome of her eggs. Indeed, her haploid egg cells were set aside long, long before she ever experienced the enriched environment and acquired the LTP trait. Rather the effect is one that seems to be dependent on her uterine environment and ability to transfer information from unterine milieu to developing offspring – whose developing brains seem to be endowed with the molecular components needed to facilitate LTP. Figure 4c of the paper shows that the LTP trait was lost in the F2 generation – therefore the effect is not stably transmitted via the germline (as plain vanilla DNA mutations are).
For an overview of the complexities of incorporating the Central Dogma of Molecular Biology into the Theory of Evolution by Natural Selection, read chapter 4 (p76) Weismann, Lamarck and The Central Dogma of John Maynard Smith‘s book “The Theory of Evolution“. Maynard Smith credits August Weismann’s germ plasm theory as a key factor in the modern synthesis since – by sequestering the germ line very early in development – acquired characteristics cannot be inherited via egg & sperm. Hence, Lamarckian evolution is (in principle) not possible. This seems to be the case here with the LTP trait. In this spirit, the authors do a great job of reviewing other similar examples of how a mother’s uterine environment can lead to epigenetic modifications (click here for review article and here for a PLoS paper on the topic) – such as the viable yellow locus in the mouse [PMID: 18673496] and the effects of endocrine disruptors on methylation of germ cells [PMID: 16973726].
Well, it is amazing indeed how Darwin’s work continues to inspire us. Happy 200th Birthday !