Epigenetics seems seems to be the top-40 hit of science media, so I thought I’d give a brief explanation of what it means and the type of changes it makes to DNA. In school you probably compared Lamarck’s and Darwin’s theories of evolution (or Noah’s ark if you lived in Kentucky) using the example of giraffes. Giraffes, with their long necks and legs, are perfectly adapted to areas with tall trees from which they eat leaves. So were they always that way?
The general theme chez Darwin follows: Earlier versions of giraffes, due to random genetic mutations, naturally came in varying heights. The tallest reached more leaves, surviving long enough to knock giraffe boots, thereby transmitting the tall trait to the next generation. Several repetitions of the tall=survival equation give us the long-necked species we know and love today.
Lamarck’s understanding, which has since fallen out of favor, sounds like this: Giraffes stretched their necks to reach higher leaves. Their heights increased throughout their lives of eating from tall trees. When they had calves, the long neck traits were passed on, making each generation taller until they were adapted to a tall-tree environment.
Until recently, the prevailing Darwinian view implied that our genetic lot determines phenotype. We’ve shown experimentally that Darwinian evolution happens all the time through the process of natural selection. But I’ll remind you that we’re not giraffes and humans need not be tall for survival. Therefore, natural selection is not the only determinant of height. Epigenetics is providing a more nuanced version of gene expression that rekindles some concepts of Lamarckian evolution, especially in regard to environmental adaptation.
Nicolas Paul Stéphane Sarközy de Nagy-Bocsa will aptly serve in my metaphor to at least partially reclaim Lamarcke’s good French name. It turns out that adapting to an environment can involve changes during an organism’s lifetime. One example of this is DNA methylation. In that old Watson and Crick model you remember from Mrs. Bormshikker’s class, you recall that DNA is composed of base pairs (C-G, A-T) and phosphodiester bonds (Mrs. Bormshikker called it a sugar-phosphate backbone). In portions of the DNA where cytosine is followed by guanine at a high frequency, the addition of methyl groups turn the cytosines into 5-methylcytosines. These normally nonmethylated areas, called CpG islands, are often where transcription is initiated. The methylation of DNA can influence how the genetic code is expressed, leading to physical changes in the organism.
One way to imagine this is with Sarkozy’s notorious
pumps Cuban heels. Though not part of his body, the lifts do make him taller. Like the addition of the methyl group to the DNA strand, the appearance of the husband of Mick Jagger’s ex changes with the addition of enhancing footwear, but his actual genetic material remains the same. The man heels can be removed (just as demethylation can occur), but monsieur is rarely seen in public without his elevator shoes. For all practical purposes, Mary Kate Olsen’s future bro-in-law can be considered taller for the duration of his life.
But here’s where it gets complicated. If epigenetic mechanisms don’t change the nucleotide sequence of DNA, can they be passed on to the next generation? Epigenetic modifications like methylation can also affect subsequent generations, even if the genetic sequence remains unchanged. This process is not well understood in humans, but some studies have demonstrated its existence. Researchers are exploring how and when these epigenetic changes can be passed on.
Let’s think back to our towering Gallic leader, Mr Sarkozy. With three sons of his own, there are several possible ways he could pass on his pension for platforms. Sarko could literally bequeath the elevator shoes to his kids or order them similar versions as gifts. Or they could sneak in through less direct routes. Perhaps the influence of growing up with a father who was constantly vertically jacked would create a latent desire to do the same. Maybe the trend of man heels would take off in society so that it would be more likely for anyone, including his children, to wear them in the future (just like environmental exposure to toxins can cause DNA methylation to occur). Eventually the trend would fade, the hand-me-down shoes would break down and the desire to walk in their father’s footsteps would wane. The heels would disappear and the children or grandchildren would remain within the bounds of their physical height. In the same way, there are higher rates of loss for epigenetic modification than for genetic mutations, so changes in gene expression (i.e. high heel wearing tendencies) are not permanent. Someday, foot binding could fall out of fashion for women too and remove any motivation for Sarkozy’s tot daughter, Giulia, to wear Blahniks.
Keep in mind that the heel-wearing is just a metaphor for epigenetic modification; DNA methylation may or may not influence stature and scientists are still discovering what types of gene expression it can produce. But Lamarck may have been onto something when he postulated that phenotypic changes can occur during an organism’s lifetime and can, in some cases, be transmitted to the next generation. Thanks to polygenic influence (the former and possibly future French president tends to couple with women 5 inches taller than him), the Sarkozy sons will most likely forgo the man heels. They’ll just have to launch tomatoes and marbles at police officers to get attention.