ChemBob @ChemBob@qoto.org

Sorry the paragraphs ran together. They weren't until I posted it.

This is an announcement I posted to my Intro to Environmental Science course this morning at one of the colleges where I teach. It relates to Lamarck, Darwin, natural selection, and epigenetics. Any comments about it?

Hi Class,
A student in an earlier course asked me a question and I thought it was worth providing all of you my answer.
The question was:
"On page 5.2 of the book, it states that Jean Baptiste’s idea of inheritance of acquired characteristics was wrong. When it gets to the section about giraffes on the same page, there appears to be a contradiction to the previous statement. It states that giraffes with long necks passed the trait to their offspring. I am confused. Can you help me understand?"
Here is my answer:
Lamarck’s concept of the inheritance of acquired characteristics has been controversial for centuries and, even though Darwin conceived of natural selection, he also thought Lamarck’s concept was correct. It turns out that neither of them are entirely wrong, but It is a bit complex, especially with certain new findings. Let’s start off as simple as I can and hope it makes sense. This is one of those things I miss about not teaching all of you in the classroom.
Essentially, the idea of acquired inheritance is that an animal that was normally adapted to its surroundings encountered a change in the environment that forced it to do something differently within its lifetime, essentially immediately, that was passed on to it’s offspring as an entirely new characteristic. For example, the leaves on the lower portions of the trees are all gone so the animal, let’s refer to them as giraffes even if they were giraffe ancestors at this time, stretches its neck to reach the leaves that are slightly higher. It has forced its neck to be longer and its its acquired longer neck will now be passed on to its offspring.
Natural selection has a different take on this that has made more sense due to genetics. In this case, if all the giraffes can already stretch their necks like that, then no big deal, right? However, if only a single or a few giraffes can stretch their necks and therefore be able to eat, while the others die from starvation, the eaters had something in their genetics, a mutation, that was different from the majority of giraffes. Because the other “normal” types had died of starvation, the longer-neck giraffes survived and reproduced more successfully, passing on the gene for the longer neck structure to it’s offspring (probably not all of them, due to how genes shuffle about during formation and joining of the gametes, but some number off them get the gene). The offspring lucky enough to get the longer-neck gene will survive while the offspring who don’t get it will mostly die off without reproducing during that generation as well. With each successive generation, the long-neck gene becomes more dominant in the population until almost all the giraffes have the slightly longer neck. One can imagine that if the leaves only become available even higher up the tree as time goes on, that giraffes with longer and longer necks are favored by selection. This has been the established orthodoxy about inheritance and is probably nearly 100% accurate for this type of change in an organism.
Now there is a concept called epigenetics, which modifies things a bit. Epigenetics addresses the idea that certain genes for characteristics of the organisms are already present in an animal line, but not being expressed (i.e., they are not turned on or they are turned off, sort of like a light switch). Here is a good definition from Collins and Roth, 2021:
“Epigenomics, a field examining changes in gene expression as a result of environmental context, has provided evidence suggesting that both intergenerational and transgenerational inheritance of stress programming are observed phenomena.”
Basically what they are saying is that certain types of stressors in the environment can actually change what is expressed by your genes by altering the chemistry of portions of the DNA that are passed on to the offspring. It causes a change in the phenotype (physical/behavioral) without changing the genotype (the sequence of genes). The environmental stressor determines what parts of the genotype are read and expressed and what parts aren’t. One chemical change (there are others) is adding a methyl group to the DNA that alters gene expression. In fact, these changes can be transmitted in the gametes and persist across generations, affecting not only the organisms offspring, but the offsprings progeny as well and so on. In other words the change is transgenerational. Some diseases can be inherited this way; certain cancers are suspected.
So a stressor in the environment can cause a change in how the genes are expressed that can then be passed on to its offspring. It sounds a little Lamarckian, doesn’t it? It’s just that this is not the type of change (as far as I know) that can result in huge anatomical changes, such as longer necks.
I will close by giving you a quote from Weinhold (2006) that provides some epigenetic examples:
“Today, a wide variety of illnesses, behaviors, and other health indicators already have some level of evidence linking them with epigenetic mechanisms, including cancers of almost all types, cognitive dysfunction, and respiratory, cardiovascular, reproductive, autoimmune, and neurobehavioral illnesses. Known or suspected drivers behind epigenetic processes include many agents, including heavy metals, pesticides, diesel exhaust, tobacco smoke, polycyclic aromatic hydrocarbons, hormones, radioactivity, viruses, bacteria, and basic nutrients.”
I hope this makes sense. Let me know if you would like to discuss it further.
Reference Links
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/inheritance-of-acquired-characteristics
Inheritance of Acquired Characteristics - an overview | ScienceDirect Topics Nicholas Collins, Tania L. Roth, in Developmental Human Behavioral Epigenetics, 2021. Abstract. Conflicting ideas surrounding the inheritance of acquired characteristics is not new; for nearly two centuries, debate surrounding the validity and mechanism behind inheritance of acquired characteristics was pioneered by Lamarck, Darwin, and Weismann. . Epigenomics, a field examining changes in ... sciencedirect.com
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1392256/

Epigenetics: The Science of Change - PubMed Central (PMC) Other Drivers of Change. Substances aren’t the only sources of epigenetic changes. The licking, grooming, and nursing methods that mother rats use with their pups can affect the long-term behavior of their offspring, and those results can be tied to changes in DNA methylation and histone acetylation at a glucocorticoid receptor gene promoter in the pup’s hippocampus. ncbi.nlm.nih.gov

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