The Argument-
Given that the epigenome can have a very strong effect on the phenotype of an organism, "Two very distinct flowers, same genetic code...difference between the two flowers in their respective epigenomes - the packaging the surrounds DNA" (p.158) and the fact that the "epigenome can be altered by the environment" (p.159), and especially the fact that "the epigenome can be inherited" (p. 159), should we rethink the way we think about natural selection and evolution? Rather than individuals of a species being culled that do not posess the required characteristics to survive and reproduce in a new environment, eventually resulting in a new species, might it be that the individuals of the species are capable of gradually changing their epigenome, reducing the number of individuals lost to the environmental stimulus? And, given that, might it be that Homo sapiens and Homo neanderthalis, commonly thought of as two different species, might only have differed in terms of their epigenome?
-Ari Bakke
Aribakke@gmail.com
Epigenetics has drastically changed our view and understanding of heredity and evolution as we now know that we can personally affect the ways our genes function and then pass on these effects to our offspring. DNA and genes are not the only things that are hereditary, but our lifestyles affect the way in which our genes function. David Shenk even writes at the top of page 161 in bold that "Lifestyle can alter heredity" (Shenk 161) and it is this concept that will change our understanding of evolution and natural selection. Since our epigenome can be altered and then effect future generations, potentially supplying them with beneficial changes in phenotype, interference occurs with the idea that animals that have undergone random mutation that is helpful will eventually spread this helpful gene throughout a population due to natural selection. Epigenetic changes give many more animals the possibility of surviving and reproducing, as the packaging around their DNA can slowly be altered to aid in their survival. David Shenk describes how in an experiment Daniel Morgan and Emma Whitelaw observed that a "batch of genetically identical mice were turning up with a range of different fur colors -- differences traced back to epigenetic alterations and passed on to subsequent generations" (Shenk 159). All of these mice were clones so their range in fur color was due to something other than the genes they had, showing the huge effect epigenetic changes can have on an organism. Pure Darwinian evolution and natural selection would not be able to explain these results unless all of the mice had different genes for fur color, which would then have been attributed to variations in a population. But it is the fact that all these mice shared the same DNA an still expressed these changes that shows how in our new thinking of evolution we must go beyond just natural selection leading to evolution due to the promotion of genes providing selective advantages to a theory in which we consider how the epigenome will also effect the variation in a population as epigenetic changes will allow more individuals in a population to survive a change in the environment. We cannot anymore assume that a changing environment will allow only those individuals in a species that have a particular gene to survive, but that a much larger part of the population will also be able to due to changes in DNA packaging aiding in the individuals survival. Now it would be wrong to completely scrap the ideas Darwin presented us as many of them still hold true or at least have some merit, but the fact is that epigenetic changes affect the ways in which differentiation of species occurs. "It [epigenetic changes] offers not just another mechanism by which species can adapt to changing environments, but also the prospect of an evolutionary process that is more interactive, less random, and runs across several parallel tracts at the same time" (Shenk 161)
ReplyDeleteEpigenetic changes have their limits though, as it is not the changing of the genes and DNA present in an individual, but the genes and DNA being expressed by an individual. With epigenetic inheritance individuals can change in many ways, but evolution will still occur, because while many new features may be created through methylation and other factors contributing to the epigenome it is sometimes necessary for new genes to be created or genes changed through mutation to bring about selective advantages. Epigenetic changes can allow more individuals in a species to survive, but this also means more mutation will occur in this species since with more in the population, this correlates to more chances of individual organisms with mutated DNA. The epigenome changes, but still aids in the process of evolution, one of our themes in biology. With the epigenome the processes of evolution become harder to understand since as Campbell says “Whereas mutations in the DNA are permanent changes, modifications to the chromatin can be reversed by processes that are not yet fully understood” (Campbell 358). We do not completely understand epigenetic inheritance, so while we know that it will still allow animals to evolve due to selective advantages found in a species, we do not know the full extent that the epigenome will take in this process. There is a chance that epigenetic inheritance will actually slow down evolution due to more changes to an animals phenotype that can be made through chromatin rearranging, or speed up making the process of copying DNA more difficult when different patterns of methylation occur and cutting out or duplicating sequences of DNA.
ReplyDeleteThe differences between Homo sapiens and Homo neanderthalensis though are too vast to simply be epigenetic differences though. These two organisms, while still being part of the same genus did evolve separately from each other with the most recent common ancestor existing between 200,000 and 500,000 years ago (from the Smithsonian Institution at http://humanorigins.si.edu/evidence/human-fossils/species/homo-neanderthalensis). While their many shared physical features may seem to support the idea that Neanderthals were related to modern humans, there differences go more than skin deep, and the extensive difference in their DNA shows that they are in fact 2 different species. The Smithsonian Institute states how “Mitochondrial DNA from… ancient humans fit within the range of modern humans, but the Neanderthals remain consistently genetically distinct” (from the Smithsonian Institution at http://humanorigins.si.edu/evidence/genetics/ancient-dna-and-neanderthals) There is a chance that there was some cross breeding between the two species, but even this would have been very minimal as modern humans were found to still be different from Neanderthals.
-Kyle Nelson (kynels21@gmail.com)
PART ONE:
ReplyDeleteNow that we know epigenetics affects gene expression and heredity, previous conceptions of natural selection and evolution need to be updated. This does not mean we should scrap the theory of natural selection altogether. Rather, the new findings about epigenetics enhance (and complicates) the concept of natural selection. “‘Contrary to current dogma, the variation on which natural selection acts is not always random…new heritable variation can arise in response to the conditions of life’” (161). The “current dogma” refers to Darwin’s theory that genes undergo random mutations that either enhance or reduce the organism’s ability to survive or reproduce (http://www.guardian.co.uk/science/2010/mar/19/evolution-darwin-natural-selection-genes-wrong).
However, the idea of epigenetics reduces the “randomness” of the evolutionary process. Epigenetics introduces the concept of “free-will” into genetics (160). If our lifestyle or environment can influence our genome and our genome can be passed down, then we do have control on our gene expression and heredity—to some extent. Thus, epigenetics complicate natural selection because the assumption that changes in heredity are simply random or based on chance is not completely accurate. It is still a foreign idea that our actions now can correlate or contribute to the success of future generations—but it is possible. Nevertheless, our understanding of epigenetics and its effect on natural selection and/or intelligence is still, as David Shenk says, “yet [to be] exactly sure” (161).
Given the potential of epigenetics, it is very possible that species can change their genome to reduce the dangers of environmental stimuli. The current view by Campbell states that species prevail because natural selection has increased the frequencies of alleles that enhance their survival and reproduction. In other words, the match between a species and its environment improves (Campbell 481). Again, epigenetics do not obviate this current knowledge—it adds flavor to it now that the natural selection—that had increased the frequencies of those alleles—is less attributed to chance and more attributed to epigenomes.
Regarding the connection between Homo sapiens and Homo neanderthalensis, it is unlikely that these two groups differed only in epigenomes. To best see the effect of epigenomes, the two individuals being compared need to be from the same species, better yet from the same DNA. For example, David Shenk introduces the idea of epigenetics with the toadflax plants—“two very distinct flowers, same genetic code” (158). Thus, the comparison must be relative to the species and DNA sequence. That’s the reason why twins are a fascinating topic to research regarding epigenomes—they have the same DNA sequence, but are completely different. On the other hand, unrelated people would not be prime examples to study epigenetics. Even though all humans share 99.99% of their DNA, there are unique variable regions in DNA called simple tandem repeats (STR). It is these segments of DNA (STR) that are used for paternity tests (http://www.medicalgenomics.co.uk/howtestingworks.html). Thus, when a child inherits “half” the DNA from his or her mother, the child is really inheriting half the STR. Changes in gene expression solely due to epigenomes are hard to track down when there’s already so many different variables in the STR for unrelated people.
Linda Xu (lindaxu22@hotmail.com)
PART TWO:
ReplyDeleteHumans and Neanderthals share 99.7 % of their DNA. Then again, humans share more than 98% of DNA with chimpanzees, 96% with gorillas, and a whopping 50% with bananas (http://www.guardian.co.uk/science/2012/mar/07/gorilla-genome-analysis-new-human-link). It only takes a few unique areas of DNA to make an individual drastically different from another—and there’s little connection in the STR between humans and Neanderthals. Furthermore, Neanderthal and human mitochondrial DNA differed 24%. All the evidence points to Neanderthals forming one clade and humans forming another—with occasional interbreeding (Campbell 732).
This response relates to evolution. Neanderthals and humans can be linked to a common ancestor, but they are not the same species. They differ in more significant ways than their epigenomes. Also, this relates to evolution because epigenetics enhance its and natural selection’s definition. Epigenetics provide an idea that people have more control over the genetic expression since it reduces the factor of chance within it. For example, it may be due to epigenetics that the frequency of alleles that promote survival and reproduction increased in species that obtained a selective advantage to prevail.
I agree with Kyle’s post that lifestyle can alter heredity—environment can influence epigenetic changes. My response differed in that I sought to redefine natural selection and evolution with epigenetics. Kyle also noted that the randomness of mutations is reduced, and that epigenetics offer organisms more ways to survive and reproduce. I also commend Kyle’s example of using the example of the mice with different fur colors—the mice had the same DNA but exhibited different phenotypic characteristics. It illustrates my point that the best way to see the effect of epigenetics is to compare individuals with the same DNA. Lastly, I agree with his opinion about how humans and Neanderthals do not differ in just epigenomes. I came to the same conclusion but in a different way—I sought to iterate why epigenomes are not enough to make up the differences between humans and Neanderthals from a genetic standpoint. Overall, I agree with Kyle’s post, but I did come to the same conclusion using different sources and a different point of view. (He just beat me to posting it. Darn.)
Linda Xu (lindaxu22@hotmail.com)