"For example, giraffes, according to Lamarck's theory, had developed longer and longer necks over the generations because of the giraffe's practice of reaching higher and higher for food." (344) Initially, that is considered incorrect because in evolution, an organism does not have a need to or has to grow or change in order to survive. However, epigenetics takes into consideration the environment. Discuss how mutations could have occurred to become beneficial for the giraffe, especially because of the environment they live in. How could Lamarck's idea be reworded to fit into the rules of evolution and factors like natural selection? It is also interesting to note that like many mammals such as humans, giraffes also have 7 vertebrae despite their very elongated necks. How do Hox genes play into that in terms of development and tie into the idea of evolution and the common ancestor of all organisms/diversity of life?
(Diana Liao - dianaliao3@gmail.com)
Although all animals have homologous Hox genes, animals show different gene expressions, even within a single species. How can this be? This is possible because of the environment in which that population lives, as evidenced through epigenetics and interplay with Hox genes. This shows the impact an environment can have on gene expression.
ReplyDeleteAn experiment was done in the Soviet Union 40 years ago to learn more about the differences between domesticated animals and those that are not. Many physical differences were noted between animals that show signs of behavioral domestication versus those that are wild and unapproachable by humans. For example, it was noted that ‘the normal pattern of coat color that had evolved as camouflage in the wild altered as well… domesticated animals are piebald, completely lacking pigmentation in specific body areas’. In the experiment, domesticated foxes were breed with domesticated foxes and wild foxes were breed with wild ones to see evolutionary differences over time between the two populations. Through this experiment, it was obvious that the behavioral patterns in the animals influenced the expression of their genes for certain physical and even non physical traits. An example of a difference that evolved between them is that ‘nondomesticated foxes first show the fear response at 6 weeks of age; domesticated ones show it after 9 weeks or even later’. Many of the physical traits that began showing up in later generations mimicked that of domesticated dogs, as the article states ‘traits such as floppy ears and rolled tails similar to those in some breeds of dog. After 15 to 20 generations we noted the appearance of foxes with shorter tails and legs and with underbites or overbites’ (http://www.hum.utah.edu/~bbenham/2510%20Spring%2009/Behavior%20Genetics/Farm-Fox%20Experiment.pdf).
This experiment and its findings prove the extent to which genes can be affected by the environment in which a population lives, and how these genes can be affected continually to create evolutionary changes. The genes are primarily affected through mutations that occur over time. If a beneficial gene is expressed from environmental stimuli, the beneficial mutation will prevail and accumulate to result in an entirely changed gene. Therefore, a beneficial mutation can prevail for positive natural selection through environmental stimuli. Mutations affect not only the DNA sequence of a gene, but the protein sequence as well. This is what likely happened with the fox populations, and with giraffes, since mutations can have huge evolutionary impacts.
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ReplyDeleteHox genes are evidence that, ‘variation of morphological form in the animal kingdom is underlain by a common set of instructions’ because Hox genes are the underlying homologies among all animals. Differences in phyla emerge from ‘differences in how the Hox genes are regulated and what target genes the Hox-encoded proteins regulate’ (http://www.ncbi.nlm.nih.gov/books/NBK9978/). The regulation of Hox genes is therefore influenced under the GxE model. Hox genes are homologous for all animals, showing we all emerged from a common ancestor. Animals begin to diverge based on their environments and the specific adaptations they acquire to them, and both Lamarck and Darwin support this. What Lamarck’s theory lacks is support from the natural selection theory. Lamarck believed that animals alter themselves through their actions in order to adapt, but Darwin proposed that organisms evolved to changing environments. Perhaps if Lamarck’s theory incorporated changes in genes from environmental stimuli that were less influenced by actions made by animals, his theory would have been more widely recognized. For example, the giraffe’s changing neck could be explained by Lamarck as him having to stretch to reach for food (the concept of use or disuse influencing genes). When an animal’s internal organs change to adapt to an environment, this was not necessarily caused by the animals actions or disuse/use of certain organs. This is where Lamarck’s theory lacks, but epigenetics and Hox genes actually support certain aspects of his theory.
The idea that we all stem from a common ancestor not only supports the theory of evolution, but can also relate to Lamarck’s theory. The evolutionary impact comes into play because ‘changes in the genes turned on by them [Hox genes], could represent a major source of evolutionary change’. (http://evolution.berkeley.edu/evosite/evo101/IIIC6cComplexity2.shtml). Lamarck’s theory supports that organisms become increasingly complex as they adapt. His theory is undermined by Darwin’s natural selection theory because this supports that individual efforts have no effect on future generations. Despite this proven theory, epigenetics actually complicates this. Hox genes, along with epigenetics, actually support Lamarckism. Changes in gene activity, according to epigenetics, can be governed by the cellular material that sits on top of the genome, not just the genetic code itself. There are ‘epigenetic "marks" that tell your genes to switch on or off, to speak loudly or whisper. It is through epigenetic marks that environmental factors like diet, stress and prenatal nutrition can make an imprint on genes that is passed from one generation to the next’ (http://www.time.com/time/magazine/article/0,9171,1952313,00.html#ixzz1rVNGHEWn). The fox experiment portrays this perfectly, since each generation of foxes was influenced by the previous in some way. Eventually, foxes were being born with multiple physical adaptations, not just one or two, proving that as the mutations built up, evolutionary changes were taking place to create an almost entirely new species of foxes, one much more like dogs.
Michaela Margolis (mmargolis989@gmail.com)