In the last chapter of the book, Shubin discusses and explains several examples of "Why History Makes Us Sick" (184). The idea behind the persistence of these evolutionary artifacts is that, as our ancestors have evolved and used old parts in new ways, some undesirable side-effects of our past have remained as a result of our extensive physiological adaptations, such as hiccups and hernias.
In general, one would expect the process of natural selection, however, to remove these negative effects over the course of evolutionary time. Why has this not happened? Why have there not been favorable mutations that have removed the diseases Shubin mentions that make us sick? Are there examples of diseases that have been selected against since humans' evolution from amphibians?
Explain why certain diseases persist in humans and why others do not. Explain the connection between your responses and evolution, the relationship between structure and function, and/or continuity and change.
- Vincent Fiorentini
(vincent@panatechcomputer.com)
Today, numerous diseases with varied symptoms permeate throughout the human race. The reality is, however, that there are many different types of disease, and to answer this question of the existence of disease, a very important distinction must be made. On one hand, there are genetic disorders that are prevalent in the human species. A genetic disorder is a disease that is caused by an abnormality in an individual's DNA (Genetic Science Learning Center – University of Utah). This abnormality is caused by a mutation in the DNA sequence of an individual. Duplication, rearrangement, and mutation of DNA contribute to the evolution of the genome (Campbell 438). As time progresses, certain mutations are inevitable. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes. Considering the number of point mutations that can occur within a sequence of DNA during DNA replication, it is understandable as to how the gene expression can be changed. When a base-pair substitution occurs in the DNA, it can cause proteins to change, seen during a missense mutation, or it can cause protein synthesis to be terminated prematurely, leading to a vastly different protein (nonsense mutation). Nearly all nonsense mutations lead to nonfunctional proteins (Campbell 345). When a gene is mutated so that its protein product can no longer carry out its normal function, a disorder can result. Additionally, insertions and deletions of specific nucleotides within the DNA sequence can have disastrous results for protein synthesis. These mutations usually cause an alteration in the reading frame of the DNA, leading to a frame-shift mutation which would radically alter the function of the protein that was created (Campbell 346). The Human Genome Project Information Center points out the four distinct forms of genetic disorder: single gene disorders, chromosomal, mitochondrial, and multifactorial. The single-gene disorders result when a mutation causes the protein product of a single gene to be altered or missing. Chromosomal diseases occur when entire chromosomes, or large segments of them, are missing, duplicated, or otherwise altered (Genetic Science Learning Center – University of Utah). Mitochondrial diseases are relatively rare type of genetic disorder and are caused by mutations in the non-chromosomal DNA of mitochondria. These three types are purely mutations of the genetic code. We must consider why any specific trait or phenotype would be eliminated from a population to understand why certain diseases are eliminated from the human population. This relates to the theme of evolution and the theme of continuity and change, where over time mutations occur that allow for a selective advantage. However, for a certain trait to cause an advantage over another, a pressure in the environment must be present. Humans are the dominant species on the Earth, and thus humans don’t have to be concerned with predation by other species of animals. In a natural environment, individuals with these genetic disorders, from Down syndrome to color-blindness, would not survive because the environment would prevent these individuals from reproducing. However, in modern society, individuals with genetic disorders are allowed to survive and reproduce, because of the lack of an environmental pressure, and the genetic mutations that cause these disorders are allowed to continue and persist. In the case of multi-factorial genetic disorders like Alzheimer’s disease and diabetes, environmental factors play a role in the development in a different way. By living sedentary lifestyles, as Shubin comments in the book, humans have brought upon disease like heart disease through the environment, but genetic factors also play a role (Shubin 188). For example, different genes that influence
ReplyDeletebreast cancer susceptibility have been found on chromosomes 6, 11, 13, 14, 15, 17, and 22 (Human Genome Project Information Center - 2008).
ReplyDeleteOverall, because society has developed in a way that protects individuals with disease, it has prevented natural selection from running its course. In the natural environment, individuals with disease would be eliminated and the healthy individuals would survive and reproduce. However, history provides us with examples where society was not so kind to individuals with disease. The Ancient Spartans, as BBC points out, commonly eliminated sickly babies to create a group of strong and healthy individuals (BBC 2011). The modern practice of eugenics seeks to manipulate DNA to eliminate undesirable traits from society. As such, these practices raise significant ethical issues. However, unless a system of eugenics was developed or a natural pressure arose in the environment, most genetic disorders would likely continue.
On the other hand, there are pathogenic diseases that result from infection by bacterial organisms or viruses. When considering bacterial or viral diseases, the development of immunity to those diseases is the only way that the human population could resist the disease would. Vaccines are developed by scientists so that humans can develop the immunity to viral diseases. A vaccine is a harmless variant or derivative of a disease-causing pathogen that stimulates the immune system to develop defenses against that specific pathogen (Campbell 391). Moreover, the human species has created antibiotics to treat bacterial infections. Antibiotic resistance is the ability of bacteria or other microbes to resist the effects of an antibiotic. Antibiotic resistance occurs when bacteria change in some way that reduces or eliminates the effectiveness of drugs, chemicals, or other agents designed to cure or prevent infections. The bacteria survive and continue to multiply causing more harm (Center for Disease Control 2010). As a result, many diseases persist in the human population because the bacteria evolve rapidly and surpass the ability of scientists to develop medication.
Vickram Pradhan 1/2a
vickram.pradhan@yahoo.com
Brief Works Cited:
Genetic Science Learning Center. "What are Genetic Disorders?." Learn.Genetics 1 April 2011
http://www.bbc.co.uk/schools/primaryhistory/ancient_greeks/sparta/
http://www.cdc.gov/getsmart/antibiotic-use/anitbiotic-resistance-faqs.html
http://www.ornl.gov/sci/techresources/Human_Genome/education/education.shtml