Sunday, April 11, 2010

Feedback and Complexity

When we think of evolution, we are usually thinking of physical traits. Coyne uses examples such as fins, wings, fur color, etc. However, it took much more complexity to create the diversity of life that we see today. When Coyne discusses complexity, it almost comes off as a five page rant about Intelligent Design (starting on 136). His chief examples are blood clotting and the eye. He shows simpler structures such as ancient proteins and eye spots. The concept is that these organs gradually evolved into their complex forms.
We have learned that it takes more than an organ's shape to make it function properly. There is a whole network of proteins, hormones, tropic hormones, pH and salinity balances at work. My question is this:

How does feedback allow for complex organs and systems to function in our own bodies? When is there a selective advantage for positive feedback, and when is there an advantage for negative feedback? Why does this difference exist?


  1. Without feedback the organs and systems of our bodies would not be able to function properly. This is because the things that our body produces are often harmful if they were to be produced in excess, hence how negative feedback helps the body to control the amount of certain substances in our bodies. Feedback is necessary in order for the body to maintain homeostasis. The feedback in our body is a system that keeps the body informed of what is going on in the body and when to release or not release hormones and/or other things.
    Negative feedback is when the output of a system works to change the input of the system and in all inhibits more output of the system to occur. The advantage of negative feedback is that it stops things in the body from accumulating to the point where they can hurt the body. This regulation is effective and when it does not occur the body can be harmed. For example, glucose is a negative feedback loop so when a certain amount of glucose is produced it inhibits itself from being produced more. If this negative feedback loop of glucose were to fail then the glucose levels in the blood could rise dramatically. This results in the disease diabetes.
    Positive feedback is when something is produced that causes more of itself. When positive feedback is strong systems can move to a limit state where the production is limited by external factors or the positive feedback is somehow negated because too much of the product is being produced. One example of a positive feedback loop is with the hormone oxytocin. This hormone is released in childbirth when a contraction results. Once the hormone is released its positive feedback loop makes it so that it triggers more release of the hormone, triggering more contractions.
    Negative and positive feedback loops are both necessary for the systems of the body to maintain equilibrium. They do different things but they are both selectively advantageous for the body. Negative feedback loops tend to reduce, inhibit, or stabilize a process in the body. On the other hand, positive feedback loops are necessary to expand, promote and in the end destabilize a process in the system of the body.
    Campbell Textbook

  2. Feedback loops, as Sami has already explained, are used by the body to maintain homeostasis within the body. They are internal regulation systems that allow us to survive and thrive depending on changes in the environment. As Sami also described, negative feedback involves a product inhibiting the initial reaction. An example of this is the control of room temperature. If the temp rises above a certain set point (homeostasis), the heater is turned off, cooling the room. And if the temp drops below homeostasis, the heater turns on to raise the temperature. The initial stimulus (a change in temperature) causes a response that dampens the initial stimulus. This is obviously a nonliving example, but you get the point. Usually, with negative feedback loops, there are two sides, for whether not homeostasis is surpassed or isnt reached. The selective advantage for negative feedback is if homeostasis must be maintained, rather than having a certain output (like in positive feedback). Therefore, if the level of a chemical or the temperature is needed to remain constant to survive, negative feedback is the most effective way of acheiving this.

    However, if there is a certain task that needs to be completed, positive feedback is your best bet. Positive feedback is when the product amplifie the stimulus, which in turn makes more product, which amplifies the stimulus and so on and so forth until a specific task is entirely carried out. An example of this is the birthing of a baby. Oxytocin stimulates contractions, which stimulate oxytocin release. This goes on until the baby is birthed. Also in child development, the release of prolactin is stimulated by suckling. Prolactin stimulates milk production, which causes more suckling. This is continued until the bab stops suckling. The task is carried out until it is completed. The selective advantage of positive feedback is that if something is disrupting homeostasis, but cant be simply controlled, then an action must be carried out to completion so that the disruption is gone.

    These feedback mechanisms are very complicated and add to the overall complication of the body. But they are absolutely necessary because they allow us to maintain or reach homeostasis, therefore, we survive.
    Sr. Erdmann

  3. The concept of feedback relates directly to the theme of regulation. Every bodily process we use is based on feedback regulation. For example, our digestive system is completely regulated by positive and negative feedback loops. Specifically, pepsinogen mixes with HCl secreted in the stomach which activates the pepsinogen which then becomes pepsin. The activated pepsin then causes more pepsiongen to be secreted by the specialized cells in the gastric pits which mixes with the HCl that is secreted by the parietal cells and together, they make more pepsin. The pepsin is responsible for protein hydrolysis because it breaks peptide bonds to produce smaller polypeptides. If there was not this positive feedback, our bodies would not know to make more pepsinogen and therefore we would not be able to break down proteins fast enough. Another piece of evidence that we have evolved from the same ancestor as chimpanzees can be seen by their feedback regulation. According to the journal article "Puberty in the Male Chimpanzee: Time-Related Variations in Circulating Inhibin" Chimpanzees have a negative feedback loop for their secretion of FSH by inhibin. In human males, FSH is also regulated by inhibin. The hypothalamus releases GnRH at puberty which causes the anterior pituitary to release FSH which targets the sertoli cells which secretes inhibin which then shuts off FSH production. This is important so that the male does not produce too much sperm. Our feedback systems are an integral part of how our body functions and the same feedback systems are present in other animals, especially our ancestors which is yet another piece of evidence that evolution is true.