Homeostasis is the regulation of the inside of a body against changes in the external environment. Organisms are subjected to the conditions of the external environment. For example, humans are able to keep their body temperature within a set range despite exposure to extreme hot or cold (within reason of course – the human body cannot sustain its body temperature if unprotected in an extremely cold environment for a long period of time). The pH of blood is also under homeostatic regulation.
An example of an evolutionary mechanism that keeps our blood pH in check is what occurs when our blood becomes to acidic. Acidosis may occur due to intake of certain drugs, organ damage (e.g. kidney failure), or bacterial infection.
Acidosis results in the patient taking deep, quick breaths (Kussmaul respiration), which takes carbon dioxide out from the blood and expels out of the body and into the atmosphere. This is beneficial because carbon dioxide concentration is related to the acidity of the blood. Carbon dioxide can be converted to carbonic acid, resulting in free hydrogen and bicarbonate ions.
Lowering the amount of carbon dioxide decreases the amount of free hydrogen ions and increases the pH. During alkalosis, breathing is shallow and slow to increase the amount of carbon dioxide in the system.
A second example of evolution of homeostasis mechanisms is iron acquisition in pathogenic bacteria (bacteria that cause disease). Iron is an essential nutrient for all organisms, including bacteria. Iron cannot be made but must be harvested from the environment. Iron is normally bound to proteins (e.g. hemoglobin in the blood) in the mammalian host, which presents a challenge of pathogenic bacteria.
Pathogenic bacteria produce siderophores, which are able to bind and collect iron from hemoglobin and other iron carrying proteins. This enables the pathogen to have enough iron to survive.
Iron, although essential, can be toxic in high quantities. Bacteria have evolved mechanisms to prevent iron toxicity. Production of siderophores can be decreased or stopped by the binding of iron to siderophore genes. Iron can thus directly regulate the amount of iron collection that occurs by the cell. If the bacterium has enough iron, it can reduce or shut down its iron collection system.