1.c.2 – Is symbiosis an important evolutionary strategy?

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December 25, 2009 — Riccardo Sabellotti - Giacinto Sabellotti


Is symbiosis an important evolutionary strategy?

When the individuals of two different species live together to obtain a mutual benefit, it is realized a phenomenon of fundamental importance defined symbiosis. This relation is sometimes so close that the two units end with forming a single body; is the case of lichens, which are bodies formed from the merger of a mushroom with a colony of unicellular algae; the fungus provides nourishment for algae, water and minerals, and support for better lighting and ventilation, while the algae in turn feed the fungus with the organic molecules produced through photosynthesis.
An example of symbiosis in the animal world is given by the Egyptian trochilus and the Nile crocodile: the trochilus is a bird that feeds cleaning the teeth of the crocodile from annoying residues of food, getting in return protection and food.
The examples of symbiosis are very numerous and some of them have obtained a great success; we can recall the symbiotic relationship between plants and fungi living in their roots that allows plants to absorb  phosphorus and nitrogen, indispensable to their metabolism, while plants allow mushrooms to eat their own sap.
Symbiosis is one of the oldest evolutionary strategies. We have seen how one of the fundamental steps in evolution of living beings is given by the appearance of protists, i.e. bodies that are distinct from bacteria by a much more complex internal structure. This structure is formed by a nucleus that joins with other parties, isolated from internal membranes and thus distinct, which, by carrying out specific functions, represent the real internal organs. Some of these organs, the mitochondria, have within them their own DNA molecule, and then a genetic heritage that reveals their true nature: they are the descendants of ancient bacteria able to use the waste products of other chemical cellular reactions to feed themselves and derive the energy necessary for life; the above is carried out with a much higher efficiency than that of the cell their host. Over time, these bacteria have become the power plant of the cell, obtaining in change a constant and secure source of nourishment.
A similar situation is in the case of chloroplasts present in the cells of all plants: they are the evolution of ancient bacteria able to carry out photosynthesis.
It is crucial to know that the mitochondria are also present in the cells of all animals and also in humans.
This is a symbiotic strategy that led to evolutionary success of protists.
The idea of creating new life forms through the merger of different species Is called symbiogenesis, and it is now clear that it has played an important role in evolution, since it allowed to combine the capabilities developed and improved by different species of bacteria (such as the use of sunlight and oxygen as a source of energy) with those of autonomous movement peculiar to complex cells. This way, chances of survival otherwise unthinkable were seized.
The symbiosis may seem a survival strategy far away from the life of human beings, but reality is quite different. For example, in human intestines, as in that of many other animals, there are colonies of bacteria that help us in digestion and in the defense from infections, having in change a safe and easy nourishment. The number of such bacteria gives us an idea of the vastness of the phenomenon: we know more than 400 different species of them.
We must also remember that every cell of the human body, like those of all animals, contains a large number of mitochondria and is thus in turn a symbiotic body . The symbiosis is therefore essential to our lives.




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