Now let’s try to understand better how the mechanism at negative retroaction or genetic feedback works. For this we’ll make use of two examples: a natural one and another where man intervention is determinant.
In the first case the number of a lot of rodents, among them Lemmings, increases as much as that an epidemic of some kind which kills nearly the whole population takes place, the increase starts again and it is followed by another epidemic and so on[1],[2].
Evidently during the fase of the highest population growth genetically inferior types increase in percentage, which, in this case, means they are susceptible to the particular virus always present in the population.
During the phase of the highest numerical reduction, on the contrary, resistant types, we have defined as genetically superior, prevail.
The frequency of resistant types is low in the original system. This is probably advantageous because it lets the mechanism at genetic retroaction work. In fact if the percentage of resistant types were high in the original system, with the numerical increase of the individuals, the types resistant to the virus would anyway prevail, therefore, without any other regulating factors, population increase would continue endlessly[3].
This example is very important to demonstrate the role and importance of the microparasite-host set in a system or ecological set.
What happened in Australia between the virus of Myxomatosis and the local population of rabbits is even more interesting: in 1859 some European rabbits, which in about 20 years became so numerous as to become dangerous for vegetation and other animals, had been introduced into Australia. Many years later, in 1949, government decided to intervene and did it by introducing into Austalia rabbits coming from South Africa where among their population the virus of Myxomatosis was endemic and not so virulent.
The meeting of this virus with the Australian rabbits caused, immediately, an epizootic with 90% mortality.
After seven years a new epizootic presented a mortality rate of about 20% and the nunber of rabbits had stabilized at about 20% of the one that was before the introduction of Myxomatosis[4],[5],[6],[7].
What had happened?
The virus gradually developed avirulence, that is there was an evolution from the most virulent type to the less and less virulent ones.
At the same time rabbit population progressively evolved towards resistance.
This probably happened because the mosquitos, Aedes and Anopheles, which would feed on living rabbits, were more probable to catch the least virulent virus from the rabbits giving it hospitality because the latter would live 26 days on average, that is the double of the ones which were infected by the most virulent type.
So we can introduce a new important concept: viruses and bacteria that attack an animal or a plant living long time, don’t evolve towards types at high reproductivity and higher pathogenity; therefore the evolution is towards repoductive types that can give a balanced economy of supply and demand among species of predators and preys[8].
The last example explains what happens when a population gets in touch with a new virus and the system or the sets of predator-prey,parasite-host haven’t yet developed homeostasis : functioning of the mechanism at negative retroaction or genetic feedback.
Before we can get a balance, mortality may be very high, so that, if a population which had already been in touch with the virus, is present in the ecosystem, the species getting in touch with it for the first time may completely disappear.
This is what happened to some bird species in New Zealand: in about one hundred years some species disappeared and so there was somebody, like Myers, who suggested they might have caught some diseases from the great number of imported foreign birds[1].
Evidently the local species were highly susceptible to the pathogens brought by the foreign birds: the result was the destruction of the first. The birds coming from abroad had learnt long time before how to live with the microorganisms they were hosts of, so they weren’t damaged by the zoonasis which destroyed the communities of the local birds.
Therefore, here is another evident example of how a microparasite(bacterium or virus) can be particularly virulent and lethal for species it has never got in touch with.
The same happened to the Amerinds when they got in touch with the viruses the Spaniards imported. The original Incas and Aztecs didn’t disappear completely, but diminished drastically before being in a dynamic balance with the new viruses the Conquistadores imported.
[1] Myers J G The present position of the endemic birds of New Zealand. J Sc and Tech.6:65-99.1923
[1] Elton C S, Plagues and the regulation of numbers in wild mammals.
J Hyg 24(2):138-163.London 1925.
[2] Elton C S: Research on rodent control by the Bureau of Animal Population. Sept 1939-July 1947. London 1954.
[3] Pimentel D. Op. e Pag.cit.
[4] Ratcliffe F N, Meyers K, Fennessy B V and Calaby J H. Myxomatosis in Australia A Step Towards the biological control of the Rabbit. Nature N. 4134 July 5, 1952 Pag.7-11
[5] Myers K, Marshall I D and Fenner F. Studies in the Epidemiology of Infectious Myxomatosis of Rabbits. III. Observations on two succeeding Epizootics in Australian Wild Rabbits on the Riverine Plain of South-Eastern Australia. 1951-1953. J Hyg 52(3):337-360 1954
[6] Fenner F, Marshall I D A comparison of the Virulence for European Rabbits ( Oryctolagus cuniculus) of Strains of Mixoma Virus Recovered in the Field in Australia, Europe and America J Hyg. 55,2:149-191.1957
[7] Fenner F, Meyers K.Myxomavirus and Myxomatosis in retrospect, the first quarter century of new disease. Pag.539-570 in Kurstak E and Maranosh EDS. Viruses and Environment 3D International Conference on Comparative Virology. Mont Gabriel, Quebec,1978.
[8] Pimentel D Op. e Pag.cit.
To be continued in :
3) Mechanism at Negative Retroaction-Genetic Feedback 3°
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Ferdinando Gargiulo offers you a new perspective on why new viral epidemics, assaults, infanticides, suicide epidemics and even environmental catastrophes. Always engaged in his research decides to create a blog to offer his readers content of high value.