Now let’s try to understand better the functioning of the mechanism at negative retroaction. For this we’ll make use of two examples: a natural one and another in which human intervention is basic.
In the first case the number of many rodents, Lemmings among them, increases up to a certain point beyond which an epidemic of some kind that kills almost the whole population, occurs. The increase starts again and is followed by another epidemic and so on[1], [2].
Evidently, during the phase of the highest expansion of the population,the inferior genetic types that, in this case, mean susceptible to the particular virus which has always been present in the population, increase in percentage.
During the phase of the highest numerical reduction, on the contrary, the resistant types, called superior genetic ones, are prevalent.
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 increase of the number of creatures, anyway types resistant to the virus should prevail and therefore, without any other factors of regulation, the increase of population would continue endlessly [3].
This example is very important to prove the role and importance of the microparasite-host set in a system or ecological set.
It’s even more interesting what happened in Australia between the virus of Myxomatosis and the local population of rabbits: in 1859, in Australia, some European rabbits had been introduced and in about 20 years they became so many as to be dangerous for vegetation and other animals. Many years later, in 1949, the government decided to intervene and did it by introducing, in Australia, rabbits coming from South Africa among which the virus of Myxomatosis was endemic and at a low virulence.
The meeting of this virus with the rabbits from Australia caused, immediately, an epizooty with a death rate of 90%.
After seven years a new epizooty presented a death rate of about 20% and the number of rabbits has stabilized at about 20% of what was before the introduction of Myxomatosis [4], [5], [6], [7].
What had happened?
The virus developed,gradually,avirulence,that is there was an evolution from the most violent type to less and less virulent ones.
At the same time the population of rabbits evolved gradually towards resistance.
Perhaps all this happned because Aedes and Anopheles mosquitos, which would feed only on living rabbits, had more chance of taking the least virulent virus from the rabbits lodging it, because they lived 26 days on average, that is the double of the ones infected by the most virulent type.
So we can introduce a new important concept: viruses and bacteria which attack animals or plants living for long don’t evolve towards types at high reproductivity and greater pathogenicity; therefore the push of evolution is towards reproductive types that can give a balanced economy of supply and demand between predatory species and preyed ones [8].
The latter example explains what happens when a population comes into contact with a new virus and the system or predator-prey and parasite-host sets haven’t yet developed homeostasis.
[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 Pgg. 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 cunuculus) 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 a new disease. Pgg.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
Translated from “Il Virus Intelligente” by Enrica Narducci
To be continued in:
3°)Mechanism at negative retroaction or genetic feedback 3/4
4°)Mechanism at negative retroaction or genetic feedback 4/4
See also :
1°)Mechanism at negative retroaction or genetic feedback 1/4
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.