This is the fourteenth in a series devoted to the theory of evolution by natural selection. We are in the book by Richard Dawkins' The Selfish Gene ", 1976. The translation is mine.
Here are links to all the episodes: 1 , 2, 3 , 4, 5 , 6, 7 , 8, 9 , 10, 11, 12 , 13, 14 .
Summary. Remember that genes create our brain is pre-programmed into them some basic trends and behaviors (strategies). The genes are then naturally selected based on the quality of programming strategies. But what kind of strategies are favored by natural selection? Episode precedente Dawkins ha accennato al fatto che le strategie che finiscono per evolversi sono le "strategie evolutivamente stabili " (abbreviate con ESS). Una ESS è una strategia che, se viene adottata dalla maggioranza della popolazione, non può essere migliorata; cioè, a nessuno conviene deviare da quella strategia per adottarne un'altra.
A partire da oggi cominceremo a capire con esempi concreti come davvero funziona la teoria del gene egoista. La parola a Dawkins.
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Come applicazione pratica [dell'idea di teoria evolutivamente stabile], considerate uno dei più semplici esempi ipotetici fatti da Maynard Smith. Supponiamo che, in a population of a given species, there are only two types of combat strategies, "dove" and "hawk". (The names refer to use of conventional human and have no connection with the habits of the birds in question: the doves are actually quite aggressive birds.) Any individual in our hypothetical population is classified as a hawk or a dove. The hawks always fighting tirelessly and with more violence possible, retreating only when they are seriously injured. Doves launch only threats, never hurt anyone. If a hawk meets with a dove, the dove runs away immediately, so there is injury. If a hawk meets with a hawk continue until one of them is seriously injured or dead. If a dove meets a dove, no one gets hurt, they continue to insult each other for a long time until one gets tired or decides not to waste any time, and then retires. For now, assume that an individual has no way of knowing in advance whether a rival is a hawk or a dove. I discovered only in battle, and has no memory of past battles with particular individuals to guide them.
Now, as a purely arbitrary convention, assigning "points" for the contenders. say 50 points for the winner, 0 for losing, -100 for those who remain seriously injured, and -10 for those who waste time making a long fight. These punti si possono pensare come direttamente convertibili nella moneta della sopravvivenza dei geni. Un individuo che vince molti punti, cioè che ha in media un guadagno, è un individuo che lascia molti geni dietro di lui, nel pool di geni. Entro certi larghi limiti, i valori numerici esatti non contano, ma ci aiutano a ragionare sul problema.
La cosa importante è che non ci interessa se i falchi tenderanno o no a sconfiggere le colombe quando ci combattono contro . Sappiamo già la risposta: i falchi vinceranno sempre. Vogliamo sapere se falco e/o colomba sono strategie evolutivamente stabili (ESS) . Se una di esse è una ESS e l'altra no, we expect that the ESS is that the two ends to evolve. It is theoretically possible that there are two ESS. This may be true if, whatever the strategy in the majority population, the best strategy for any given individual had to adapt to the majority. In this case the population tends to stay at any of the two stable states were randomly reached first. However, as we shall see, none of the two strategies, hawk or dove, is in fact evolutionarily stable by itself, and therefore must not expect either to evolve. To show this we calculate the gain medium.
Suppose we have a population consisting entirely of doves. Whenever fighting, nobody gets hurt. The battle is long tournament rituals, for example consisting of eye contact, ending when one of the two rivals at a time. Then the winner earns 50 points because it gets the resource that was the subject of contention, but will pay a penalty of -10 for wasting time in a long battle of looks, it scores 40 points less. The loser is penalized -10 points for wasting time. On average, an individual can expect to win half the fights and lose half. Therefore its average profit for each battle is the mean value between +40 and -10, which is +15. Then, each individual in a population of pigeons seems rather passarsela bene.
Ma ora supponiamo che un falco mutante compaia nella popolazione. Poiché è l'unico falco nei paraggi, tutti i combattimenti che fa sono contro una colomba. I falchi battono sempre le colombe, così vince +50 ad ogni combattimento, ed è questo il tuo tornaconto medio. Egli ottiene un enorme successo rispetto alle colombe, il cui tornaconto netto è solo +15. Come risultato , i geni del falco si diffonderanno rapidamente nella popolazione. Ma adesso ogni falco non può più confidare che tutti i rivali che incontrerà saranno colombe. Per fare un esempio estremo, se il gene del falco si diffondesse con tale successo che l'intera popolazione finisse per essere di hawks, all fights would now be between hawks. In short, things are now very different. When a hawk meets a hawk, one of them is seriously wounded, totaling -100, while the winner scores +50. Each hawk in a population of hawks can expect to win half of his fights and lose half. Its average expected profit is therefore somewhere between +50 and -100, which is -25. Now consider a single dove in a population of hawks. Certainly lose all the battles, but on the other hand is never bad. His average gain is 0 in a population of hawks, while the average profit of a hawk in the same population is -25. genes are therefore likely to spread the dove in the population.
For as I told the story it seems that there should be a continuous oscillation in the population. The genes of the Falcon will rise rapidly and then, as a result of the majority of hawks, the genes of the dove will get an advantage and increase in number until once again the genes of the hawk began to prosper, and so on. On the other hand, a similar response should not necessarily take place. There is a stable ratio between the hawks and doves. In arbitrary point system we are using, this stable relationship, if we calculate it, turns out to be 5 / 12 doves to 7 / 12 hawks. When this stable relationship is reached, the gain medium for hawks is exactly equal to the average gain for the doves. Therefore, the selection does not favor either of the other. If the number of hawks in the population began to rise so that the ratio was no longer 7 / 12, the doves begin to have an extra advantage, and the proportion would return soon to the stable. Just as we will find that the stable relationship between the sexes is 50:50, so the stable relationship between hawks and doves in this hypothetical example is 7:5. In each instance, if there are oscillations around the stable point, you need not be very large.
Superficially, this looks a bit 'to group selection, but in reality it is not anything like that. It looks like group selection because it allows us to think of a population as having a stable equilibrium which tends to return when disturbed. But the ESS is a more subtle concept of group selection. It has nothing to do with groups that are more successful than others. This can be illustrated well by the arbitrary system of points in our hypothetical example. The average gain of an individual in a stable population made up of 7 / 12 hawks and 5 / 12 doves appears to be 6 * 1 / 4. This is true whether the individual is whether it is a hawk a dove. Now 6 * 1 / 4 of the gain medium is much less of a dove in a population of doves (which is 15). Se solo tutti si accordassero per essere colombe, ogni singolo individuo ne trarrebbe beneficio. Secondo la semplice selezione di gruppo, qualunque gruppo in cui tutti gli individui si accordano per essere colombe avrebbe molto più successo di un gruppo rivale che si trova in una situazione ESS. (Tra parentesi, una cospirazione fatta solo di colombe non è il gruppo di maggior successo possibile. In un gruppo che consiste di 1/6 falchi e 5/6 colombe, il tornaconto medio per combattimento è 16 * 2/3. Questa è la cospirazione di maggior successo possibile, ma per gli scopi attuali possiamo ignorarla. Una più semplice cospirazione di sole colombe, con un tornaconto medio doi 15 per ogni individual, it is much better than the ESS for each individual.) The theory of group selection and then predict the evolution of a conspiracy of doves sun, as a group that contains a proportion of 7 / 12 Hawks would have less success . But the problem of conspiracies, including those that make long-term interest of all is that they are vulnerable to betrayal. It 's true that everyone benefits in a group of doves sun more than they would in a group ESS. But, unfortunately, in a conspiracy of doves, a single hawk has been so successful that nothing can stop the evolution of hawks. The conspiracy is thus meant to be destroyed by betrayal from within. An ESS is stable, not because it is especially good for individuals who take part, but simply because it is immune to treachery from within.
(continued)
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