There are four possible types of interactions among individuals living together in a population. First, cooperation or mutualism, in which both the participants gain from the act as in the nest building by both male and female birds, or cooperation in the colony of social insects.
Second, altruism in which the actor (individual that carries out the action) pays fitness cost to the recipient that gets the benefit as in social insects.
Third is the selfishness, in which the actor gains but the recipient loses in terms of fitness. Fourth interaction, which is rather rare in nature, is spite in which both the participants lose in terms of fitness. As for example in the case of two eagles fighting in the air for the possession of a killed prey, which ultimately falls down and is taken away by a fox.
KINSHIP & ALTRUISM
Kinship is a phenomenon that occurs in social animals or in closely knit populations which are genetically related to one another. In these populations kin selection operates and traits that result in decreased personal fitness but increase the survival and reproductive fitness of the species are favoured by natural selection. Kin selection works not on individuals but on genotypes.
Altruism evolved in colonies that show kinship. An altruist by way of helping other individuals increases the fitness of its own genome. A honey bee worker is a sterile female and shares at least 50% of its genotype with its sisters even when its mother and father are unrelated. If a worker decides to breed on its own, its diploid daughters and haploid sons will never be more than 50% related to it. So, the workers choose to become sterile and ensure survival of their genetically identical sisters, because the queen can produce more offspring than workers can do individually.
Kin selection leads to altruism in a colony and fitness is direct when it gives the individual personal benefit and reproductive advantage, and indirect when the reproductive benefit goes to the colony or relatives. Kinship favours the spread of an allele that increases the indirect component of fitness of an individual and in most instances it gives rise to altruism.
The gene that favours altruism could spread when participants are related and the cost to the individuals is low as compared to the benefit to recipient. Therefore, altruism is promoted by kin selection and close genetic kinship.
In a large number of bird species, especially those in which nesting opportunities are limited, young ones help their parents in rearing their own sisters and brothers by way of nest building, nest defence and feeding the chicks, although they are themselves capable of breeding. In such birds, as for example in bee-eaters help is always given to their kin.
RECIPROCAL ALTRUISM WITHOUT KINSHIP
The theory of group selection was championed by Wynne-Edwards (1962). Altruism has evolved among the related individuals by means of kin selection. But there are also instances of cooperation among the unrelated individuals. Altruistic act towards non-kin is possible only if the recipient is likely to return the favour at a later date, in a ‘Tit for Tat’ manner.
Natural selection will favour altruism among unrelated individuals only if they reciprocate. Non-reciprocating or selfish individuals of the population are selected out. Robert Trivers (1971) proposed that reciprocal altruism can develop in the following conditions:
- If interacting individuals remain together for considerably longer period of time.
- If frequency of altruistic attempts is high.
- If the cost and benefit to both individuals are more or less equal.
- If selfish individuals that fail to reciprocate are punished in some way, such as withdrawing the benefits in future.
Species which have mutual dependence in defence, foraging, territoriality etc. are most likely to develop reciprocal altruism, as in monkeys, baboons, chimpanzees and man. Kin selection and reciprocal altruism are sometimes found to coexist in many social groups of animals and at times it is difficult to distinguish between the two or measure them independently. Altruism is promoted by group selection but when it benefits close relatives it is promoted by kin selection.
This is a phenomenon in which food is offered by one individual to the other which is not its own offspring. This is very common in social insects where feeding is done by specialized individuals of the colony called workers. In chimpanzees distribution of meat among individuals after collective hunting of monkey has been recorded.
Wilkinson (1984) studied blood-sharing in vampire bats (Desmodus rotundus) in Costa Rica. The bats demonstrated altruistic behaviour by regurgitating blood meal and sharing it with others. Trophollaxis is essential for the survival of species which do not find enough food and starving individuals must be helped for the benefit of the species. Wilkinson also found that bats regurgitate food more frequently to relatives and rarely to non-relatives, since relatives are likely to reciprocate when they do not find meal themselves. This is called reciprocal altruism.
COST OF SELFISHNESS
Individuals living in groups enjoy the advantage of protection from the predators as some members out of hundreds will spot the predator and give alarm calls to alert the others. Similarly, prey hunted by a predator group can be shared by all members and sometimes even injured individuals that cannot hunt themselves can also get food. But this favour must be returned by individuals when their turn comes.
If the favour is not returned then the individuals are labelled selfish and will be selected out of the group. Herding protects the herbivore animals from predators but an individual straying away from the group will be killed by predators and eliminated from the population. Selfishness is therefore punished by natural selection. Selfishness is also found in the prides of lions where male lion kills all the cubs after dethroning a lion and taking over his pride.
This is done to bring the lionesses to oestrus so that he can have his own progeny quickly. Selfishness is also seen in protocooperation in which only one individual derives benefit, as in the case of suckerfish attached to the shark. Shark does not get any benefit but the suckerfish gets leftover food from shark’s mouth. Group selection and kin selection, therefore, demand faithfulness to the society and selfish individuals are selected out and eliminated from the population.