Lecture 2: The Sometimes Counter-Intuitive Darwinian Logic of how Natural Selection Acts on Behaviour


The below set of notes is from APS209: Lecture 2: The Sometimes Counter-Intuitive Darwinian Logic of how Natural Selection Acts on Behaviour.

Group selection

Group selection is the idea that certain characteristics of individuals are adaptive not at the individual level (i.e., helping individuals to survive and reproduce) but at the group level (i.e., helping groups survive and reproduce). The group-selected individuals would be doing something good for the group but bad for their own reproduction, such as exercising self restraint so that the resources on which everyone depends are not overexploited. The problem is that individuals that don’t show self-restraint will prosper within the group. To counter this, and so keep the self-restraint genes from going extinct, there must be an advantage at the group level. That is, groups with more self-restraining individuals survive or “reproduce” (i.e., divide to produce several groups) better. The main problem is not that group selection can’t work in principle (because it is logical and can work), but that the dynamics of actual groups are not suitable. Groups are not sufficiently well differentiated from each other, nor do they die out and have offspring groups at sufficient rates, to make between-group selection a significant evolutionary force.

Many behaviours that were formerly given a group selection interpretation, can be better explained by individual selection. One example is infanticide. Male lions and langurs kill unweaned offspring so that their mothers become sexually active sooner. In this way the male can start reproducing sooner and increase his lifetime reproductive success. Infanticide also occurs in meerkats (Young & Clutton Brock 2006). It is carried out by pregnant females on the newly born litter of another female. Meerkats live in groups, and different sets of offspring would compete for limited resources to feed the young. So infanticide can benefit either individual males or individual females depending on circumstances.

Counter-intuitive logic

If a behaviour helps an animal to survive or to reproduce then this is what we expect from natural selection. But selection can also result in counter-intuitive or unusual behaviours, as is seen in organisms that sacrifice their lives. Thinking about these behaviours and how they could be selected for will deepen your understanding of both natural selection and animal behaviour. In addition, many of the examples are interesting in their own right.

When a honeybee, Apis mellifera, worker stings a vertebrate intruder her sting detaches from her body. It becomes caught in the flesh of the intruder by means of backward-pointing spines. The sting itself exhibits complex adaptive behaviour even when detached from the bee. The sting contains muscles, a nerve ganglion, a venom sac, and an alarm pheromone gland. The muscles and nerves cause the 3 sting stylets to drill into the intruder’s flesh and to pump venom. The alarm pheromone, which is very volatile and smells of bananas (the main chemical is isoamyl acetate, which is the flavour in some sweets like pineapple cubes), guides other worker bees to the intruder and makes them much more prone to sting. That is, it “releases” highly defensive behaviour. Alarm pheromones are highly volatile so that they spread but then dissipate rapidly. The honeybee worker is selected to sacrifice her life for two reasons. First, she is protecting her family hence helping to pass on copies of her genes by helping her family to survive a predator attack. Second, the defence is more effective when the sting can continue its work when detached from the worker.

Another example of self-sacrifice is the male red back spider (Andrade 1996), who somersaults into the female’s jaws during copulation. If a male is eaten during copulation, it prolongs the copulation, and increases the male’s paternity via two mechanisms. If the male is the first male (female is virgin), then the female is less likely to remate. If the male is the second male (female already mated) he will father a greater proportion of the offspring.

Reproduction is in large part a cooperative act between mating partners. But there is also conflict. In particular, the male usually benefits if he can reduce the chance that the female re-mates. The male redback spider does this by sacrificing his life to prolong the copulation, which presumably results in greater sperm transfer. Male bean beetles Callosobruchus maculatus adopt a different strategy (Crudgington & Siva-Jothy 2000). A male damages the female’s genital tract with his spiny penis during copulation. Females who mate twice die sooner than those who mate with one male. Females try to shorten the mating by kicking the male off, and females with their kicking legs removed have longer matings. Males may damage females in other ways, such as toxic seminal fluids, which may benefit the male but not the female.

Unicellular: slime moulds appear to exhibit altruism during sporulation. Those forming stems will die. Those forming fruiting bodies will propagate. Because these microorganisms are genetically similar, it is beneficial for some to sacrifice such that others can duplicate. Such instinctive behaviour is detrimental to the sacrificing individuals but favors the survival or spread of their genetics by benefitting its relatives. This enhances the reproductive fitness of genetically-related kins.

Multicellular: Reciprocal altruism occurs in social animals with mechanisms for recognising individuals. This cannot be considered kin selection as the individuals do not share genotypes. There is a safeguard or punishment to prevent cheaters from abusing the altruistic behaviours of others. Such as vampire bats. Feeding at night on victims whose skin is penetrated. Usually overfeed until they are almost unable to fly. Roost in communal caves with related and unrelated bats. Those that don’t feed will die. Lick the faces of other individuals, to beg for blood. Sharing blood meal. Repay the help next time. Only involves permanent members of the community.

Can increase fitness, because cost of sharing is low when stomach is full. Benefit of receiving food in unsuccessful nights is high, because it avoids starvation. Cheating (only receiving but not giving food) unlikely because the bats know each other.

Altruism (Temporary help, eusociality (haplodipoidy), reciprocal altruism)

Temporary help (kinship selection)

Permanent help (eusociality)

References Andrade, M. C. B. 1996. Sexual selection for male sacrifice in the Australian redback spider. Science 271: 70-72. Crudgington, H., Siva-Jothy, M. T. 2000. Genital damage, kicking and early death. Nature 407: 855-856. Young, A. J., Clutton-Brock, T. 2006. Infanticide by subordinates influences reproductive sharing in cooperatively breeding meerkats. Biology Letters (published on line).