Reading: Chapter 12 pg.

For all laboratory sections:Remember to complete problem set 1 (problems 1-15) for laboratory next week.

Problems can be found on the last pages of the supplement.

You should read the laboratory on demography (pg. 36 of the supplement) and, if possible, begin gathering data in your working groups from one or more cemetaries.

Why do most species live in groups?

There are many reasons for species tending to occur in groups:

• increased chance of surviving - e.g.probability of being predated lower on average, most predation occurs on animals at the periphery of the group

• increased chance of finding a mate -higher density in groups than if scattered

• increased chance of finding food and feeding -some keep watch while others feed, jobs exchanged over time, everyone gets more time to feed

There are costs, as well as benefits...

When each individual has to defend itself,it has to look around for predatorsfrequently. When group defense occurs,each individual has to check much lessfrequently, even though the group as awhole has increased its defense.

Therefore, each can spend more time feeding.

Even with the limited range of flock sizesexamined here, increased numbers meanmore effort and time to find foodresources.

This is one form of altruism that is seemingly easy to understand…

It is called reciprocal altruism.

In an unrelated group of birds, e.g. crows or the graphicalexample shown...

One bird acts as a sentinel for the group. The remainder forage.

There might be a small chance that a predator would find and attack the sentinel, since it ‘crows’ to warn others. The reciprocal benefit gained is the long stretches of time when this crow can feed while others act as sentinels.

Meerkats and the effect of groups on foraging and group defense

The advantage, measured by the presence of at least one sentinel first increases when flock size is small…

but decreases again when flock size is large.

The cause: rapid decrease in local food supply means more time the group spends searching for and moving between patches of food.

Effect on donor fitness-- +

Effect on -- spitefulness selfishnessrecipientfitness + altruism cooperation

There are numerous observations of cooperation, bothwithin related groups and among unrelated individuals. That’s logical from an evolutionary (fitness) perspective.There are no observations of spitefulness that I know of.That, too, is logical. So is selfishness.

We can make a 2 x 2 table of the possible interactions of individuals in a group, measured by the effects of the interaction on the fitness of participants…

Altruism is a more difficult question.

To understand altruism that is not reciprocal, you need to understand the idea of inclusive fitness.

Evolutionary fitness is measured by the numbers of copiesof ‘your’ genes in the subsequent generation compared tothose of others.

Note that it doesn’t matter whether you or a relative supplied those genes.

An extreme example: You are one of 4 children of a family in a war zone. You are playing together when some-one throws a live hand grenade through your door. Wouldyou achieve higher fitness by jumping through a windowor jumping on the hand grenade?

The answer: If you survive, you leave behind onecopy of your genes.

Jump on the grenade! If you act in an altruistic way, each of your siblings shares (on average) ½ of your genes by descent from the parents. Saving 3 of them would leave 1½ copies of your genes, and give you a higher inclusive fitness.

To assess the contribution to inclusive fitness made by arelative, you need to know the coefficient of relationship(the fraction of your genome shared by descent from ancestors in common).

Proportion shared

Self 100%father 50%mother 50%full sibling 50%half sibling 25%offspring 50%grandparents 25%niece/nephew 25%uncle/aunt 25%first cousin 12.5%

Again, cost/benefit ratios determine whether altruismshould arise in the behaviour of an individual.

Altruism should arise when the cost to the donor of thebehaviour is less than the benefit to recipient(s), measured by the summed change in their individual fitnesses multiplied by their coefficients of relatedness (in other words their contribution to your inclusive fitness)…

Or C < Br

where C is cost, B benefit, and r the relatedness of thosewho ‘receive’ the behaviour

Similarly, relatedness affects the likelihood of selfishbehaviour. Since it has negative impact on recipients, if theyare related, it reduces inclusive fitness…it should evolveonly when benefits exceed costs (to the relatives)...

or B > Crrearranging… C/B < 1/r

Selfish behaviour occurs only in this region

Altruistic behaviour can evolve in this region

There is a name for the pattern of evolution indicated inthese cost/benefit relationships…

Kin Selection

Hamilton’s definition of kin selection:

...selection operating between closely related individualsto produce cooperation

Closely related individuals are more likely to benefit from(pseudo)altruistic behaviour than distantly related ones.

Fisher, who has been critically important in the developmentof so much of modern population genetic theory, anticipated ideas about kin selection at least 30 years inadvance of anybody else…

Think of warning colouration in insects, e.g. the larvae ofthe monarch butterfly. It contains toxins poisonous tobird predators. The colouration warns them off…

but how could this evolve? The first brightly coloured larvae would have attracted predators, and suffered totalloss of individual fitness. Siblings (and they would be numerous) are ‘protected’ by bird learning. Inclusivefitness increases through kin selection.

Another example of kin selection is helping at the nest inwhite-fronted bee eaters in Kenya…

These birds live in extended, multi-generational family groups of from 3 – 17 birds.

Interactions among members of these families follow the predictions of theory closely.

Siblings help each other, but cousins are unlikely to be helped, and are treated much like non-relatives.

These birds nest colonially. Helpers tend to assist closerelatives more often than distant relatives or unrelated birds.

Breeders coeff. of relatedness % cases to a particular offspring

father x mother 0.5 44.8father x stepmother 0.25 9.8mother x stepfather 0.25 9.2son x nonrelative 0.25 10.3uncle x nonrelative 0.125 1.1grandmother x nonrelative 0.125 0.6

From the occurrence of kin selection and the occurrence ofunrelated groups living together arose the notion ofgroup selection.

The usual, commonsense view has a fatal flaw…

“Robins (or any other bird) lay fewer eggs in a drought year because competition for limited food supplies would be detrimental to the group…”

but under those conditions in such a group, a cheater who laid more eggs would have a higher fitness.

The simplistic view of group selection does not makeevolutionary sense, since it works in the opposite directionas individual selection.

There are two recent approaches to group selection thatcould work:

1) a population exists as a set of groups. Among those groups, isolated, selfish subgroups must go extinct faster than selfishness arises among initially altruistic subgroups. Most newly founded subgroups must be altruistic. Classical Darwinian selection says that eventually selfishness will develop.

2) Populations come together and separate into subgroups. Some subgroups have different rates of survival and/or reproduction. More successful groups contribute larger numbers to the population at re-coalescence.

Group selection is controversial because conditions thatlead to cooperation among unrelated individuals are veryrestrictive in evolutionary terms. Self-interest is thedominant force in Darwinian selection.

Ricklefs uses game theory, and the hawk-dove game todemonstrate this.

Hawks always behave selfishly in conflicts.Doves never compete for resources, instead sharing them evenly with other doves.

Now consider the costs and benefits in this system whenindividuals must share or compete for resources.

The payoff to a ‘contestant’ depends on the behaviour ofthe other individual…

2 hawks fight over resources - each will end up with halfthe benefits less the cost of the physical conflict, or

1/2B - C

when a hawk and a dove conflict, the hawk gets it all, the dove gets nothing, or the hawk gets

B

when 2 doves share a resource, there is no physical conflict,so each gets half at no cost, or

1/2B

Who is better off? It depends on the proportions in the population. If the population is all hawks, then each gets

1/2B - C

If the population is all doves, each gets 1/2B, a higher rewardand doves have the advantage.

If the population is a mixture of hawks (proportion p of thetotal) and doves (proportion (1 - p), then the reward to ahawk is:

p (1/2B - C) + (1 - p) Band to a dove:

1/2 (1 - p) B

You can try to solve this for equilibrium, and you will fail.Why?

Try thinking about the rewards to a single hawk in a population otherwise comprised of doves. All its encounterswill be with doves, giving a reward of B, while doves willalmost always encounter other doves, and get a reward half as large, 1/2B

This sort of result indicates that selfish behaviour ultimatelywins, and a mixed population is an evolutionarily unstablesystem. Dovish behaviour is an evolutionarily unstablestrategy.

The text demonstrates this graphically...

Is there any circumstance that can lead to persistence of amixed strategy?

Yes. If the cost of the contest to hawks is very high. Hawkishness is advantageous unless the cost is greater than1/2B. If B < 2C then doves can invade an all hawkpopulation…an equilibrium mixture exists as an evolutionarilystable strategy.

A last form of group/society structure - the eusocial insects

Almost all eusocial species are hymenoptera.They have certain characteristics in common:

1) adults live together in groups (e.g. hives)2) the group includes overlapping generations (parents

& offspring)3) there is cooperation among members of the group in

efforts supporting reproduction4) there is reproductive dominance by (at most) a few

individuals (frequently one individual)

What makes this remarkable is the sacrifice of individualfitness by whole sterile castes. This can be explained onlyby kin selection.

As an example, the structure of a bee colony…

There is one queen, only she is reproductively active as afemale.

She only mates once, gathering enough sperm to continueproducing fertilized eggs through her lifespan.

A) Some of her eggs undergo the equivalent of partheno-genetic development - no fertilization and a haploid genome. They become ‘male’ drones.

B) Some eggs are fertilized, but exudates produced by thequeen arrest their development prior to sexual maturity. Theyare ‘female’ workers.

C) Some (only a few) are specially fed and allowed to mature.They will disperse to found new colonies as queens.

Why should drones be dispersed, and a female biased sex ratio (by weight) occur in a colony?

Kin selection…

The queen has a coefficient of relatedness of 0.5 tooffspring of either sex.

A female worker has a coefficient of 0.25 with malesiblings, but a coefficient of 0.75 with femalesiblings (sperm is produced by mitosis, therefore all sperm are genetically identical.). She is more closely related to femalesiblings than she would be to her ownoffspring. Kin selection is, therefore, thelogical result.

Parent-offspring conflict…

A parent would maximize its fitness by producing as manysurviving offspring as possible (and considering bothcurrent and future reproductive value)…

but the offspring is best served (in the selfish sense) bybeing given more resources, and having what is availabledivided up among as few offspring as possible.

Think about this and consider what’s happening between a plant parent and the offspring (seeds, fruits, nuts, …) it’s producing simultaneously. What control does a parent have? What might an offspring do to get a larger share of resources?