week 7- evolutionary stable stratergies Flashcards
what does the success of an organism depend on
how its behaviour interacts with that of others
how must fitness be evalutated
in the context of the population in which it lives
key terms of the game theory
Players: individuals playing a strategy in the game
Strategies: the possible actions each player can take
payoffs: the outcomes or rewards resulting from the combination of players strategies
maximisation: assumes players maximize their playoffs (fitness)
interdependence: each players payoff depends on the strategies chosen by all players
equillibrium: a state where no player can benefit by changing their stratergy while others keep theirs unchanged (e.g. nash equilibrium)
what is the game theory
introduced into evolutionary biology by john maynard smith and george price in 1970s, after pioneering work by bill Hamilton
adapted from game theory as developed in economics in the 1950s
what is the evolutionary stable stratergy (ESS) and biological analogy of it
Strategy A is an ESS if:
-when A is rare, it can invade a population in which another strategy is common
-when A is common, it cannot be invaded by any other strategy present in the game
biological analogy of it:
if A arises as a rare mutant, it will increase in frequency by selection. if A is common in the population, rare mutants will not increase in frequency
what is ESS concerned with vs allele dynamics
The stability of strategies (phenotypes) in a population, while allele dynamics focuses on the genetic level and how allele frequencies change
what is ESS used in the context of vs allele dynamics
behavioural/phenotypic stratergies and their evolutionary stability, whereas allele dynamics are used to understand genetic changes and evolutionary forces acting on populations
explain body size ESS in beetles
summary
-a few large beetles introduced into a population consisting of small beetles
-large beetles will do really well:
-they rarely meet each other
-they get most of the food in most competitions
-population of small beetles cannot drive out the large ones
-so small is not evolutionary stable
-a few small beetles will do very badly as lose almost every competition for food
-a populationm of large beetles resists the invasion of small beetles
-large is therefore evolutinoary stable
look at ppt more for in depth
explain hawk-dove game and ESS
table on ppt
parameters:
V= value of resource
C=cost of living
strategies
hawk= always fights
dove= displays, retreats
if V>C:
hawk is the ESS
if V<C:
no pure ESS
what does hawk dove game mean biological
in order for lethal fighting ti be stable the value of the resource must be very high
the greater C relative to V the more it pays to display rather than fight
mixed ESS:
a population with equillibrium of hawks and doves can be stable
a population of individuals who play hawk some percentage of time and dove some percentage of the time can also be stable
explain sex ratio- a mixed ESS
every individual has one mother and one father so each sex contributes equally, on average, to subsequent generations therefore males and females have the same average fitness
if 25% males and 75% females who has higher fitness
males will have high fitness because they mate with multiple females and vice versa
members of rarer sex experience increased reproductive success relative to common sex
frequency-dependent selection results in stable equillibrium 1:1 sex ratio- a mixed ESS
explain rock paper scissors game (orange, yellow, blue lizards)
-orange lizards are territory holders
-yellow lizards are female mimics
-blue males hold territories but unlike the orange ,ales, blue males can recognise yellow males and kick them out
-orange can recognise blue males, and since orange is more aggressive, they can increase when rare when blue is common
-the system is expected to cycle
explain sequential assessment cycle
-individuals benefit by avoiding fights they are likely to lose (essentially they asses whether V<C)
-honest signals of size and strength: resource holding potential/power
-parallel walk and roaring contests in stags
-head-butting in rams
-eye-to-eye comparisons in flies
sequential assessment:
-Individuals start with the least costly display and escalate until they have enough information about the opponent to decide whether fighting will pay off
what are two factors of sequential assessment game
probability of winning
-correlated with specific strategy or measurable individual character state
resource holding power (RHP)
-experience
-size
-physiological state
animals should be able to measure RHP
why did fight duration change in shore crabs
fight duration was highest with equal competitors
fight duration decreased with decreasing resource value
probability of winning is highest if opponent is smaller
summary of fighting in shore crabs
-effect of resource value on fight duration
-effect of RHP on fight duration: size-matched crabs fight longer
-no effect on fight intensity (number of strikes)
war of attribution without assessment (WOA-WA):
male competition in dung flies
-soon after a cowpat has been deposited, female dung flies lay eggs
-males accumulate at the pat to intercept females and copulate with them. males guard females from pother males until eggs have been laid
-as time goes on pat dries and less suitable for egg laying, so arrival rate of females declines
-problem facing males is to decide how long to stay at the pat
-answer depends on what other males are doing
-if most males stay for short time, might pay a male to stay for longer and copulate with late-arriving females
-if most males stay longer then males will move to new pat and mate with early arriving females
-random distribution suggests that males stay randomly distributed lenghts of time predicted by WOA model
-mixed ESS of randomly distributed stay times
charachteristics of WOA-WA
-non-injurious agnostic behavour
-waiting games
-decision to give up fight based on level of costs accured
-WOA-WA: costs in terms of time or energy expenditure
-loser ends fight when its cost have exceeded its individual threshold
-RHP is therfore equivalent to a threshold of maximum cost that an individual can allocate to the contest
-threshold can be influenced by resource value
explain energetic WOA (e-WOA)
-no link between time and energy expenditure
-rates of costs accrued can differ between individuals
-contests are about stamina
-no mutual assessment
-very difficult to distinguish from WOA-WA
explain asymmetric WOA (A-WOA)
-contestants value resource differently
-mutual assessment: maximum potential endurance is advertised
-as in sequential assessment game, contest duration increases with RHP symmetry
explain cumulative assessment game (CAM)
costs of fighting accumulate in two ways:
1. energetic and time costs
2. costing inflicted directly e.g. injuries
-no mutual assessment
what is the bourgeois stratergy
-animals do not always behave the same; animals sometimes seem to exhibit “respect for ownership”
-without anthromophising, can it be determined if it is evolutionary stable
bourgeois:
-play hawk if territory owner, play dove if intruder
-assume that bourgeois is a territory owner 50% of the time
bourgeois payoff when focal is dove:
1/2 x0 +1/2 x V/2 = V/4
is bourgeois an ESS
yes
if 0<V<C
