Adaptation and Optimality Theory

Transcription

1 Adaptation and Optimality Theory

2 Prisoner s Dilemma game: An optimality model What is the optimal strategy when you are playing with different people of unknown reputation? Defect: We call this the evolutionarily stable strategy. It is adaptive in a world with the rules defined by this game. Cooperators can t invade the system. Or put another way, in a system with cooperators, defectors will always invade. The strategy of defecting should not change even if you know your partner is a cooperatin type. Cooperators can t increase in a population of defectors because they always get 0. If group selection is in action, a cooperating group might beat a defecting group, but we re talking about individual selection here.

3 Hawk-Dove Game another model by John Maynard Smith Would you rather be a hawk or a dove? What factors will influence your preference? Hawk Dove Hawk Dove 50, -100 (loser is severely injured, possibly I am too) 0, 50 (I run away, you win) 50, 0 (I win, you run away) 25, 25 (We split 50 evenly)

4 Average Payoff to: A hawk in a population of hawks: ( )/2 = -25 A hawk in a population of doves: 50 A dove in a population of doves: 25 A dove in a population of hawks: 0 Is there an optimal ratio of hawks to doves, where payoffs are about even on average for hawks or doves? Optimal ratio is slightly skewed towards hawks. (When ratios are even hawks have a slight advantage, on average).

5 Solution to the Hawk-Dove Game: A mixed strategy. What does this look like in nature? Frequency Dependent Selection could maintain a stable genetic polymorphism in population Or, any individual could employ plastic development depending on the ratio of hawks to doves at birth. Non-reversible phenotypic plasticity! See forthcoming examples. Or, any individual can assess the situation and employ the strategy that best suits that situation. Reversible phenotypic plasticity! Examples: chameleons, learning, etc.

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7 Which genetic line shows greatest and most consistent phenotypic plasticity? Note: Plasticity itself is under selection. This is non-reversible plasticity in this case. Daphnia are genetically fixed to be plastic in their development or not, depending on whether they evolved in a lake with fish or not.

8 This is amazing: clones are taken from different layers of the mud that correspond to the years above. Depending on the environment (fish or no fish) associated with the year, there has been selection for different levels of plasticity.

9 Then transplant into a high density envi or low density envi.

10 What are the predictions of this experiment if plasticity is adaptive or if it is not adaptive? Could you draw a plot showing the predicted fitnesses of each phenotype in each environment (high density or low density).

11 Red Crossbill Note handedness in bills At least 7 different populations throughout N. America with distinct calls and bill morphology in N. America

12 Benkman 2003

13 Once you start to climb one peak, difficult to get to another because you need change in several correlated traits at once.

14 South Hills crossbills are not at bill depth optimum. Why? A trade-off

15 Which graph best represents a population with phenotypic plasticity? = genotype 1 = genotype 2 Phenotype a b Reaction norms Environmental State

16 Which is a population of ecological generalists and which is a population of ecological specialists? = genotype 1 = genotype 2 Fitness Environmental State Genotype by environment interaction

17 Nature vs. Nurture: Which graph demonstrates which as the main influence on phenotype?

18 Sexual selection Works just like natural selection Variation, heritability, differential reproduction It arises over competition for mates Two forms: Intersexual competition, aka choice Intrasexual competition Because getting your genes into the next generation is the ultimate goal, selection on traits that ensure this happens leads to some of the most bizarre, beautiful, and ingenious traits (physical or behavioral) known in biology!

19 male ornaments and weapons

20 Bower Bird ornaments an extended phenotype

21 Anisogamy unequal gametes this is the key to sexual selection. Eggs are expensive. Sperm is cheap. Females make a greater Parental Investment (PI) than males

22 By the numbers Splendid fairy-wren: Male: approximately 8 billion sperm per ejaculate Female: 6 eggs per nesting attempt. Salmon: Males: 100 billion sperm per batch of 3500 Female eggs. Humans: Every healthy male makes enough sperm in a month to (theoretically) fertilize every fertile woman in the world. Females have a few hundred cells that ever develop into eggs. Eggs are expensive, sperm is cheap: It s not just an analogy

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24 Robert Trivers: Key to sexual selection is parental investment (PI) = effort put into helping your offspring reach reproductive age. Typically: Female PI > Male PI 1. Sex investing less will compete for sex investing more (intrasexual selection) 2. Sex investing more will be discriminating in choosing mates (intersexual selection)

25 Example: How do you get this male seal onto a scale to weigh him? Put a plastic decoy female out and let him try to mate with it!

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28 Variation in RS in male vs. female elephant seals

29 Bateman s fruitfly experiment

30 Bateman s Principle Reproductive success on average equal between males and females VARIANCE in reproductive success differs between males and females

31 Females and males have very different perspectives on sex FEMALES limited by the number of eggs they can produce--more reluctant and extremely choosy. MALES limited by the number of mates they can encounter--more ardent and indiscriminant.

32 *Female PI > Male PI, Female chooses *Males selected to impress, females selected to assess But, what if Male PI > Female PI? Female PI = Male PI?

33 Choosy males: Mormon crickets (neither mormon nor crickets) Male produces large spermatophore. 25% of body weight! In this case, sperm is expensive. Do they get a benefit from this? (extra 18 eggs! >50% more than if they d mated with rejected females)

34 Which species below represents a sex role reversed species? Choice One 2. Choice Two 50% 50% 1 2

35 Evolution of elaborate male traits: The handicap principle Females are interested in signals that are costly to produce Only high quality males can produce the signal (so a low quality male can t fake the signal) Examples: Long tails that make flight more difficult Weight lifting Song repertoires that require larger brain capacity (also correlated with nutrition)?

36 Example: Red-collared widow bird males Cost to tail is demonstrated here: Experimentally shortened tail Body condition Control (normal long tail) Only the healthiest (genetically superior) males can sport the longest tails Time Natural selection will set a limit on tail length Females prefer long tailed males (why? It is an honest indicator of male quality) Tail length has nothing to do with territory quality. Males with shortened tails did just as well at obtaining a territory. (Pryke and Andersson 2005)

37 Black Wheatears Males carry stones up to a quarter of body mass -this signal cannot be faked! # of stones correlates with earlier laying date and greater production of fledglings. But females probably cue in on size of stones: Males carrying largest stones have highest T cell count (greater immune capacity) is this heritable? If so, this would confer an advantage on offspring.

38 Evolution of elaborate male traits: The runaway model Traits have no cost at least not initially Traits are initially correlated with reproductive benefit Trait and preference for the trait become linked, i.e. because males who have the trait and females who choose males with the trait leave more surviving offspring offspring inherit the trait and the preference together. Males with the most exaggerated traits are being chosen preferentially for breeding, and their offspring are more viable. The next generations always consist of males with more and more exaggerated traits (and females that carry the genes). It s sort of like the selection experiment on oil content in corn, breeding only those with the most exaggerated ornament. So it s a positive feed back loop. Hence runaway.

39 Propaganda? Honest signal? Runaway Selection? =VjE0Kdfos4Y

40 Males are randomly paired with females. Eggs are incubated and offspring raised in a common garden to control for environmental effects and maternal effects. Eye spot area is correlated with survival for some reason. Probably linked to other more beneficial traits this starts the runaway process. No initial cost demonstrated therefore, not a handicap model. Petrie 1994