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Natural Selection Or, how did we get here…. Natural Selection • The Theory of Natural Selection is so simple that anyone can misunderstand it…. (Anonymous) • Charles Darwin (1809-1882) saw three problems in need of a solution. – Darwin was not the only one to see these problems BTW – Other ‘Naturalists’ were struggling with the same issues Problem the First • There is change over time in the flora and fauna of the Earth – What we would commonly call ‘evolution’ today – The fossil record showed this to be pretty clear, even to people in the mid 1800s – This was not controversial in Darwin’s time, and is not now. The Second Problem • There is a taxonomic relationship among living things – People were big into classifying stuff – It was pretty obvious that there was a relationship between different species • Different birds, different grasses, different cats etc The Third Problem • Adaptation – Different kinds of teeth for different animals, say carnivore ripping teeth and herbivore grinding teeth – Different tissues within species • Heart vs. eye etc. The Solution! • Natural Selection provides a mechanistic account of how these things occurred and shows how they are intimately related. • It is one of those ‘oh man is that ever easy, why didn’t I think of that?’ type things. How’s it work? • There is competition among living things – More are born or hatched or whatever, than survive and reproduce • Reproduction occurs with variation – This variation is heritable – Remember, there was NO genetics back then, Chuck knew, he just knew…. – Realized that is wasn’t ‘blending’ How’s it Work? • Selection Determines which individuals enter the adult breeding population – This selection is done by the environment – Those which are best suited reproduce – They pass these well suited characteristics on to their young How’s it Work? • REPRODUCTION is the key, not merely survival • If you survive to be 128 but have no kids, you are not doing as well as I am • I have reproduced… • Assuming the traits that made me successful will help them then I amore fit NOW than the 128 year old guy This lecture keeps evolving….. • Survival of the Fittest (which Chucky D NEVER said) means those who have the most offspring that reproduce • So, the answer to the trilogy of problems is: • ‘Descent with modification from a common ancestor, NOT random modification, but, modification shaped by natural selection’ Different types of selection • Directional Selection • What most of us think about when we think about selection • An extreme value is selected for • Human brain size is a nice example Different kinds of selection • Stabilizing or Normalizing selection • The middle is selected for • Many examples here – Symmetry – Two eyes Different kinds of selection • Disruptive selection • Extremes are selected for • Might be where the two sexes come from • Trait was probably gamete sized • Two ‘mating types’ But… • Darwin was troubled by the preponderance of behaviour that seemed to be of no benefit to the actor • Examples – Sterile insect castes – Bee stings – Alarm calling – Reproductive restraint Reproductive restraint • Birds can produce many more eggs than they actually do! • Wynne-Edwards beleied that selection also acted on the species level to stop massive overpopulation Reproductive Restraint? • Lack argued that animals are doing what is best for the INDIVIDUAL • Big clutch means possible exhaustion, death • LIFETIME fitness, not just this season Lack • Well, if clutch size depends on the environment, then clutch size should vary with quality of resources Hamilton figured it out • Hamilton’s Response was theoretical • Look at behaviour from the gene’s point of view • Inclusive fitness = direct fitness + effect on fitness of others Hamilton • C < rb • Or r > c / b • Remember, r is relatedness, c is cost b is benefit • Would you give your life for a brother? • No, but maybe 2 brothers, or 8 cousins • Two uncles and one mother…. Mechanism • For this to work there must be some sort of proximate mechanism • Easy for mother/father and child • How about everyone else? • ‘Green Beard’ hypothesis – Allele produces some phenotype – Also allows for recognition • Tough for a single gene…. The elusive mechanism • Proximity – Families – Little dispersal – So if close by, you are related • Example: – Leaf eating ants – Far nests more hostile – Different plants more hostile More on mechanism • Those crazy ants – Split colony and put on different plants – Hostile, but non injurious! • Gene environment interaction baby! • Vervet monkeys – 2 year olds scream – Mother comes – Others watch the mother! Mechanisms, pheremones and bees • Greenberg looked at bee relatedness and allowing bees in or not • Probably genetically determined odour Conclusions about inclusive fitness • • • • • • Group selection is silly Individual selection is cool Gene level selection is VERY cool Hamilton is a genius Don’t over use this Gives us some insight into some nasty human behaviour Games are fun • Animals tend to behave in ways that maximize their inclusive fitness • Usually pretty straightforward • But, sometimes we must know what others are doing before we adopt a strategy • What if your mating call is drowned out by others’ calls, what to do, ahh what to do… Fitness and Strategies • In certain cases payoffs, and hence fitness maximization, depend on what other populations are doing • When the payoff to one individual depends on the behaviour of others we cannot use the principle of fitness maximization until we know: – What the alternatives are – P(encountering alternatives) – Consequences of encounter Game Theory • Think of it like a game • Each individual’s behaviour is its strategy, payoffs are in units of fitness • Players produce more players (offspring) • Changes in fitness are directly proportional to payoffs • An evolutionary Stable Strategy is one that, when adopted by enough individuals, maximizes payoff Pure Strategy • One that cannot be replaced • Food storing • Recover your own seeds (Anderssen and Krebs, 1978) • If they recovered communally, a selfish hoarder would replace the communals damned quckly Mixed Strategies • Hawks and Doves – Not real hawks or doves, strategies • • • • Always fight, or always give up Look at the payoffs Look at the costs Determine what proportion should be hawks and should be doves Hawks and Doves • • • • • • • Say its all Doves Hawk shows up, wins resource Spreads genes Now more hawks Oh oh, now you are fighting, P(injury) = .5 Now being a dove pays Either strategy good when rare, bad when common Doves and Hawks • • • • V = Value of resource for winner W = cost of a wound T = cost of display (no fighting) (John Maynard Smith, 1978) Whoa, I know Kung Fu • Set up a payoff Matrix Opponent in the contest Hawk Dove Payoff Hawk ½(V-W) V Received By Dove 0 ½V-T ESS as easy as 123 • If W > V then there can be no pure ESS – In a population of hawks, a small number of doves do better than hawks • E(dove,hawk) > E(hawk, hawk) • E(dove, hawk) = 0 • E(hawk, hawk) = ½(V-W) – W > V, therefore ½(V-W) < 0 Pure Doves don’t do it either • Payoff to Hawk is V • Payoff to doves is less than that – (½W – T) • Hmmm • So, what proportion of hawks and doves balances it out? What is theoretical population biologist to do? • Find the proportion (p) of hawks of hawks such that the following equation balances: • p ½(V-W) = (1-p) V = p (0) + (1-p) (½V– T) • Simply (?) solve for p • p = (V+2T) / (W+ 2T) Apply it, sort of • Say V = 10 • W = 20 • T=3 Opponent in the contest Hawk Dove Payoff Hawk -5 10 Received By Dove 0 2 Now, sub that back into the formula • P = 16/26 or 8/13 • 8/13ths of the population, with these payoff values, must be hawks • The values are not that important really, the point is that you can determine the point at which a strategy can coexist with another strategy as an ESS • Could be percentage of population, or percentage of time each animal adopts a given strategy So? • It is actually applicable that’s so • Toads looking for breeding grounds (Davies and Hallaway, 1979) • Payoffs determined Another so • Dungflies • Should a male hang around poo as it gets older? Conclusions • This is a very brief intro to game theory • This stuff is way powerful • You have to sit and think some about the payoffs and costs • Dynamic programming models are becoming more popular Other Evolutionary Theories • Lamarckism – Inheritance of acquired characteristics • E.g., giraffes really wanted leaves, so they stretched their necks and….. • Sounds crazy, but a lot of people think this way • ‘We will all have giant heads and tiny bodies someday’ • ‘Cave swelling fish don’t use their eyes so they disappear’ • ‘We don’t use our appendix so it is disappearing’ Silly incorrect evolutionary theories and ideas • Orthogenesis – – – – There is some plan to evolution. NO WRONG INCORRECT, THANKS FOR PLAYING The idea of an ‘evolutionary ladder’ fits in here It is wrong too…… Still another silly idea • Intelligent Design • Just Creationism with a fancy name • God does not belong in a science class, any more than experiments belong in church • NOT A SCIENTIFIC THEORY