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 ( ) 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 HawkDove PayoffHawk½(V-W)V Received ByDove0½V-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 HawkDove PayoffHawk-510 Received ByDove02

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