2Natural Selection It is a two step process: The unequal survival and reproduction of organisms due to environmental forces, resulting in the preservation of favorable adaptations.It is a two step process:1. The production of variation in a population2. Non-random aspects of survival and reproduction
3Natural Selection is a two step process: Step One: The Production of Variation. (Chance)MutationsMeiosis:recombination due to crossing-over in 1st divisionrandom movement of chromosomes in 2nd divisionRandom mate selection & fertilizationStep Two: Non-random aspects of survival and reproductionSuperior success of certain phenotypesNonrandom mate choice
6Humans select traits for dogs, pigeons and other animals when they breed them. Bred Pigeons came from a single original speciesWho selects the traits for wild plans & animals?
7Natural Selection is Survival of the fittest NOBODY!!There is no agent involved in natural selection.Natural selection is a process of eliminationINDIVIDUALS THAT HAVE TRAITS THAT ARE BEST ADAPTED FOR THE CURRENT ENVIRONMENT ARE THE ONES THAT SURVIVE TO BREED AND PASS ON THEIR GENES TO THE NEXT GENERATION.Organisms not possessing the beneficial traits either die or don’t have as many offspring.Natural Selection is Survival of the fittest
8Natural Selection is a mixture of both Chance and necessity Natural Selection is not goal directed. It does not have a long term goal.
9What acts as a selection pressure on a population? Competition for foodCompetition for a mateChanges in the environmentPredatorsParasitesExample of Natural Selection in Action: Monarch / milkweedCard gameVideo of Darwin’s Finches
10Main Types of Selection Pressures Directional SelectionNatural selection favors one extreme of the population for that traitoften happens when environment changes in a consistent way- e.g.climate gets colder.Disruptive SelectionNatural selection favors both extremes selectedCauses species to divergeStabilizing SelectionNatural selection favors the average for population selectedSexual Selection
11Directional Selection Neck of GiraffeAntibiotic resistance of bacteriaMoth color (melanin)Camouflage/MimicsMany sexually selected traits
13Stabilizing Selection When the extremes of the trait aren’t as well suitedExamplesbird clutch sizeElk Antlers sizeGiraffe neck lengthTail length in birds
14Disruptive Selection Causes divergence within the species Occurs when two different types of resources in one areaResults in specialization for each branched groupMay lead to formation of new speciesE.g. Darwin’s Finches
16Examples of selection pressures... Predators - variants with adaptations allowing them to escape predators have more offspring- e.g. speed, defensive weapons, camouflage, mimicryPrey/Food - variants with adaptations allowing them to obtain food have more offspring- e.g. Speed, senses for finding prey/food, weapons for killing prey or obtaining food, camouflage for stealthClimate - those who can survive new climate best have more kids- e.g. ice age, change in climate due to migration.Mates - variants with adaptations allowing them to attract a mate to have offspring- e.g. strong, attractive, good provider
17Example #1: Escaping Predation Peppered Moth (see video clip) Early trees had light-colored barkOnly the light-colored moths survived. Selection was for less melanin.After industrialization, the tree bark was darker.Only the darker colored moths now survived. Selection was for more melanin.New info on the Pepper Moth experiment
22Natural SelectionThe 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 BTWOther ‘Naturalists’ were struggling with the same issues
23Problem the FirstThere is change over time in the flora and fauna of the EarthWhat we would commonly call ‘evolution’ todayThe fossil record showed this to be pretty clear, even to people in the mid 1800sThis was not controversial in Darwin’s time, and is not now.
24The Second ProblemThere is a taxonomic relationship among living thingsPeople were big into classifying stuffIt was pretty obvious that there was a relationship between different speciesDifferent birds, different grasses, different cats etc
25The Third Problem Adaptation Different kinds of teeth for different animals, say carnivore ripping teeth and herbivore grinding teethDifferent tissues within speciesHeart vs. eye etc.
26The 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.
27How’s it work? There is competition among living things More are born or hatched or whatever, than survive and reproduceReproduction occurs with variationThis variation is heritableRemember, there was NO genetics back then, Chuck knew, he just knew….Realized that is wasn’t ‘blending’
28How’s it Work?Selection Determines which individuals enter the adult breeding populationThis selection is done by the environmentThose which are best suited reproduceThey pass these well suited characteristics on to their young
29How’s it Work?REPRODUCTION is the key, not merely survival
30This lecture keeps evolving….. Survival of the Fittest (which Chucky D NEVER said) means those who have the most offspring that reproduceSo, the answer to the trilogy of problems is:‘Descent with modification from a common ancestor, NOT random modification, but, modification shaped by natural selection’
31Other Evolutionary Theories LamarckismInheritance of acquired characteristicsE.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’
32Silly incorrect evolutionary theories and ideas OrthogenesisThere is some plan to evolution.NO WRONG INCORRECT, THANKS FOR PLAYINGThe idea of an ‘evolutionary ladder’ fits in hereIt is wrong too……
34Problem 3If you observe a population and find that 16% show the recessive trait, you know the frequency of the aa genotype. This means you know q2. What is q for this population?
35Evolution of populations Evolution = change in allele frequencies in a populationhypothetical: what conditions would cause allele frequencies to not change?non-evolving populationREMOVE all agents of evolutionary changevery large population size (no genetic drift)no migration (no gene flow in or out)no mutation (no genetic change)random mating (no sexual selection)no natural selection (everyone is equally fit)
365 Agents of evolutionary change Gene FlowNon-random matingGenetic DriftSelectionMutation36
37Application of H-W principle Sickle cell anemiainherit a mutation in gene coding for hemoglobinoxygen-carrying blood proteinrecessive allele = HsHsnormal allele = Hblow oxygen levels causes RBC to sicklebreakdown of RBCclogging small blood vesselsdamage to organsoften lethal
38Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = HbHsunusual for allele with severe detrimental effects in homozygotes1 in 100 = HsHsusually die before reproductive ageSickle Cell:In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite.Cystic fibrosis:Cystic fibrosis carriers are thought to be more resistant to cholera:1:25, or 4% of Caucasians are carriers CcWhy is the Hs allele maintained at such high levels in African populations?Suggests some selective advantage of being heterozygous…
39MalariaSingle-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells123
40Heterozygote Advantage In tropical Africa, where malaria is common:homozygous dominant (normal)die or reduced reproduction from malaria: HbHbhomozygous recessivedie or reduced reproduction from sickle cell anemia: HsHsheterozygote carriers are relatively free of both: HbHssurvive & reproduce more, more common in populationHypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.Frequency of sickle cell allele & distribution of malaria