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Natural Selection It is a two step process:

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Presentation on theme: "Natural Selection It is a two step process:"— Presentation transcript:


2 Natural 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 population 2. Non-random aspects of survival and reproduction

3 Natural Selection is a two step process:
Step One: The Production of Variation. (Chance) Mutations Meiosis: recombination due to crossing-over in 1st division random movement of chromosomes in 2nd division Random mate selection & fertilization Step Two: Non-random aspects of survival and reproduction Superior success of certain phenotypes Nonrandom mate choice

4 Lemurs of Madagascar

5 Amazonian Frogs

6 Humans select traits for dogs, pigeons and other animals when they breed them.
Bred Pigeons came from a single original species Who selects the traits for wild plans & animals?

7 Natural Selection is Survival of the fittest
NOBODY!! There is no agent involved in natural selection. Natural selection is a process of elimination INDIVIDUALS 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

8 Natural Selection is a mixture of both Chance and necessity
Natural Selection is not goal directed. It does not have a long term goal.

9 What acts as a selection pressure on a population?
Competition for food Competition for a mate Changes in the environment Predators Parasites Example of Natural Selection in Action: Monarch / milkweed Card game Video of Darwin’s Finches

10 Main Types of Selection Pressures
Directional Selection Natural selection favors one extreme of the population for that trait often happens when environment changes in a consistent way- e.g.climate gets colder. Disruptive Selection Natural selection favors both extremes selected Causes species to diverge Stabilizing Selection Natural selection favors the average for population selected Sexual Selection

11 Directional Selection
Neck of Giraffe Antibiotic resistance of bacteria Moth color (melanin) Camouflage/Mimics Many sexually selected traits

12 Directional Selection: Mimicry (mimic environment)

13 Stabilizing Selection
When the extremes of the trait aren’t as well suited Examples bird clutch size Elk Antlers size Giraffe neck length Tail length in birds

14 Disruptive Selection Causes divergence within the species
Occurs when two different types of resources in one area Results in specialization for each branched group May lead to formation of new species E.g. Darwin’s Finches


16 Examples of selection pressures...
Predators - variants with adaptations allowing them to escape predators have more offspring - e.g. speed, defensive weapons, camouflage, mimicry Prey/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 stealth Climate - 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

17 Example #1: Escaping Predation Peppered Moth (see video clip)
Early trees had light-colored bark Only 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


19 Example #2: Obtaining Food
The neck of the Giraffe Co-evolution with Acacia Trees Selection pressure is source of food The Red Queen Hypothesis...

20 Example #3 The leaf bug The selection pressure is predators
It’s strategy is to mimic a leaf Pray Mantis Camouflage

21 Natural Selection Or, how did we get here….

22 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

23 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.

24 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

25 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.

26 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.

27 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’

28 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

29 How’s it Work? REPRODUCTION is the key, not merely survival

30 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’

31 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’

32 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……


34 Problem 3 If 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?

35 Evolution of populations
Evolution = change in allele frequencies in a population hypothetical: what conditions would cause allele frequencies to not change? non-evolving population REMOVE all agents of evolutionary change very 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)

36 5 Agents of evolutionary change
Gene Flow Non-random mating Genetic Drift Selection Mutation 36

37 Application of H-W principle
Sickle cell anemia inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = HsHs normal allele = Hb low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs often lethal

38 Sickle cell frequency High frequency of heterozygotes
1 in 5 in Central Africans = HbHs unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs usually die before reproductive age Sickle 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 Cc Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous…

39 Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3

40 Heterozygote Advantage
In tropical Africa, where malaria is common: homozygous dominant (normal) die or reduced reproduction from malaria: HbHb homozygous recessive die or reduced reproduction from sickle cell anemia: HsHs heterozygote carriers are relatively free of both: HbHs survive & reproduce more, more common in population Hypothesis: 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

41 Hardy-Weinberg lab pod cast

42 Lab—Part A Non-Tasters=Homo recessive (aa)
PTC Control Non-Tasters=Homo recessive (aa) Tasters=Homo Dominant (AA) or Heter (Aa) Figure out the p2 and 2pq for our class

43 Results and Discussion
5 Hardy Weinberg calculations Show work—organzied part A-E Discussion: In part A evolution did/did not happen because…. In part B…. In part C Explain WHY, use terms

44 Part B—Testing an ideal Population
Initial Class Frequencies GG____ Gg____ gg___ My initial genotype ___ F1____ F2_____ F3_____ F4_____ F5______ Final Class Frequencies GG____ Gg____ gg ___

45 Part C—Selection (homo recessive selected against)
Initial Class Frequencies GG____ Gg____ gg___ My initial genotype ___ F1____ F2_____ F3_____ F4_____ F5______ Final Class Frequencies GG____ Gg____ gg___

46 Part D—Heterozygous Advantage (Homo dom—may die of maleria (flip coin; homo recessive—die of sickle cell) Initial Class Frequencies GG____ Gg 24/48 gg___ My initial genotype ___ F1____ F2_____ F3_____ F4_____ F5______ Final Class Frequencies GG 16/48 Gg 32/48 gg 0/48

47 Part E—Genetic Drift (break into 3 smaller populations—make hypothesis)
Initial Class Frequencies GG____ Gg____ gg___ My initial genotype ___ F1____ F2_____ F3_____ F4_____ F5______ Final Class Frequencies Frequencies GG____ Gg____ gg___


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