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UW-Madison ENVIRST 260 - Evolution and Natural Selection

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ENVIR ST 260 1st Edition Lecture 3Outline of Last Lecture I. PopulationsA. What Controls Size and GrowthB. Impact of Competition and DensityC. What We Learn from Studying PopulationsII. EvolutionA. DefinitionB. Observation of Genetic ChangesOutline of Current Lecture I. EvolutionA. Working DefinitionB. Null ModelII. Natural SelectionA. RequirementsB. Examples/Things to Think AboutIII. Information about Files on MoodleCurrent LectureEvolution Definition of Evolution (Working Definition) Change in the genetic makeup of a population overtime.  Changes in DNA. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. We have to have the whole population; individuals can have changes but it doesn’t really matter.  And then there’s the time component: generational time Example from Tuesday: If there are flowers of two different colors and if a pollinator has a preference to one flower color (the purple flower), we can look at how the allele frequencies change in this population. There would be a greater proportion of purple flowers.  Null Model: (5 criteria that if in place will lead to a population that is static and not evolving) No Selection- You can select for a trait or against a trait Predators- Predators can select against slow easy to spot individuals Climate- One trait as cold tolerant or not cold tolerant. If it gets colder, you’ll be selecting for the cold tolerant genotype and against the genotype that is not cold tolerant Resistance to disease - Antibiotic resistance is a huge problem in health care. You’re selecting for antibiotic resistant bacterial strains No Mutation- These occur in the background all the time. They’re not necessarily bad; they canbe good. A gene can mutate and give an individual an advantage. But, if it’s not occurring, you don’t see evolution. No gene flow- This is individuals moving from one population to another. This is either immigration (joining a population) or emigration (leaving a population) Clicker Question: What do mutation and gene flow have in common?- Correct Answer: These events can introduce new genotypes Mutation rate: generally mutations are rare, but that’s what we see and experience in a day to day basis. Typically, organisms with big genomes have slow mutation rates, while those with smaller genomes mutate a lot faster Random Mating The idea that all individuals have an equal opportunity to mate and produce offspring. An example of nonrandom mating would be sexual selection which happens in peacocks. Males with big flashy tales get all the ladies. Nonrandom mating takes place with most organisms that have males and females. There’s some sort of mate selection going on.  Large Population Size Large populations “buffer” against random events. So, if you had some sort of disturbance (introduce a predator, disease, fires, floods) and you have thousands ortens of thousands of individuals a lot will stay alive in comparison to a population with around 10 individuals.  Genetic drift happens with small populations. It’s a random occurrence. If you have abig population and you reduce it, you get a skewed subset of the population. From these individuals you may have more of one genotype than another, so surviving individuals can really change the genotype of the population over time when they start to breed. This is still evolution, but there’s no selection pressure that’s making these individuals more fit. As you can see, ALL of these assumptions are violated in the real worldNatural Selection Requirements for Natural Selection Variation in traits If you have a population of flowers that are all the same color, there’s no variation in the trait so there’s nothing that’s going to be selected for or against. Having a lot of variation is a good thing from an evolutionary perspective. If you have species that lack genetic differences and key traits (ability to avoid predator) (no variation in color) they are doomed to go extinct. Differences in fitness of genotypes You might have insects with two different phenotypes of color but they’re both easy to spot so there’s no difference in fitness. Fitness isn’t about strength, it’s about being able to breed and pass on genetic material Heritability of traits These traits have to be inheritable. If you have a great genetic make-up but no offspring, the population doesn’t benefit Examples/Things to Think about Population of beetles If the predator bird prefers green beetles, that trait is selected against. The preference for pray changes how the population looks. By the end, it’s all brown beetles. Once we have a bunch of brown beetles, the birds might change and eat brown beetles or if the predator won’t change, they’ll die out Properties of natural selection speeding it up artificially (corn example) Ancestral corn: people saw it was good and tasty to grow and started selecting for good traits. So they take the good seeds out of the harvest and harvest those with the trait for more kernals. It’s not a natural selection in that it’s human intervention, but it’s still an example of a species that’s evolving. Modern corn: We’ve completely altered what the species looks like over time. Same thing can be said for what animals humans have domesticated Evolutionary arms race: Futurama video clip Two species: Every time one adapts to have the upper hand there’s some individuals of another species that can adapt and outsmart the one with the upper hand. They’re not adapting these traits consciously. Clicker Question: Does selection act upon the genotype or phenotype? Phenotype-the expressed trait- Example: population of mice- AA-long legs, fast mice- Aa-long legs, fast mice- aa- short legs, slow mice- Predator: he’s going to catch the slow mice. We’re selecting against a slow trait- What happens if a fast mouse has a slow day? He’s got long legs and the right genotype, so he’s supposed to have a higher fitness. However, if the cat catches him he eats him anyway. In the rare circumstances where the genotype and phenotype don’t match, natural selection goes towards the phenotype. It’s all about the expressed traits. Chapter 8: Intersection of Ecology and evolution “An ecologist naïve


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UW-Madison ENVIRST 260 - Evolution and Natural Selection

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