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Transcript
Tuesday, February 7th Genes Within Populations slash The Nature of Evolution What We’ll Learn Today • How did things get like they are now? – Evolutionary Change – Genetic Variation • Hardy-Weinberg Principle – The Five Assumptions – (And how those 5 assumptions are pretty hypothetical) Let’s Get Some Definitions Out of the Way: • Genetic Variation – differences in alleles of genes found within individuals of a population •Let’s pretend our class is a population. •There is variations among the alleles in our genotypes (some differences we can see in our phenotype) •Thus, there is genetic variation within our population Let’s Get Some Definitions Out of the Way: • Evolution – refers to how an entity (like a species, a social system, a gas, a planet, etc) changes through time – Darwin actually didn’t use the term “Evolution” – He used the phrase “descent with modification” Two Evolutionary Ideas • Jean-Baptiste Lamarck was a prominent biologist who believed that evolution occurred by the inheritance of acquired characteristics. -Characteristics or changes an individual acquired over its lifetime were passed onto its offspring The Human Equivalent • Let’s say you were a wood-worker. You are married and have one child, who, like you, has 5 fingers on each hand. • One day, tragically, you have an accident at your work and two of your fingers on your right hand are cut off. • Any child you have after this point will inherit this change you have recently acquired and will be born with 8 fingers. The Second Theory: Darwin’s • Darwin was boasting another idea: I say dear chap, Natural Selection is the mechanism of evolution! Darwin • Natural Selection - the differential reproduction of genotypes caused by factors in the environment • Wait… What? • Let’s break it down… Natural Selection, the Break Down • “differential reproduction” = some animals are better at making offspring than others • “of genotypes” = those new offspring will inherit the genes of its parents • “caused by factors in the environment” = the differential reproduction often depends on an organism’s ability to live in its environment (how good is it at hiding from predators? Reaching or finding food? Protecting itself/ its young?) So… What? • Some organisms are better suited for their environment, or sudden changes their environment recently underwent, and they are more successful at having offspring • Ability to reproduce successfully is called an organism’s fitness • Anyone heard the song “Fit But You Know It?” Reference: Urban Dictionary • Fit is defined as: • The British version of Hot. – “Dude that girl is so fit!” “Right?” • Extremely good looking, synonymous with “hott” (why two “t’s”? IDK) – “Whoa, that model is fit!” You should remember this definition: • Fitness – genetic contribution of an individual to succeeding generations • i.e. Passing down your genes • i.e. Reproducing • i.e. Having kids… • You get the idea. • So that song is really saying that some girl has a lot of kids…? …Yeah, I guess so. Which Giraffe Is More Fit? Anyway… • Darwin purposed that Natural Selection is the mechanism for Evolution • Some individuals in a population who are more suited for the environment are more fit, passing on their advantageous genes to more offspring than those less-fit losers. • As a result, the population gradually comes to include more and more individuals with the advantageous characteristics Hardy-Weinberg (and Castle) Equilibrium They asked “Why doesn’t a population, after a long time, just consist of individuals with the dominant phenotype?” Well, they realized the original proportions of the genotypes will remain constant as long as…. Five Ideal (and unrealistic) Assumptions • 1. No mutation takes place. Ever. • 2. No genes are transferred to or from other sources (no immigration or emigration takes place) • 3. Random mating is occurring. • 4. The population size is LARGE • 5. No selection occurs. So in ideal conditions, let’s check out the Hardy-Weinberg Equations • These equations let us calculate the allele frequencies of the two alleles in a population • “p” is a variable used to denote the frequency of one allele (usually the dominant one) • “q” is the variable used to denote the frequency of the other allele (usually the recessive one) • The first equation : p + q = 1 • So if 75% of alleles are dominant, p = 0.75 • That leaves us with 25% recessive, q = 0.25 • Thus 0.75 + 0.25 = 1 So in ideal conditions, let’s check out the Hardy-Weinberg Equations • • • • • The Second Equation looks at genotype p2 represents Homozygous Dominant (AA) q2 represents Homozygous Recessive (aa) 2pq represents Heterozygotes (Aa) The equation is: • p2 + 2pq + q2 = 1 • This is derived from squaring both sides of the first equation: (p + q )2 = 12 Let’s Try an Example • BB = a black cat • Bb = a black cat • bb = a white cat 84% are black 16% are white Well bb = all white cats, which is homozygote recessive (which = q2) So q2 = 0.16 So q = 0.4 You Finish It • With a partner / group: • Find the frequency of p • Then find the frequency of: – Homozygote Dominants (BB or p2) – Heterozygotes (Bb or 2pq) – Homozygote Recessives (bb or q2) Let’s Try One More • RR = Can Roll • Rr = Can Roll • rr = Can NOT Roll Who CAN Roll their tongue? Who CANNOT Roll their tongue? Use this information to find the allele frequencies of R and r in this classroom.