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Lecture 8

SFWRENG 3RA3 Lecture 8: 2. Evolutionary Processes

11 Pages

Software Engineering
Course Code
Ryszard Janicki

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Bio 1M: Evolutionary processes Evolution by natural selection • Is something missing from the story I told last chapter? – Heritable variation in traits – Selection (i.e., differential reproductive success) based on these traits Some genetics • Our basic traits are determined by genes • A location where a gene can occur is called a locus (pl. loci) • A particular version of a gene is called an allele • Complex organisms usually have two alleles at each locus – These can be the same, or different Loci • Complex organisms usually have two alleles at each locus – These can be the same, or different • An organism with different alleles at a particular locus is referred to as heterozygous (adj., n. form heterozygote) • An organism with two copies of the same allele at a particular locus is referred to as homozygous (adj., n. form homozygote) Two definitions of evolution • Lecture: heritable changes in species traits over time • Book: changes in allele frequencies • These definitions are consistent; use the one which helps you think clearly 1 Analyzing genotype frequencies — S25.2 Genotypes and phenotypes • A genotype is the collection of an individual’s genes 1 • A phenotype is the collection of an individual’s physiological and physical traits – What we can observe about an individual – Phenotype is largely (but by no means entirely) determined by genotype Example: peppered moths • Check “peppered moths” or “Kettlewell’s experiment” on wikipedia • Two different alleles possible at the wing color gene1 A and2A . – Individuals with A A 1 1 genotype have light-winged phenotype – Individuals with 2 2 genotype have dark-winged phenotype. – Individuals with A1 2genotype ??? • If individuals with genotype XY have the same phenotype (on average) as those with XX, we say that X is a dominant allele and Y is a recessive allele. • If XY individuals have an intermediate phenotype (between XX and YY , we say X and Y are co-dominant. Analyzing genotype frequencies • We analyze genotype frequencies as follows: – Make simple assumptions about how frequencies work – Calculate expected frequencies under our assumptions – Measure observed frequencies in the population – Look for evidence of systematic (not random) difference between expected and observed frequencies Making simple assumptions • Expected frequencies are usually calculated by assuming that alleles assort randomly and independently, like flipping two coins, or rolling two diCoin flipping • I flip two fair coins (ie., each coin will land heads with probability 1/2). • What is the probability of: – Two heads – Two tails? – One of each? • 2 Professional coin flipping • A professional gambler can flip a coin so that it lands heads 70 the time. She flips two coins. • What is the probability of: – Two heads – Two tails? – One of each? • Hardy-Weinberg distribution • The Hardy-Weinberg distribution is the distribution expected if alleles work like coins (random and independent). • If p is frequency of allel1 A and q is frequency of all2le A , then: – Frequency of genotype A A1 1 p . – Frequency of genotype 2 A2 2is q . – Frequency of genotype A A1 2 2pq. • Why the 2? – Example: calculating allele frequencies • Icollect20pepperedmothsfromaparticularplace,andfindthat4havegenotypeA A , 1 1 8 have genotype A1 2, and 8 have genotype A 2 2 • What is the observed frequency of each allele? • What is the expected frequency of each genotype under the Hardy-Weinberg assumptions? • Is this population in Hardy-Weinberg equilibrium? – – What do we mean by expected? • If we flip a fair coin 100 times, what is the expected number of heads? – What if we flip it 25 times? • We don’t expect to get exactly the expected value. 3 • The ‘expected value’ is an average of what is expected under our assumptions How do you know a coin is perfectly fair? • You can never be sure that a coin is perfectly fair, you can only evaluate your evidence that it’s more or less close to fair. • Similarly, we never have evidence that a population is exactly in Hardy-Weinberg equilibrium, we can only evaluate our evidence that it is not in equilibrium, or our evidence that it is close to equilibrium. Hardy-Weinberg equilibrium • When do we expect genotype frequencies to behave like coins? • Alleles selected at random from the previous generation: – – • If these assumptions hold, we expect Hardy-Weinberg equilibrium – Hardy-Weinberg distribution, with no change in allele frequencies from generation to generation. Differences from equilibrium • If we observe large differences from the Hardy-Weinberg equilibrium, this is usually a sign that mating is not random, or that natural selection is operating • The analysis tells us that something is going on, but not what • Hardy-Weinberg is a null model: it tells us what to expect if complicating effects are absent Example: Human blood groups • MN blood groups in different human populations are very close to Hardy-Weinberg equilibrium — Table 25.1 – No evidence for non-random mating, or for fitness differences. • What about MN blood groups in the global human population? – – Example: Human HLA genes — Table 25.2 • HLA genes are used by the immune system to recognize disease-causing organisms 4 • Researchers hypothesized that heterozygous individuals may recognize more bacteria and viruses • Data shows that more people are heterozygous for HLA genes than would be expected under the Hardy-Weinberg assumption Heterozygous HLA genes • Why might more people be heterozygous for HLA genes than predicted by HardyWeinberg? -Heterozygous people might be more likely to survive -Heterozygous people have more offspring • Effects of this are complicated • Heterozygotes don’t necessarily have heterozygous offspring -People might be more attracted to people withdifferentHLAtypes(Maybeevolveddue to above reasons) 2 Types of natural selection — S25.3 2.1 Directional selection • Directional selection tends to move a population in a particular direction – Giraffe necks – Human brains Multi-directional selection • Directional selection can change through time with the environment – Swallows may get larger during extreme cold spells, smaller again during normal weather ∗ But we need to know whether the changes we saw were heritable – Finch beaks get thicker when food is scarce, and smaller when food is abundant – Why might small-beaked finches have advantages? *Lots of reasons* 2.2 Stabilizing selection • Stabilizing selection tends to keep the population where it is 5 – Example: human birthweights — Fig 25.4 Connections between selection types • What happens if the target of directional selection stays the same for a long time?
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