Chapter 25 Evolutionary Processes

Chapter 25 Evolutionary Processes - Evolution by natural...

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Evolution by natural selection Is something missing from the story I told last chapter? 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 diFerent Loci Complex organisms usually have two alleles at each locus These can be the same, or diFerent An organism with diFerent 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 defnitions oF evolution Lecture: heritable changes in species traits over time Book: changes in allele frequencies These de±nitions are consistent; use the one which helps you think clearly 1
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1 Analyzing genotype frequencies * (pp. 504–508) Genotypes and phenotypes A genotype is the collection of an individual’s genes 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 Two diFerent alleles possible at the wing color gene: A 1 and A 2 . Individuals with A 1 A 1 genotype have light-winged phenotype Individuals with A 2 A 2 genotype have dark-winged phenotype . Individuals with A 1 A 2 genotype ??? 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) diFerence between expected and observed frequencies 2
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Making simple assumptions Expected frequencies are usually calculated by assuming that alleles assort randomly and independently, like Fipping two coins, or rolling two dice Coin fipping I Fip 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? ProFessional coin fipping A professional gambler can Fip a coin so that it lands heads 70% of the time. She Fips two coins. What is the probability of:
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Chapter 25 Evolutionary Processes - Evolution by natural...

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