10.31.07 - Natural Selection

10.31.07 - Natural Selection - Darren Rothberg Biol 105...

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Biol 105 – Evolution 10.31.07 I) Natural Selection – Darwin’s Grand Idea 1) Natural Selection a Differential reproductive success (i) Higher mean reproductive success of one type relative to another b Due to relationship between phenotype and environment (i) Does not require genetic differences between the phenotypes (variants) (ii) EVOLUTION does require genetic differences between the phenotypes c Unlike genetic drift, produces directional change d Favors one phenotype/genotype/allele over others in the population (i) (Steady pattern of change over time towards favored allele) 2) Fitness a Reproductive success of an individual or phenotype compared to others in the population (i) Relative reproductive success Comparing two things: are the reproductive success the same or is one greater? (ii) Differential reproductive success (iii) Equal reproductive success b An individual that produces more offspring than another in a population is said to have greater fitness or relative reproductive success c A phenotype that produces on average more offspring than another in a population is said to have greater fitness or relative reproductive success 3) How did Darwin come to it? a Observation 1: (i) All organisms have the potential for very rapid increase in numbers - Thomas Malthus b Observation 2: (i) Yet, natural populations tend to be stable in size c Deduction: (i) Many more offspring are produced than survive to reproduce d Observation 3: (i) Variation exists in the traits of individuals in populations e Observation 4: (i) Some of this variation is inherited f Deduction (final): (i) Individuals that inherit characters that confer higher reproductive success will replace those with characters that confer lower reproductive success
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10.31.07 - Natural Selection - Darren Rothberg Biol 105...

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