Chapter 23 The Evolution of Populations

Chapter 23 The - Chapter 23 The Evolution of Populations The Smallest Unit of Evolution Individual organisms do not evolve populations evolve

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Chapter 23: The Evolution of Populations The Smallest Unit of Evolution - Individual organisms do not evolve, populations evolve o Natural selection works on organisms - Evolution on its smallest scale, or the change in allele frequencies in a population over generations is microevolution 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible - Darwin couldn’t explain how organisms pass heritable traits onto their offspring o A few years later after he published The Origin of Species , Mendel wrote a paper about inheritance in pea plants, which provided an explanation Genetic Variation - Only the genetic part of variation can have evolutionary consequences Variation within a population - Discrete characters can be classified on an either-or basis (like the colors of Mendel’s pea plants) o Many of these characters are determined by a single gene locus with different alleles that produce distinct phenotype - Most heritable variation involves quantitative characters, which vary along a continuum within a population o This usually results from the influence of two or more genes on a single phenotypic characters - Biologists can measure genetic variation in a population at both the whole- gene level (gene variability) and the molecular level of DNA (nucleotide variability) o Gene variability is quantified as average heterozygosity, which is ha average percent of loci that are heterozygous Usually estimated using gel electrophoresis, but it can’t detect silent mutations To include silent mutations in the estimates of average heterozygosity, scientists much use PCR and restriction fragment analyses o Nucleotide variability is measure by comparing the sequences of two individuals in a population and then averaging the data from many comparisons o Gene variability is often greater than nucleotide variability because a difference at only one nucleotide is enough to make two alleles of that gene different and therefore increase gene variability Variation between populations - There is also geographic variation betwe en s ep arat e populations of the sa m e species o These se e m to b e the result of chance events rather than natural selection - Another exa m ple of g eographic variation occurs a s a cline, which is a graded chang e in cha g eographic axis Mutation - Mutations are the ultimate source of n ew alleles o Mutations often occur in so m atic cells a nd are not p ass ed on to offspring o In plants and fungi, m a ny different kinds of cells can produce ga m et es Point Mutations - A chang e in only one b as e, which can have a hug e effect on an individual o Ex: sickle cell dise as e - Since organisms are often reflections of thousands of g en erations of p ast selection, it’s rare
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This note was uploaded on 04/21/2009 for the course BISC 120Lg taught by Professor 11:00-01:50pm during the Fall '06 term at USC.

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Chapter 23 The - Chapter 23 The Evolution of Populations The Smallest Unit of Evolution Individual organisms do not evolve populations evolve

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