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Chapter 23 Notes.pdf

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BIOL 226
Robert Weladji

CHAPTER 23. THE EVOLUTION OF POPULATIONS Overview: The Smallest Unit of Evolution • Natural selection acts on individuals, but only populations evolve • The proportion of individuals with the “favored traits” in the population increase over generations • Genetic variations in populations contribute to evolution What Did Darwin’s Theory Lack? • Coherent theory of inheritance • There are heritable variations among individuals in a population (eyes color: brown, blue, …) • These are transmitted from parents to offspring • How do heritable variations appear in populations? • Blending theory? • ? Blending hypothesis • It proposes that the genetic material contributed by each parent mixes in a manner analogous to the way blue and yellow paints blend to make green • This means: over many generations, a freely mating population should give rise to a uniform population of individuals – NOT TRUE What Did Darwin’s Theory Lack? • Coherent theory of inheritance - blending theory - refuted • Darwin had no alternative hypothesis Gregor Mendel proposed a model of inheritance that supported Darwin’s theory. • Mendel’s particulate hypothesis of inheritance • States that parents pass on discrete heritable units (genes) that retain their identities in offspring • Mendel’s work rediscovered in 1900’s • Modern synthesis: Darwin + Mendel will lead to population genetics Some key terms • Each chromosome carries units of heredity called genes • Genes are paired because chr. are paired • Position of a gene on a chromosome=locus • Genes occupying the same locus on a pair of chromosomes are termed alleles • If each member of the pair of alleles affects a given traits in the same manner, the two alleles are called homozygous • As opposed to heterozygous Genetic variation • Within a Population: Variation among individuals within a population in discrete and quantitative character, and • Between Populations: Geographic variation between populations Genetic variation – discrete vs quantitative characters • Discrete characters: classified on either-or basis– often single gene locus with different alleles that produce distinct phenotypes • e.g. flower color • Quantitative character: varying along a continuum within a population– often from 2 or more genes influencing a single phenotypic character • e.g. plant height (ranges from short to tall) Gene variability vs nucleotide variability • Biologist can measure genetic variation at:  The whole gene level (gene variability) or at  The molecular level of DNA (nucleotide variability) • Average heterozygosity measures gene variability, the average percent of gene loci that are heterozygous • Nucleotide variability measure the mean level of difference in nucleotide sequences (base pair differences) among individuals in a population Source of genetic variation that makes evolution possible • Processes producing the variation in gene pools that contributes to differences among individuals include: • Mutation (including gene duplication and other similar processes) and • Sexual reproduction, Mutations • ? • Mutations are changes in the nucleotide sequence of an organism’s DNA • Mutations cause new genes and alleles to arise • Only mutations in cells that produce gametes can be passed to offspring Mutation rates • Vary from organisms to organisms? • Tends to be low in plants and animals • Average about 1 per 100000 genes per generation • Short generation span of prokaryotes makes them prone to quick generation of genetic variation • E.g. virus such as HIV (generation span of 2 days) • Hence need to use drug ”cocktails” as treatment against AIDS – several medications 2. Sexual reproduction  Sexual reproduction can shuffle existing alleles into new combinations  In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make evolution possible  Three mechanisms: crossing over, independent assortment of chromosomes, and fertilizations (read about genetic variation from sexual recombination in chapter 13, pp 257-259) Hardy-Weinberg equation • A population is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring • A gene pool consists of all the alleles for all loci in a population • A locus is fixed if all individuals in a population are homozygous for the same allele • Each allele has a frequency (proportion) in the population The frequency of an allele in a population can be calculated – For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 – The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles R W • E.g. Imagine a population ofW50W wildflower with 2 alleles C andRC Rcoding flower pigment) • 4 homozygous (20; white; C C ), 64% homozygous (320; red; C C ) and 32% heterozygous (160; pink; C C ). • Total number of alleles=? • 500 X 2 = 1000 (plants are diploid) • Number of dominant alleles C =? R R
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