A4.2ch23 - Chapter 23 Overview The Smallest Unit of...

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Chapter 23 Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variation in populations contribute to evolution Microevolution is change in the allele frequencies in a population over generations Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible Two processes, mutation and sexual reproduction, Genetic Variation Variation in individual genotype leads to Not all phenotypic variation is heritable Natural selection can only act on Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis (purple flowers or white flowers) Quantitative characters Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals Variation Between Populations Most species exhibit geographic variation , differences between gene pools of separate populations or population subgroups Some examples of geographic variation occur as a cline , Mutation Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring
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Point Mutations A point mutation is a change in one base in a gene The effects of point mutations can vary: Mutations in noncoding regions of DNA are often harmless Mutations in a gene might not affect protein production because of redundancy in the genetic code The effects of point mutations can vary: Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Duplication of large chromosome segments is usually harmful Duplication of small pieces of DNA is sometimes less harmful and increases the genome size Duplicated genes can take on new functions by further mutation Mutation Rates Mutation rates are low in animals and plants Often lower in eukaryotes The average is about one mutation in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses Sexual Reproduction Sexual reproduction can shuffle existing alleles into new combinations and doesn’t create new alleles Mutations create new alleles In organisms that reproduce sexually, recombination of alleles is more important than mutation in
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A4.2ch23 - Chapter 23 Overview The Smallest Unit of...

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