TAILIEUCHUNG - Lecture Biology: Chapter 23 - Niel Campbell, Jane Reece

Chapter 23 - The evolution of populations. In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how sexual recombination generates genetic variability; define the terms population, species, gene pool, relative fitness, and neutral variation; list the five conditions of Hardy-Weinberg equilibrium;. | Chapter 23 The Evolution of Populations Overview: The Smallest Unit of Evolution Natural selection acts on individuals, but only populations evolve. Genetic variations in populations contribute to evolution. Microevolution is a change in allele frequencies in a population over generations. Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals. Genetic Variation (a) (b) Figure Nonheritable variation 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. Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis. Cline 0 46 44 42 40 38 36 34 32 30 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency Figure A cline determined by temperature 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. 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. Mutations that result in a change in protein production are often harmful. Mutations that result in a change in protein production can sometimes increase the fitness of the organism in its environment. Mutations That Alter Gene / Chromosome Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful. Mutation rates are low in animals and plants. Mutations rates are .

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