evolution

• The theory of evolution is based on the idea that all living things are related by common descent.
• Two main sources of genetic variation:

  • Mutations: any change in a sequence of DNA, occurs because of mistakes in DNA replication or as a result of radiation or chemicals in the environment
  • Genetic shuffling: results from sexual reproduction, includes independent assortment, crossing over, and random fertilization
• Gene pool: all genes, including all different alleles, present in a population

  • Contains two or more alleles for each gene
• Evolutionary forces can change allele frequencies in a population

  • Five principle evolutionary forces:

  1. Mutation

  2. Gene flow (migration)

  3. Nonrandom mating

  4. Genetic drift

  5. Natural selection
• Natural selection:

  • Acts on phenotypes, slowly reduces the frequency of harmful recessive alleles
  • Very few individuals are homozygous recessive, so very few express the allele
• Genetic equilibrium:

  • Situation where allele frequencies remain constant
  • Maintenance requires conditions like random mating, large population, no movement in or out, no mutations, and no natural selection
• Trait expression:

  • Number of phenotypes for a trait depends on how many genes control it
  • Single-gene trait: controlled by a single gene with two alleles, leads to two distinct phenotypes
  • Polygenic traits: influenced by two or more genes, can have many genotypes and phenotypes
• Stabilizing selection:

  • Individuals near the center of the curve have higher fitness
  • Proportion of similar individuals increases
• Genetic drift:

  • Random change in allele frequency in small populations
  • Can cause an allele to become common over time
• Speciation:

  • Formation of new species
  • Species defined as organisms that breed with one another and produce fertile offspring
• Reproductive barriers:

  • Reproductive isolation prevents interbreeding and fertile offspring
  • Develops through behavioral isolation, geographic isolation, and temporal isolation
• Evolution is the process by which modern organisms have descended from ancient organisms
• A theory is a well-supported, testable explanation of phenomena
• Charles Darwin concluded that animals on the coast of the Galápagos Islands resembled those on nearby islands and evolved differences after separating from a common ancestor
• Natural selection is the process by which populations change in response to their environment
• Survival of the fittest refers to individuals that are better suited to their environment surviving and reproducing most successfully (fitness)
• Natural selection causes changes in the inherited characteristics of a population, eventually leading to new species
• Adaptation is the changing of a species that results in it being better suited to its environment
• Isolation leads to two isolated populations of the same species becoming more different over time, potentially resulting in the inability to breed with one another
• Darwin's Theory of Evolution:

  • More organisms are produced than can survive, leading to competition for limited resources
  • Variation exists within the genes of every population or species due to random mutations
  • Some individuals of a population are better suited to survive in a particular environment, leading to more offspring
  • Surviving organisms pass their heritable traits to their offspring
  • Natural selection causes species to change over time
  • Species alive today are descended with modification from ancestral species (common ancestors)
• Evidence of Evolution:

  • Paleontologists study fossils
  • Radiometric dating enables paleontologists to arrange fossils in sequence
  • Orderly change can be seen when fossils are arranged according to their age
• Biological Molecules Contain a Record of Evolution:

  • Differences in amino acid and DNA sequences are greater between species that are more distantly related than between species that are more closely related
  • Homologous structures and vestigial structures suggest that all vertebrates share a common ancestor
  • Homologous structures share a common ancestry
  • Vestigial structures are organs reduced in size and function, considered evidence of an organism's evolutionary past
• Anatomy and Development Suggest Common Ancestry:

  • Similarities in embryology show that the same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates
• Embryology:

  • Nature provides the variation while humans select useful variations through artificial selection
• Relative frequency: the number of times an allele occurs in a gene pool compared to other alleles for the same gene
• Directional selection:

  • Individuals at one end of the curve have higher fitness
  • e.g. pesticide resistance in disease-causing bacteria

  
• Stabilizing selection:

  • Individuals near the center of the curve have higher fitness
  • Proportion of similar individuals increases

  
• Disruptive selection:

  • Individuals at upper and lower ends of the curve have higher fitness
  • Can cause a single curve to split into two

  
• The ability of an organism to survive and reproduce in its natural environment is called natural selection/fitness
• Lamark formulated the theory of evolution through the inheritance of acquired traits
• Example of Lamark's theory: A giraffe stretching its neck to reach leaves from a tall tree, passing on this acquired trait to its offspring
• Populations of the same species living in different places become increasingly different as each becomes adapted to its own environment, eventually leading to speciation
• Scarcity of resources and a growing population are most likely to result in competition
• Reproductive isolation is necessary for the formation of a new species