تطور نظري 6

• Mutation

  An alteration in the nucleotide sequence of the genome of an organism (an error in copying the DNA)

  
• Somatic mutation

  A mutation that occurs in body cells that are not passed along to subsequent generations

  
• Gamete mutation

  A mutation that occurs in a gamete or in a cell that gives rise to gametes, which can impact the next generation

  
• If the mutation has a deleterious effect on the phenotype of the offspring

  It is referred to as a genetic disorder

  
• If the mutation has a positive effect on the fitness of the offspring

  It is called an adaptation

  
• All mutations that affect the fitness of future generations are agents of evolution

  
• Gene mutation

  An alteration in the DNA nucleotide base sequence of an allele

  
• Gene mutations

  • Change in a single DNA base spells the difference between a normal hemoglobin molecule and sickle-cell hemoglobin
  • Change in a regulatory gene can increase or decrease the expression of a structural gene

  
• Mutations

  • Can be neutral in their effect or even harmful to an organism in its present environment
  • Beneficial mutations can lead to the reproductive success and adaptability of an organism to its environment

  
• Chromosomal mutations

  An alteration in the number of chromosomes inherited
• Chromosomal mutations

  • Inversions (a segment of chromosome is inverted)
  • Translocations (exchange of chromosomal segments between nonhomologous chromosomes)

  
• Recombination

  1. Pieces of DNA are broken and recombined to produce new combinations of alleles

  2. Occurs during meiosis when homologous pairs of maternal and paternal chromosomes align and the arms of the chromosomes can overlap and temporarily fuse, causing a crossover

  3. Crossovers result in the exchange of genetic material between the maternal and paternal chromosomes

  
• Gene flow

  The transfer of alleles or gametes from one population to another
• When individuals of one population migrate to another population

  The allele frequency (the proportion of individuals carrying the same allele) of the population changes
• Factors affecting gene flow rate

  • Mate choice
  • Distance between populations
  • Physical barriers like mountains, rivers, or man-made structures
• Genetic drift

  A change in allele frequency of a new population as compared with the origin population due to random events or chance
• Genetic drift

  • In a small population, some individuals may just by chance leave behind a few more descendants (and genes) than other individuals, so the genes of the next generation will be the genes of the "lucky" individuals, not necessarily the healthier or "better" individuals
• Genetic drift is a mechanism of evolution, but it doesn't work to produce adaptations