Natural Selection

• Natural selection


  The process by which certain traits become either more or less common in a population based on their relative fitness

  
• Concepts recalled

  • Mendelian genetics
  • Population genetics
• Gene


  Occurs in pair, and alternative form is called an allele
• Complete dominance


  When one allele is fully expressed and the other is completely masked
• Genotypes


  • Homozygous dominant
  • Heterozygous
  • Homozygous recessive
• Genotype


  The genetic constitution that an individual inherited
• Phenotype


  The form that is shown or the appearance of the organism: morphology, physiology & behavior
• Hardy-Weinberg law


  A population is said to be at equilibrium when a large, randomly mating population will not change its gene frequency from generation to generation provided that no outside forces acting on its gene frequency to change it
• Mechanisms of evolution


  • Mutation
  • Selection
  • Migration or gene flow
  • Genetic drift
• Mutation


  Must be recurrent to have effect on the population

  Forward mutation: A

  Reverse mutation: a

  Mutation proposes, selection disposes
• Types of selection

  • Artificial selection
  • Natural selection
• Artificial selection


  The most favored genotypes are propagated as made by plant & animal breeders
• Natural selection


  Favoring the survival & reproduction of the most fit genotypes as the guiding force of evolution
• 3 basic kinds of selective effects


  • Directional
  • Disruptive
  • Stabilizing
• Directional selection


  One of the extremes in the phenotypic range becomes fit & thus preserved
• Disruptive selection


  Both extremes of the phenotypic range are selected for, thus preserving the differences in the gene pool of a population. Sub-speciation ensued which eventually leads to the formation of new species

  
• Stabilizing selection


  Tends to eliminate phenotypic extremes
• Relative fitness


  Reproductive success (number of progenies) of one phenotypes as opposed to alternative phenotypes
• Selection pressure


  The force acting on the phenotype to reduce its fitness or adaptive value (W) as represented by selection coefficient (s) = 1 - W
• Migration


  Or gene flow occurs when individuals move from one population (of different gene frequency) to another and interbreed with the latter
• Genetic drift


  A non-directional force that changes gene frequency in an unpredictable rate between generations
• Effective population size


  The number of parents that contribute offspring to the next generation, which is important in determining genetic drift
• Founder principle


  A new population arose from few founders sent forth in a particular area like an isolated island
• Causes of speciation


  • Geographic isolation
  • Reduction of gene flow
• Allopatric speciation


  Speciation that occurs in separated groups due to geographic isolation
• Sympatric speciation


  Speciation that occurs in area of overlap due to reduction of gene flow when individuals exploit a new niche
• Fitness


  The ability to survive and reproduce, which depends on the environment
• Adaptation


  The 'match' between an organism and its environment, which can be genetic or induced by the environment (phenotypic plasticity)
• Darwin's Postulate on Natural Selection:

  Individuals within species vary, some variations are heritable, more offspring are produced than can survive, and survival & reproduction are nonrandom
• Natural selection


  Differential survival and reproduction of individuals based on genetic differences in some trait(s)

  
• For adaptations to evolve, differences in the trait must cause differences in fitness and the differences must be heritable
• Poecilia reticulata


  • Lives in gravel-bottomed forest streams: headwaters to lowlands
  • Great variation in coloration from population to population, even within the same stream
• Natural predators of Poecilia reticulata


  • Up to 7 natural predators
  • Generally more and the most ferocious are at lower elevations (downstream)
• Brightest guppy populations


  Tend to live upstream: populations downstream are duller in color
• Females of Poecilia reticulata


  Dull gray (and blend in with the stream bed) in all populations
• Necessary conditions for evolution by natural selection


  • Variation
  • Heritability: offspring resemble parents
  • Selection: consistent relationship between phenotype and survival and reproduction
• Adaptation


  The phenotypic variant that results in the highest fitness in a given environment
• Endler's experiments


  1. The phenotype of each population is adapted, and is the result of the local balance of sexual and predator selection

  2. If the balance of sexual and predator selection is changed, the phenotype should evolve in response

  3. Environment-dependent
• Endler's results


  • Ponds with no predators (K) showed a steady increase in the # of spots
  • Little change in spot # (compared to the control) after addition of the innocuous predator (R)
  • Pools that received a voracious predator treatment (C) showed a marked decrease in the # of spots
• Phenotypes evolved in response to changes in the balance of sexual and predation selection