Evolution

• parsimony principle


  1.) simplest explanation

  2.) tree with the fewest character transition

  3.) fewest homoplasies
• natural selection


  1.) differential reproductive success of certain individuals with more or less desirable traits

  2.) acts on heritable variation in a way that favors the traits that lead to reproductive fitness and promotes adaptive evolution

  3.) only acts on existing variation

  4.) DOES NOT act on genotypes, favor the perfect phenotype, or create genetic variation
• heritable


  can be passed from parent to offspring
• differential reproductive success (DRS)

  individuals with more desirable traits will be more likely to pass those traits to future generations
• evolution


  changes in biological populations (NOT individuals) over time
• 3 things necessary for natural selection to occur

  1.) Variation in the trait

  2.) Heritability

  3.) Impact on reproductive success
• gene pool


  sum of all of the alleles for a particular locus in a population
• mutation


  change in the nucleotide sequence (random)
• 2 sources of phenotypic variation

  V(g) - genetic variation

  V(e) = environmental variation

  V(p) = V(g) + V(e)
• heritability


  1.) (h^2) = V(g)/V(p)

  2.) Fraction of variability NOT fraction of trait that is genetic
• parent-offspring regression line


  relates traits of offspring to biological parents

  ex: slope 0f 0.5 -> 1/2 of the variability is due to the parents
• Breeder's equation

  1.) quantify response to natural selection in terms of heritability and selection differential (S)

  2.) R = (h^2)(S)
• selection differential (S)


  measure of the intensity of selection on a particular trait
• fitness


  genotype and phenotype's contribution to the genetic composition of future generations; which organisms are more likely to survive AND reproduce
• Directional selection


  one extreme phenotype is the fittest

  mean changes, variance does not

  ex: TX longhorns
• Disruptive selection


  1.) trait tends toward BOTH extremes

  2.) Causes the emergence of two species

  3.) bimodal distribution leads to eventual sympatric speciation

  4.) rarest of all the selections

  ex: bird beaks
• frequency-dependent selection (FDS)


  occurs when a phenotype's fitness depends on how common it is in the population
• positive FDS


  most common phenotype is favored
• negative FDS


  rare phenotype is favored in the poulation
• fixation


  a given allele is the only allele in the population and all other variations have been lost
• sexual selection


  1.) non-random mating in which an organism's phenotype impacts its choice of mating

  2.) traits favored are often costly to males
• population genetics


  study of how evolutionary forces cause allele frequencies to change over time
• allele frequency


  # copies of a particular allele / # copies of all alleles
• random mating


  when a member of one sex is equally likely to mate with any member of the other sex
• gene structure


  frequency of different alleles and genotypes in a population
• fixed allele


  p=1 or q=1 -> only one allele at a locus

  population is considered monomorphic
• polymorphic vs. monomorphic


  monomorphic - fixed allele, polymorphic (p<1 and q<1) - more than one allele at a locus
• Hardy-Weinberg Equilibrium equation


  p^2 + 2pq + q^2 = 1
• 5 Hardy-Weingberg assumptions


  1.) No mutations

  2.) Random mating

  3.) Large population

  4.) No gene flow (migration) between populations

  5.) No selection (natural, sexual, etc.)
• Charles Darwin


  credited with the theory of evolution by natural selection
• heterozygote advantage

  1.) heterozygous individuals are more fit than homozygous individuals

  2.) Maintains polymorphic loci

  3.) Common in highly variable environments

  4.) Occurs in cases of co-dominance or incomplete dominance
• assortative mating


  individuals are more likely to mate when they have traits in common
• selfing


  self-fertilization in plants (why nonrandom mating doesn't work); common in tetraploids
• genetic drift

  1.) unpredictable, random fluctuations in allele frequencies from one generation to the next because of small population size

  2.) most significant in small populations

  3.) decrease in genetic variation

  4.) can eliminate beneficial alleles
• bottleneck effect


  1.) genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population

  2.) Reduces genetic variability

  3.) Few heterozygous individuals

  ex: cheetahs
• founder effect


  1.) few individuals become isolated from a larger population, the new population gene pool is not reflective of the original population

  2.) incorporates the concepts of gene flow and genetic drift

  ex: ladybugs
• genetic polymorphism


  1.) two or more genetic variations persist in a population over time

  2.) maintains genetic variability
• gene flow


  - additions or subtractions from a population resulting from the movement of fertile individuals or gametes

  - exchange of genetic material from one population to another

  - increase variation within population, decreases variation between populations
• processes that create/maintain variation

  1.) mutation

  2.) migration (gene flow)

  3.) disruptive or negative FDS

  4.) heterozygote advantage
• synonymous substitution


  1.) neutral

  2.) no effect on phenotype

  3.) caused by silent mutations