BIOL 150 Chapter 2

• Plato's first understanding of species


  Perfect essence, type created by God (unchanging)
• Typological thinking


  Notion of higher and lower species, including separation of humans from other animals
• Use and Disuse Theory


  Organs/traits used are more developed and will grow and function, while organs/traits not used deteriorate until they're gone
• Inheritance of acquired characteristics


  Acquired traits organisms get from use will be passed down to offspring
• Darwin and Wallace


  • Found that species change through time and common ancestry
  • Species change through time because of natural selective forces
• Descent with modifications


  Organisms change through time and linked by common ancestry
• Variation


  • Important for why species can change over time
  • Observed in nature could be caused by processes similar to artificial selection, but driven by environmental forces
• Levels of evolution


  • Macroevolution
  • Darwinism descent with modification
  • Mendelian inheritance and molecular variation
• Evolution


  Change in allele frequencies in a population over time
• Modern synthesis


  Put evolution in a population genetics framework
• Hardy-Weinberg equilibrium


  • Allele and genotype frequency in a population reach equilibrium in one generation and stay at genetic equilibrium generation after generation
  • Allele and genotype frequency do not change over time
• Hardy-Weinberg assumptions


  • Mating is random
  • Population is infinitely large
  • No natural selection
  • No mutation occurs
  • No migration
• Mechanisms of evolution


  • Mutation
  • Gene flow
  • Genetic drift
  • Selection
• Nonrandom mating


  Doesn't lead to evolution on its own because it doesn't change allele frequency
• Types of nonrandom mating


  • Positive assortative mating
  • Negative assortative mating
  • Inbreeding
• Positive assortative mating


  Similar genotypes are more likely to mate than dissimilar ones, which increases homozygosity
• Negative assortative mating


  Dissimilar genotypes are more likely to mate than similar ones, increasing heterozygosity
• Inbreeding


  Mating individuals are related, which happens most often in small populations, increasing homozygosity
• Nonrandom mating does change genotype frequency, causing population to deviate from Hardy Weinberg equilibrium
• You can deviate from HW equilibrium and not evolve
• Inbreeding


  • Doesn't lead to evolution, but it can have negative impacts on organisms in population
  • Increases probability that an individual will be homozygous at many loci: rare alleles are brought together at many gene loci, so new combinations are seen
  • Causes inbreeding depression which reduces survival and fertility of offspring of related individuals, and has the appearance of harmful recessive traits
• Mutation


  • A change in DNA
  • When mutations create new alleles, allele frequency in population will change slightly
  • Mutation is the only evolutionary mechanism that creates new alleles (and adds new variation) in gene pool
  • Mutation alone is usually inconsequential in changing allele frequency of a particular gene because mutation rates are too slow and mutations combine with other processes of evolutionary change in populations
  • Mutation is the ultimate source of genetic variation because it is the only process that creates new alleles, genetic recombination only shuffles existing alleles
  • Mutations are random with respect to phenotype and fitness
  • When mutations occur, they can be beneficial, neutral, or deleterious
  • Mutations just happen, organisms can't create mutations because they "want" or "need" them
• Gene flow


  • Occurs when genetic material moves between populations via movement of individuals or gametes
  • Results in genetic mixing of populations, leading to evolution in population that receives new genetic material
  • Can introduce new alleles to a population but cannot create new alleles, increasing genetic variation
• Genetic drift


  • Chance events determine which alleles are passed to the next generation
  • Chance fluctuations that result from sampling error during reproduction
  • Overtime, the allele frequency of small populations will fluctuate, reducing genetic variation
  • In small populations, because some alleles are lost, genetic variation of the population is reduced and the frequency of harmful alleles can increase
• Bottleneck effect


  Occurs when random subsampling of a population occurs due to a decrease in population size
• Founders effect


  Occurs when random subsampling of a population occurs due to dispersal and founding of a new population by a few individuals
• Natural selection


  • Occurs when organisms with a particular phenotype survive at a higher rate than organisms with a different phenotype
  • The environment selects for the favored phenotype and selects against other phenotypes
  • Individuals with the favored phenotype will leave behind more offspring with the phenotype, and because of this, favorable phenotypes increase over time, increasing the frequency of genotype and alleles that result in this phenotype
• Requirements for natural selection


  • Variation- individuals that form a population of a species are not identical
  • Inheritance- some variation among individuals is heritable
  • Struggle for Existence- more offspring are born than can survive in an environment
  • Differential Survival and Reproduction- individuals vary in the number of descents they leave behind
• Adaptation


  • A trait of an organism that improves its ability to survive and reproduce within its environment
  • Adaptations improve an organism's fitness
• Natural selection is the only evolutionary mechanism that consistently causes adaptive evolution
• Misconceptions about natural selection


  • Natural selection changes individuals
  • Natural selection is goal oriented
  • Natural selection leads to perfection
  • Natural selection is the only process of evolution
• Evolution is change in allele frequency in a population over time, changes in individuals across their lifespan is not evolution
• Mutations occur by chance, not on purpose, so organisms can't "will" evolution and natural selection can only act on existing variation
• Selection is not progressive and different environments and different times will exert different evolutionary pressures
• Fitness trade-off


  A compromise between two traits that cannot be optimized simultaneously
• Traits are genetically constrained, selection on one trait causes a change in another trait
• Traits evolve from previously evolved traits
• Types of selection


  • Directional
  • Stabilizing
  • Disruptive
• Directional selection


  Individuals at one phenotypic extreme are favored
• Stabilizing selection


  Individuals with an intermediate phenotype are favored