Macroevolution 

  • Unlike microevolution, macroevolution can lead to the formation of a species
  • Species: A group of populations that is, or could reproduce with each other and is isolated from other groups
  • 4 of the microevolutionary mechanisms can lead to the creation of a species from a population
    • Natural selection
    • Mutation
    • Non-random mating
    • genetic drift
  • For species to be distinct they must have reproductive isolation
  • Reproductive isolation: Reproduction between two groups results in a viable and fertile offspring.
  • Prezygotic: Prevents the formation of a zygote
    • Ecological isolation
    • Temporal isolation: different mating seasons
    • Behavioural isolation: different mating rituals
    • Mechanical isolation: Mismatched reproductive structures
    • Prevention of gamete fusion: Mismatched gametes
  • Postzygotic: Prevents the zygote from functioning properly
    • Genetic isolation: mismatched chromosomes prevents embryonic development
    • Hybrid in-viability: Zygote does not reach maturity
    • Hybrid sterility: Sterile offspring are produced
    • Ex. mule
    • Hybrid breakdown: Offspring has reduced fertility
  • Speciation can occur in two ways:
    • Allopatric speciation: Physical or geographic isolation leads to different characteristics acquired through microevolutionary mechanisms
    • Peripheral isolates: Occurs at the outskirts of the parent population. The further the distance from the center of the the population the greater likelihood that individuals will stop breeding with those at the center. The center of the population likely has the best resources creating a selection pressure on the outskirts. Creates multiple founder effects.
    • Sympatric speciation: Individuals that are in the same geographic location but are isolated by other means (reproductive isolation)
    • Autopolyploidy: Doubling of chromosomes to create tetraploid organisms, all copies of the chromosomes are from the same species. Tetraploids can mate with diploids but their offspring are sterile due to the odd number of chromosomes
    • Allopolyploidy: Doubling of chromosomes when to species cross to create a hybrid. The hybrid is often sterile, but miotic disjunction can create a fertile diploid hybrid, allowing it to interbreed with other similar hybrids.
    • Both autopolyploidy and allopolyploidy are mostly seen in plants.
  • The rate of macroevolutionary changes is dependent on:
    • The number of species in the area, the larger the number, the greater the opportunity for new species to form​
    • The range of species in an area, the more species the greater the likelihood of a barrier, or distance, is to geographically subdivide it.​
    • The generation time, shorter times increases the rate of speciation.​
    • Short generation in bacteria resulting in a high number of mutations
    • Complexity of mating, more complex mating may cause more reproductive isolation.
    • Rapid changes in environmental conditions can cause rapid change, that are either successful or lead to extinction.
    • Antibiotic resistance in bacteria
  • Phyletic gradualism​: Based off of Darwin’s work that suggested that change slowly accumulates as adaptation occurs. According to this hypothesis there should be intermediate forms, which is not supported by the fossil record.
  • Punctuated equilibrium:​ Change occurs quicker in smaller population and is more gradual in large populations. Long periods of stability punctuated by short periods of change, which is better supported by the fossil record.
  • Most likely both phyletic gradualism and punctuated equilibrium occur to some degree. Creating small changes over a long period of time, and large changes in a short period