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ever been found in birds. Two mammalian species are suspected tetraploids, the red vizcacha rat (Tympanoctomys barrerae) and the golden vizcacha rat (Pipanacoctomys aureus), however, some disagreement exists in the literature. 4N = 100 + XY
What happens When Newly Divergent Species Reconnect?•Mechanisms of Reducing Fitness of Hybrids–Prezygotic isolating–Postzygotic isolating•Postzygotic Isolating Mechanisms reinforce Prezygotic Isolating Mechanisms•Hybrid Zones may develop if Reproductive Isolation is Incomplete
Summary of Pre- and Postzygotic Mechanisms That Contribute to Continued Reproductive Isolation
Prezygotic Isolating Mechanisms are Acquired Traits which Prevent Mating Between Two PopulationsEcological Isolation:Species geographical ranges overlap but they have become adapted to different habitats Behavioral Isolation:There is a behavioral component to mating that is species specificMechanical Isolation:Incompatible reproductive structuresMale reproductive organs from different species of damsel flies
Postzygotic Isolating Mechanisms are the Result of Genetic Differences Which Inhibit Full Hybridization•Hybrid Inviability–Hybrids fail to develop to reproductive maturity•Hybrid Infertility–Hybrids become reproductively mature but are sterileHorses have 64 chromosomes (n=32)Donkeys have 62 chromosomes (n=31)Mules have 63 chromosomes (n+1 or n-1)Why is this a problem in Meiosis but NOT Mitosis?
Postzygotic Isolating Mechanisms are the Result of Genetic Differences Which Inhibit Full HybridizationHybrid InstabilityF1Fertile but F2has issues•lethality•infertility•reduced fitness
Postzygotic Isolation Reinforces Prezygotic Isolation•So even if the adaptation differences can be overcome (Timing, habitat preference, behavioral cues and physical differences) reduced fitness of hybrids prevents gene flow–therefore postzygotic isolation is a selection force that reinforces and increases the fitness of individuals that mate based upon the prezygotic isolation mechanisms
If Reproductive Isolation is Incomplete Hybridization Can Occur•Allopolyploidy is an example of formation of hybrids which can be adaptive•For ring species there are intermediate “hybrids” found in the overlapping ranges of the morphologically different subspecies (populations)
Species Specific Traits May Limit or Accelerate Speciation•Dietary Specialization•Pollinator Specialization•Sexual Selection•Dispersal Ability•Evolutionary Radiation–Adaptive Radiation
Dietary Specialization•Individuals within a population may have allelic combinations which allow them to prey on or consume organisms which are newly introduced into a population–Akin to the formation of a new habitat but specific to acquiring nutrients•i.e. Apple Maggot Flies
Pollinator Speciation•Coevolution drives selection–plants and pollinators adapt to newly acquired traits found in populations of the “partner”Speciation driven by pollinatorsH = hummingbirdB = batM = moth Note colors and shape of flowers
Sexual Selection•One sex preferentially mates with the other sex based upon an inherited physical trait•This can lead to a rapid change because it is directly affects reproductive fitness