Lecture 19 Speciation 2 Flashcards
Two ways to disrupt gene flow
Reproductive isolation (intrinsic) and geographic isolation (extrinsic)
Disrupting gene flow normally occurs in a 3 step process
1) Isolation of populations (reduce gene flow)
2) divergence of populations (selection/drift)
3) evolution of reproductive barriers
Allopatric
Gap between population’s
Peripatric
Peripheral isolate; extreme case; Isolate’s outside main range and no gene flow after
Parapatric
Non-overlapping without gap
sympatric
overlapping
Some species should be in intermediate stages of formation. How often do we see them and what do they look like?
Many species are found in mid-process, making it difficult to decide on the number of species
Two types of divergence of interest
1) Divergence that has no effect on ability to interbreed
2) Divergence that does
Divergence is typically a
Slow process of accumulating differences
Types of divergence (3)
Local adaptation
Sexual Selection
Drift
Some divergence in characters affecting the ability to interbreed can occur due to
Drift or selection while populations are isolated
Types of isolating mechanisms
Prezygotic (premating)
Postzygotic (postmating)
Types of prezygotic mechanisms
Temporal or habitat isolation
Behavioral or sexual isolation
Mechanical isolation
Gametic incompatibility
Temporal or habitat isolation
Mates do not meet when reproductive
Behavioral or sexual isolation
Mates meet but do not choose the mate
Mechanical isolation
Mating occurs, but gamete are not transferred
Gametic incompatibility
Chemical differences between species preventing fertilization
As you move along the two scales (pre-post zygotic) there is
And more investment from parents
Postzygotic mechanisms
Zygote mortality (zygote dies)
Hybrid inviability
Hybrid sterility
Hybrid breakdown
Hybrid inviability
F1 has reduced survival (severe survival maladaptation
Hybrid sterility
F1 viable but reduced fertility (sterile)
Hybrid breakdown
Reduced viability or fertility in F2 or backcross (selective effects isn’t perceived until generation or two later)
Reinforcement
The process by which selection acts to reduce interbreeding by assortative mating so as to decrease the formation of hybrids
(Evolution of pre-zygotic elements when post-zygotic mechanisms occur)
Prezygotic isolating mechanisms evolve much faster in
Sympatric species than allopatric species
Instantaneous isolating mechanisms
Chromosomal rearrangements
Polyploidy
Hybridization (More common in plants)
Chromosomal rearrangements
Disrupts pairing during meiosis
Can be instantaneous with some translocations or inversions or gradual after several fusions/fissions
