BIOL 2040 Lecture Notes - Lecture 15: Allopatric Speciation, Alpheidae, Erythranthe Guttata
27 Apr 2018
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Lecture 15 - Speciation (pt. 2)
Three classically defined "modes" of speciation
• Allopatric - separate species ranges
o Regions are separated by a physical barrier
o External factor independent of species creates a barrier
• Parapatric - adjacent (or slightly overlapping) ranges
o Don’t need to focus on this
• Sympatric - same range (substantially overlapping)
o Two populations somehow separate without being geographically different
Focus only on allopatric and sympatric
Allopatric speciation
• Conceptually simple
• Barrier to gene flow is external - physical separation in some way
• Divergence can be adaptive or neutral
• Speciation by vicariance
o Start out on an island
o Rising water restricts gene flow from one end of the island to the other
o Barrier doesn't have to restrict all gene flow - just makes it unlikely the populations would
overlap
o Ex. central American snapping shrimp
• A barrier prevented the populations from mingling - this is allopatric speciation
• Speciation by dispersal
o Some individuals from a population disperse onto a habitat that is already isolated from
original habitat
o Barrier doesn't split up population - population moves from one isolated habitat into
another isolated habitat
• Example: allopatric speciation with dispersal in Hawaiian Drosophila
o Many different species - dispersed from old islands onto younger islands
o As islands were formed, from the west (oldest) to east (formed more recently), Drosophila
species dispersed from those old islands towards the more recent ones
• Predictions
o If speciation happens in allopatry (where species diverge because they are separated), then
close relatives will have no overlapping ranges
o Species with overlapping ranges will be distant relatives
o Species that diverged in allopatry must have done so because of a physical barrier - if close
relatives overlapped, then they couldn't have diverged due to allopatry
Sympatric speciation: Predictions
• If speciation happens in sympatry (related species diverge in the same region), then species with
overlapping ranges will be close relatives (sister taxa)
Test of prediction: Grass Finches
• Small ground dwelling Finches are native to Australia
• Some species ranges overlap, some don't
o None of the overlapping ranges belonged to close relatives - distantly-related taxa have
overlapping ranges
• Same results in leopard frogs
Document Summary
Sympatric - same range (substantially overlapping: two populations somehow separate without being geographically different. Allopatric speciation: conceptually simple, barrier to gene flow is external - physical separation in some way, divergence can be adaptive or neutral. If speciation happens in sympatry (related species diverge in the same region), then species with overlapping ranges will be close relatives (sister taxa) Small ground dwelling finches are native to australia. Some species ranges overlap, some don"t: none of the overlapping ranges belonged to close relatives - distantly-related taxa have overlapping ranges. Allopatric speciation: predictions: recent species pairs have little/no range overlap, older species pairs have more range overlap, populations separated by allopatric speciation eventually will come in contact again, even if it"s only a little bit. Sympatric speciation: predictions: recent species pairs have overlapping ranges, older species pairs disperse apart. Allopatric speciation prediction: early on the species don"t overlap (due to barrier), later on there is a little overlap.
