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NCEA Style Question – Allopatric Speciation of stag beetles
The four species of stag beetle in the upper North Island are thought to have evolved from a
single ancestor during the Pliocene rise in sea level.
Discuss allopatric speciation using the NZ stag beetles as an example. Refer to:

The causes of allopatric speciation
Define allopatric speciation. Talk about Sea level rising in the Pliocene, which could have
been the geographical barrier which started/caused the process of speciation.
Isolated groups/populations within the ancestor population… begin to adapt to different
environments… different selection pressures in different areas … gain different adapations…
reduced gene flow between different groups/populations.

The relevant reproductive isolating mechanisms that develop during the process
Two main types of reproductive isolating mechanisms: prezygotic and postzygotic
mechanisms.
Prezygotic: Temporal (eg breed at different times of the year; active at different times of
day). Mechanical (genital structure may no longer fit like a lock and key). Behavioural:
different mating displays not recognised by the other group. Gamete incompatibility: eg.
different number of chromosomes in the sperm & egg of each population so can’t make a
zygote.
Postzygotic: Hybrid sterility (offspring can’t breed); Hybrid breakdown (offspring is weak,
deformed, often stillborn); Hybrid inviability (may produce zygote but genetic
incompatibility stops development).
Some or all of these repro iso mechanisms could be acting on the isolated groups of stag
beetles over time….. causing changes to each group…. Further reducing gene flow between
the populations… you could insert diagram from Page 120 Biozone to show the splitting of
the ancestral pop’n into diff’t species over time.
Isolated groups turned into isolated subspecies and finally separate species x 4, all
genetically isolated from each other due to the various factors outlined above. No gene
flow is now possible between species.

Why two species, which are now overlapping in their distribution, remain separate
species.
Link back to the repro isolating mechanisms (above), when the geographic barrier is
removed and 2 species now overlap, they can no longer interbreed successfully due to these
factors. Now called “sympatric species”.