Allopatric speciation can begin when a geographic barrier reduces gene flow. Different selection pressures, mutations and drift may then cause populations to diverge. If reproductive barriers develop, they may remain distinct species even after contact is restored. Geographic separation alone does not guarantee speciation: the key outcome is sufficient reproductive isolation, not merely living on separate sides of a map.
Divergent evolution produces differences among lineages sharing an ancestor. Adaptive radiation is diversification of an ancestral lineage into multiple forms associated with different ecological opportunities. Convergent evolution produces similar traits independently in less closely related lineages exposed to comparable pressures. Similar function therefore does not always mean a recently shared origin of that trait. For example, a streamlined aquatic body can evolve independently.
Example 1
A river separates a population. Later the groups cannot interbreed successfully. Which process fits?
Reduced gene flow followed by divergence and reproductive isolation is consistent with allopatric speciation.
Example 2
Two distantly related aquatic lineages independently evolve streamlining. Name the pattern.
Convergent evolution: similar pressures favour similar form without a recent shared origin of that particular adaptation.