11 Geographic Factors in Speciation
Summary
Define sympatric and allopatric speciation and describe how each might
occur.
Sympatric speciation occurs in situations in which a geographic barrier does not isolate the populations
undergoing speciation. The two or more species arising from sympatric speciation overlap in their
geographic ranges. Allopatric speciation occurs in populations that are separated geographically. In either
case, reproductive isolation prevents interbreeding, even though populations in sympatry could freely mate.
Evidence for sympatric speciation is rare, but in those cases in which it is likely to have occurred,
reproductive barriers typically include assortative mating (individuals are more likely to mate with similar
individuals) and sometimes small-scale habitat differentiation.
Recognize the scientific controversy surrounding sympatric speciation.
Although theoretically possible, the conditions required for sympatric speciation are difficult to achieve.
Sympatric populations can successfully interbreed unless reproductive barriers are strong (such as different
timing of breeding or microsite variation). Most potential cases of sympatric speciation are ruled out
because closer investigation often reveals some degree of geographic isolation.
Discuss factors that may weaken or strengthen reproductive barriers in
hybrid zones.
Reproductive barriers in hybrid zones can be weakened by fusion, in which two species produce fertile,
successful offspring. The barriers can be strengthened by reinforcement, when hybrids are less fit than
either of the parent species.
Key Terms
allopatric speciation
Speciation associated with geographical isolation of the populations involved.
fusion
Two recently diverged species start exchanging genes again.
habitat differentiation
Sympatric speciation scheme in which mutations in a population allow individuals to exploit different
conditions within the same environment.
hybrid
An offspring resulting from the mating of individuals of different species.
polyploidy
Condition in which more than two sets of chromosomes are present in an individual. For example, the
strawberry is octoploid, or 8N.
reinforcement
Hybrids are less fit and therefore selected against.
sympatric speciation
Speciation from a single ancestor in the same geographic region.