23.2 How Do New Species Arise?
Allopatric speciation requires almost complete genetic isolation
•Speciation that results when a populations is divided by a physical barrier.
•Is thought to be the dominant mode of speciation among most groups of
•The populations separated by such barriers are often, but not always, initially
•They evolved difference for reasons including gene drift, but especially
because the environments in which they live are, or become, different.
•Allopatric speciation may also result when some members of a population
cross an existing barrier and found a new, isolated population.
•A physical barrier’s effectiveness at preventing gene flow depends on the
size and mobility of the species in question.
Sympatric speciation occurs without physical barriers
•A partition of a gene pool without physical isolation.
•What is required is some form of disruptive selection in which certain
genotypes have high fitness on one or the other of two resources.
•Sympatric speciation via ecological isolation may be widespread among
insects, many of which feed on a single plant species.
•BUT most common means of sympatric speciation is polyploidy—the
production within an individual of duplicate sets of chromosomes.
•Polyploidy can arise from chromosomes duplication in a single species
(autopolyploidy) or from the combing of the chromosomes of two different
•Allopolyploids may also be produced when individuals of two different species
interbreed or hybridized.
23.3 What Happens when Newly Formed Species Come Together?
•Reproductive isolation can evolve as an incidental by-product of genetic
changes in allopatric populations.
•Geographic isolation does not necessarily lead to reproductive isolation,
however, b/c genetic divergence does not cause reproductive isolation to
appear as a by-product.
Prezygotic barriers operate before fertilization
•Mechanisms that operate before fertilization—prezygotic reproductive
barriers—may prevent individuals of different species or populations from
Postzygotic barriers operate after fertilization
•If individuals of two different populations lack complete prezygotic
reproductive barriers, postzygotic reproductive barriers may still prevent