Biology Reference
In-Depth Information
hybridization, polyploidy, or both. Polyploidy can be the result in an
increase in the number of sets of chromosomes in individuals of one
species, a process termed autopolyploidy, or an increase in the number of
chromosome sets in individuals produced by interspecific hybridization, a
process termed allopolyploidy. For polyploids to be fertile, an even num-
ber of sets of chromosomes must usually exist, so that chromosome pair-
ing in meiosis is possible.Triploid forms resulting from autopolyploidy or
allopolyploidy are usually sterile. Homoploid hybrid speciation involves
the hybridization of species with the same chromosome number, leading
to a form that is distinct and has strong reproductive isolation without the
occurrence of polyploidy (Rieseberg 1997; Rieseberg et al. 2003). Speci-
ation can also result from the appearance of a genetic characteristic that
leads to reproductive isolation between sets of individuals in a formerly
fully interbreeding population.
For animals, various processes may lead to rapid speciation. Polyploidy
is less common in animals but occurs in many molluscs, crustaceans,
insects, fish, amphibians, and a few higher vertebrates. Many of the poly-
ploid forms do not reproduce sexually, so forms different in chromosome
number are essentially reproductively isolated. For animals, however, spe-
ciation results more often by divergent natural selection. Allopatric and,
possibly, sympatric speciation have apparently resulted in the recent ori-
gin of hundreds of species of cichlid fishes in East African lakes such as
Lake Malawi (Owen et al. 1990) and Lake Victoria (Seehausen and Van
Alphen 1999). The Lake Victoria basin is inferred to have completely
dried out in the Late Pleistocene, so the endemic species of cichlids have
apparently arisen within the past 12,000 yr (Johnson et al. 1996).The pre-
cise mechanisms involved in these remarkable radiations are still some-
what unclear but may include disruptive sexual selection, as well as selec-
tion for adaptation to diverse habitats. Seehausen et al. (2003) showed that
the ancestors of this species flock were members of the genus
Thora-
cochromis
of the Congo and Nile river systems and that much of the
genetic diversity that permitted rapid speciation in Lake Victoria proba-
bly arose much earlier. Extensive hybridization and genetic introgression
likely occurred among species colonizing the reformed Lake Victoria. In
any case, the evidence is now strong that unusual biogeographic events,
such as the massive introduction of species to new geographic areas, can
lead quickly to the origin of new species.
Darwin's finches, confined to the Galapagos Islands and Cocos Island,
720 km to the north, are another group of rapidly evolving species
(Petren et al. 1999). Differentiation of populations into reproductively iso-
